View source for Mosasaurus

{{Short description|Extinct genus of marine squamate reptile from the Late Cretaceous}}
{{Distinguish|Mesosaurus{{!}}''Mesosaurus''}}
{{Use mdy dates|date=April 2021}}
{{Automatic taxobox
|fossil_range = [[Campanian]]-[[Maastrichtian]], {{fossil range|82.7|66.0|ref=<ref name=GeologicTime>{{citation|author1=James G. Ogg|author2=Linda A. Hinnov|chapter=Cretaceous|year=2012|pages=793–853|editor1=Felix M. Gradstein|editor2=James G. Ogg|editor3=Mark D. Schmitz|editor4=Gabi M. Ogg|title=The Geologic Time Scale|publisher=Elsevier|place=Oxford|doi=10.1016/B978-0-444-59425-9.00027-5|isbn=978-0-444-59425-9|s2cid=127523816}}</ref><ref name=Gallagher2005>{{cite journal|author=William B. Gallagher|title=Recent mosasaur discoveries from New Jersey and Delaware, USA: stratigraphy, taphonomy and implications for mosasaur extinction|year=2005|journal=Netherlands Journal of Geosciences|volume=84|issue=3|pages=241–245|doi=10.1017/S0016774600021028|doi-access=free|bibcode=2005NJGeo..84..241G }}</ref><ref name=Gallagher1984>{{cite journal|author=William B. Gallagher|title=Paleoecology of the Delaware Valley region, Part II: Cretaceous to Quaternary|year=1984|journal=The Mosasaur|volume=2|issue=1|pages=9–43|url=https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxkdnBzcGFsZW98Z3g6NzMyMmYyZDM2ZGE0MWE5MA}}</ref><ref name=Obasietal>{{cite journal|author1=Christian C. Obasi|author2=Dennis O. Terry Jr.|author3=George H. Myer|author4=David E. Grandstaff|title=Glauconite Composition and Morphology, Shocked Quartz, and the Origin of the Cretaceous(?) Main Fossiliferous Layer (MFL) in Southern New Jersey, U.S.A.|year=2011|journal=Journal of Sedimentary Research|volume=81|issue=1|pages=479–494|doi=10.2110/jsr.2011.42|bibcode=2011JSedR..81..479O}}</ref>}}
|image = Mosasaurus hoffmannii - skeleton.jpg
|image_upright = 1.15
|image_caption = Reconstructed skeleton of ''M. hoffmannii'' at the [[Maastricht Natural History Museum]]
|taxon = Mosasaurus
|authority = [[William Daniel Conybeare|Conybeare]], 1822
|type_species = {{extinct}}'''''Mosasaurus hoffmannii'''''
|type_species_authority = [[Gideon Mantell|Mantell]], 1829
|subdivision_ranks = Other species
|subdivision =
*{{extinct}}'''''M. missouriensis''''' {{small|([[Richard Harlan|Harlan]], 1834)}}
*{{extinct}}'''''M. conodon''''' {{small|([[Edward Drinker Cope|Cope]], 1881)}}
*{{extinct}}'''''M. lemonnieri''''' {{small|([[Louis Dollo|Dollo]], 1889)}}
*{{extinct}}'''''M. beaugei''''' {{small|[[Camille Arambourg|Arambourg]], 1952}}
{{collapsible list|title=Species pending reassessment|
*{{extinct}}'''''[[Antipodenectes|M. mokoroa]]''''' {{small|Welles & Gregg, 1971}}
*{{extinct}}'''''[[Moanasaurus hobetsuensis|M. hobetsuensis]]''''' {{small|Suzuki, 1985}}
*{{extinct}}'''''M. flemingi''''' {{small|[[Joan Wiffen|Wiffen]], 1990}}
*{{extinct}}'''''[[Umikosaurus|M. prismaticus]]''''' {{small|Sakurai ''et al.'', 1999}}
}}
| synonyms =
{{collapsible list|title=List of synonyms|
{{collapsible list|title=Synonyms of genus<ref name=StreetandCaldwell /><ref name=Leidy1864>{{cite book|author=Joseph Leidy|title=Cretaceous Reptiles of the United States|year=1864|publisher=Smithsonian Contributions to Knowledge|volume=14|pages=30–120|url={{google books|plainurl=yes|id=Nb1BAQAAMAAJ|page=30}}}}</ref>|
*''Batrachiosaurus'' {{small|Harlan, 1839}}
*''Batrachiotherium'' {{small|Harlan, 1839}}
*''Macrosaurus'' {{small|Owen, 1849}}
*''Drepanodon'' {{small|Leidy, 1856}}
*''Lesticodus'' {{small|Leidy, 1859}}
*''Baseodon'' {{small|Leidy, 1865}}
*''Nectoportheus'' {{small|Cope, 1868}}
*''Pterycollosaurus'' {{small|Dollo, 1882}}
}}
{{collapsible list|title=Synonyms of ''M. hoffmannii''<ref name=StreetandCaldwell /><ref name=HarrellandMartin>{{cite journal|author1=T. Lynn Harrell Jr.|author2=James E. Martin|title=A mosasaur from the Maastrichtian Fox Hills Formation of the northern Western Interior Seaway of the United States and the synonymy of ''Mosasaurus maximus'' with ''Mosasaurus hoffmanni'' (Reptilia: Mosasauridae)|year=2014|journal=Netherlands Journal of Geosciences|volume=94|issue=1|pages=23–37|doi=10.1017/njg.2014.27|s2cid=131617632|doi-access=free}}</ref>|
*''Lacerta gigantea'' {{small|von Sömmerring, 1820}}
*''Mososaurus hoffmannii'' {{small|Mantell, 1829}}
*''Mosasaurus belgicus'' {{small|Holl, 1829}}
*''Mosasaurus camperi'' {{small|Meyer, 1832}}
*''Mosasaurus dekayi'' {{small|Bronn, 1838}}
*''Mosasaurus hoffmanni'' {{small|Owen, 1840}}
*''Mosasaurus major'' {{small|De Kay, 1842}}
*''Mosasaurus occidentalis'' {{small|Morton, 1844}}
*''Mosasaurus meirsii'' {{small|Marsh, 1869}}
*''Mosasaurus princeps'' {{small|Marsh, 1869}}
*''Mosasaurus maximus'' {{small|Cope, 1869}}
*''Mosasaurus giganteus'' {{small|Cope, 1869}}
*''Mosasaurus fulciatus'' {{small|Cope, 1869}}
*''Mosasaurus oarthus'' {{small|Cope, 1869}}
}}
{{collapsible list|title=Synonyms of ''M. missouriensis''<ref name=CaldwellandBell /><ref name=Konishietal />|
*''Ichthyosaurus missouriensis'' {{small|Harlan, 1834}}
*''Ictiosaurus missuriensis'' {{small|Harlan, 1834}}
*''Batrachiosaurus missouriensis'' {{small|Harlan, 1839}}
*''Batrachiotherium missouriensis'' {{small|Harlan, 1839}}
*''Mosasaurus maximiliani'' {{small|Goldfuss, 1845}}
*''Mosasaurus neovidii'' {{small|Meyer, 1845}}
*''Pterycollosaurus maximiliani'' {{small|Dollo, 1882}}
*''Mosasaurus horridus'' {{small|Williston, 1895}}
}}
{{collapsible list|title=Synonyms of ''M. conodon''<ref name=IkejiriandLucas />|
*''Clidastes conodon'' {{small|Cope, 1881}}
}}
}}
}}

'''''Mosasaurus''''' ({{IPAc-en|ˌ|m|oʊ|z|ə|ˈ|s|ɔːr|ə|s}}; "lizard of the [[Meuse (river)|Meuse River]]") is the [[type genus]] (defining example) of the [[mosasaur]]s, an extinct group of aquatic [[Squamata|squamate reptiles]]. It lived from about 82 to 66 million years ago during the [[Campanian]] and [[Maastrichtian]] [[Stage (stratigraphy)|stages]] of the [[Late Cretaceous]]. The genus was one of the first Mesozoic marine reptiles known to science—the first fossils of ''Mosasaurus'' were found as skulls in a chalk quarry near the Dutch city of [[Maastricht]] in the late 18th century, and were initially thought to be crocodiles or whales. One skull discovered around 1780 was famously nicknamed the "great animal of Maastricht". In 1808, naturalist [[Georges Cuvier]] concluded that it belonged to a giant marine lizard with similarities to [[monitor lizard]]s but otherwise unlike any known living animal. This concept was revolutionary at the time and helped support the then-developing ideas of [[extinction]]. Cuvier did not designate a scientific name for the animal; this was done by [[William Daniel Conybeare]] in 1822 when he named it ''Mosasaurus'' in reference to its origin in fossil deposits near the Meuse River. The exact affinities of ''Mosasaurus'' as a squamate remain controversial, and scientists continue to debate whether its closest living relatives are monitor lizards or [[snake]]s.

The largest species, ''M. hoffmannii'', is estimated to measure up to {{convert|12|m|ft|sp=us}} in maximum length, making it one of the largest mosasaurs. The skull of ''Mosasaurus'' had robust jaws and strong muscles capable of powerful bites using dozens of large teeth adapted for cutting [[prey]]. Its four limbs were shaped into paddles to steer the animal underwater. Its tail was long and ended in a downward bend and a paddle-like fluke. ''Mosasaurus'' possessed excellent vision to compensate for its poor sense of smell, and a high metabolic rate suggesting it was [[endotherm]]ic ("warm-blooded"), an adaptation in squamates only found in mosasaurs. There is considerable morphological variability across the currently-recognized species in ''Mosasaurus''—from the robustly-built ''M. hoffmannii'' to the slender and serpentine ''M. lemonnieri''—but an unclear [[Species description|diagnosis]] (description of distinguishing features) of the type species ''M. hoffmannii'' led to a historically problematic classification. As a result, more than fifty species have been attributed to the genus in the past. A redescription of the [[type specimen]] in 2017 helped resolve the taxonomy issue and confirmed at least five species to be within the genus. Another five species still nominally classified within ''Mosasaurus'' are planned to be reassessed.

Fossil evidence suggests ''Mosasaurus'' inhabited much of the Atlantic Ocean and the adjacent seaways. ''Mosasaurus'' fossils have been found in North and South America, Europe, Africa, Western Asia, and Antarctica. This distribution encompassed a wide range of oceanic climates including tropical, subtropical, temperate, and subpolar. ''Mosasaurus'' was a common large predator in these oceans and was positioned at the top of the [[food chain]]. Paleontologists believe its diet would have included virtually any animal; it likely preyed on bony fish, sharks, [[cephalopods]], birds, and other marine reptiles including [[sea turtle]]s and other mosasaurs. It likely preferred to hunt in open water near the surface. From an ecological standpoint, ''Mosasaurus'' probably had a profound impact on the structuring of marine ecosystems; its arrival in some locations such as the [[Western Interior Seaway]] in North America coincides with a complete turnover of [[faunal assemblage]]s and diversity. ''Mosasaurus'' faced competition with other large predatory mosasaurs such as ''[[Prognathodon]]'' and ''[[Tylosaurus]]''{{Em dash}}which were known to feed on similar prey{{Em dash}}though they were able to coexist in the same ecosystems through [[niche partitioning]]. There were still conflicts among them, as an instance of ''Tylosaurus'' attacking a ''Mosasaurus'' has been documented. Several fossils document deliberate attacks on ''Mosasaurus'' individuals by members of the same species. Fighting likely took place in the form of snout grappling, as seen in modern crocodiles.
{{TOC limit|3}}

==Research history==
{{Main|Research history of Mosasaurus|l1=Research history of ''Mosasaurus''}}

===Discovery and identification===
[[Image:Mosasaurus hoffmanni first specimen.jpg|thumb|left|TM 7424, the first known specimen of ''M. hoffmannii'']]
The first ''Mosasaurus'' fossil known to science was discovered in 1764 in a [[chalk]] quarry near [[Maastricht]] in the Netherlands in the form of a skull, which was initially identified as a [[whale]].<ref>{{cite journal|author=Martinus van Marum|year=1790|title=Beschrijving der beenderen van den kop van eenen visch, gevonden in den St Pietersberg bij Maastricht, en geplaatst in Teylers Museum|trans-title=Description of the bones of the head of a fish, found in the St Pietersberg near Maastricht, and placed in Teylers Museum|journal=Verhandelingen Teylers Tweede Genootschap|volume=9|language=Dutch|pages=383–389|url=https://www.delpher.nl/nl/boeken/view?coll=boeken&identifier=dpo:1297:mpeg21:0388}}</ref> This specimen, cataloged as TM 7424, is now on display at the [[Teylers Museum]] in [[Haarlem]].<ref name=OOKMosasaurus1>{{cite web|author=Mike Everhart|title=''Mosasaurus hoffmanni''-The First Discovery of a Mosasaur?|date=May 14, 2010|website=Oceans of Kansas|access-date=November 6, 2019|url=http://oceansofkansas.com/mosahoff.html|archive-url=https://web.archive.org/web/20190904012849/http://oceansofkansas.com/mosahoff.html|archive-date=September 4, 2019|url-status=live}}</ref><ref name=Mulder /><ref name=Pietersetal /> Later around 1780,{{efn|The exact year is not fully certain due to multiple contradicting claims. An examination of existing historical evidence by Pieters et al., (2012) suggested the most accurate date would be on or around 1780.<ref name=Pietersetal /> More recently, [[Limburg (Netherlands)|Limburg]] newspapers reported in 2015 that [[Ernst Homburg]] discovered a [[Liège]] magazine issued in the October 1778 reporting in detail a recent discovery of the second skull.<ref name=Limburg>{{cite news|author=Vikkie Bartholomeus|title=Datum vondst mosasaurus ontdekt: in oktober 1778|trans-title=Date of mosasaur discovered: in October 1778|language=Dutch|work=1Limburg|date=September 21, 2015|url=https://www.1limburg.nl/datum-vondst-mosasaurus-ontdekt-oktober-1778|archive-url=https://web.archive.org/web/20200307171343/https://www.1limburg.nl/datum-vondst-mosasaurus-ontdekt-oktober-1778|archive-date=March 7, 2020}}</ref>}} the quarry produced a second skull that caught the attention of the physician [[Johann Leonard Hoffmann]], who thought it was a [[crocodile]]. He contacted the prominent biologist [[Petrus Camper]], and the skull gained international attention after Camper published a study identifying it as a whale.<ref name=Mulder /><ref name=Camper>{{cite journal|author=Petrus Camper|title=Conjectures relative to the petrifactions found in St. Peter's Mountain near Maestricht|year=1786|journal=Philosophical Transactions of the Royal Society of London|volume=76|issue=2|pages=443–456|doi=10.1098/rstl.1786.0026|issn=2053-9223|doi-access=free}}</ref><ref name=PietersEssay>{{cite book |author=Florence F. J. M. Pieters|chapter=Natural history spoils in the Low Countries in 1794/95: the looting of the fossil ''Mosasaurus'' from Maastricht and the removal of the cabinet and menagerie of stadholder William V|title=Napoleon's Legacy: The Rise of National Museums in Europe, 1794-1830 |editor1=Ellinoor Bergvelt|editor2=Debora J. Meijers|editor3=Lieske Tibbe|editor4=Elsa van Wezel|pages=55–72|year=2009|publisher=G+H Verlag|location=Berlin |isbn=978-3-940939-11-1|hdl=2066/79083|s2cid=164995758|chapter-url=https://pure.uva.nl/ws/files/1052778/79068_325600.pdf}}</ref> This caught the attention of [[French Revolution|French revolutionaries]], who looted the fossil following the [[Siege of Maastricht (1794)|siege of Maastricht]] during the [[French Revolutionary Wars]] in 1794.<ref name=PietersEssay /><ref name=Pietersetal /> In a 1799 narrative of this event by [[Barthélemy Faujas de Saint-Fond]], the skull was allegedly retrieved by twelve [[grenadier]]s in exchange for an offer of 600 bottles of wine.<ref>{{cite book|language=French|author=Barthélemy Faujas de Saint-Fond|title=Histoire naturelle de la montagne de Saint-Pierre de Maestricht|trans-title=Natural history of the Mount Saint Peter of Maestricht|chapter=Tête du crocodile|trans-chapter=Crocodile head|pages=59–67|publisher=H. J. Jansen|location=Paris|oclc=22224984|year=1799|chapter-url=https://www.biodiversitylibrary.org/item/280833#page/75/mode/1up|url=https://www.biodiversitylibrary.org/item/280833#page/9/mode/1up}}</ref> This story helped elevate the fossil into cultural fame, but historians agree that the narrative was exaggerated.<ref name=PietersEssay /><ref name=Pietersetal>{{cite journal|author1=Florence F. J. M. Pieters|author2=Peggy G. W. Rompen|author3=John W. M. Jagt|author4=Nathalie Bardet|title=A new look at Faujas de Saint-Fond's fantastic story on the provenance and acquisition of the type specimen of ''Mosasaurus hoffmanni'' Mantell, 1829|year=2012|journal=Bulletin de la Société Géologique de France|volume=183|issue=1|pages=55–65|doi=10.2113/gssgfbull.183.1.55|s2cid=130192719|issn=0037-9409}}</ref>

{{multiple image
|align = right
|total_width = 500

|image1 = Mosasaurus2.JPG

|image2 =MosasaurDiscovery.jpg

|footer = MNHN AC 9648, the second skull and [[holotype]] of ''M. hoffmannii'', which was nicknamed the "great animal of Maastricht" (left) and [[Barthélemy Faujas de Saint-Fond|Faujas']] 1799 interpretation of its excavation (right)
}}
After its seizure, the second skull was sent to the [[National Museum of Natural History, France]] in 1795 and later cataloged as MNHN AC 9648.<ref name=Pietersetal /> By 1800, Camper's son [[Adriaan Gilles Camper]] concluded that the fossil, which by then was nicknamed the "great animal of Maastricht", belonged to a [[marine reptile]] sharing affinities to monitor lizards, but otherwise unlike any modern animal.<ref name="AGCamper1800">{{cite journal|author=Adriaan Gilles Camper|title=Sur les ossements fossiles de la montagne de St. Pierre, à Maëstricht|trans-title=On the fossil bones of the Saint Peter mountain, in Maastricht|journal=Journal de Physique, de Chimie d'Histoire Naturelle et des Arts|year=1800|volume=51|pages=278–291|url=https://www.biodiversitylibrary.org/item/29658#page/290/mode/1up}}</ref> [[Georges Cuvier]] confirmed the observations of Camper Jr. in a more in-depth study which was published in 1808.<ref name="Cuvier1808">{{cite journal|language=French|author=Georges Cuvier|title=Sur le grand animal fossile des carrières de Maestricht|trans-title=On the great fossil animal of the quarries of Maastricht|journal=Annales du Muséum d'Histoire naturelle|year=1808|volume=12|pages=145–176|url=https://www.biodiversitylibrary.org/item/91672#page/148/mode/1up}}</ref><ref name=OOKMosasaurus1/><ref name=Mulder>{{cite book|author=Eric Mulder|chapter=Maastricht Cretaceous finds and Dutch pioneers in vertebrate palaeontology|title=Dutch Pioneers of the Earth Sciences|editor1=Jacques L. R. Touret|editor2=Robert P. W. Visser|isbn=978-9-069-84389-6|year=2004|publisher=[[Royal Netherlands Academy of Arts and Sciences]]|pages=165–176|chapter-url=https://www.researchgate.net/publication/270286099}}</ref> The skull became part of Cuvier's first speculations about the conception of [[extinction]], which later led to his theory of [[catastrophism]], a precursor to the theory of [[evolution]]. At the time, it was not believed that a species could go extinct, and fossils of animals were often interpreted as some form of an [[extant taxon|extant]] species.<ref name=Evans2010>{{cite journal|author=Mark Evans|s2cid=84158087|title=The roles played by museums, collections and collectors in the early history of reptile palaeontology|year=2010|journal=Geological Society, London, Special Publications|volume=343|issue=1|pages=5–29|doi=10.1144/SP343.2|bibcode=2010GSLSP.343....5E}}</ref> Cuvier's idea that there existed an animal unlike any today was revolutionary at the time, and in 1812 he proclaimed, "Above all, the precise determination of the famous animal from Maastricht seems to us as important for the theory of zoological laws, as for the history of the globe."<ref name=PietersEssay /><ref name=Pietersetal /> In a 1822 work by [[James Parkinson]], [[William Daniel Conybeare]] coined the genus ''Mosasaurus'' from the [[Latin]] ''Mosa'' "[[Meuse]]" and the [[Ancient Greek]] σαῦρος (''saûros'', "lizard"), all literally meaning "lizard of the Meuse", in reference to the river where the holotype specimen was discovered nearby.<ref>{{citation|author=William D. Conybeare|chapter=Fossil crocodiles and other saurian animals|editor=James Parkinson|title=An Introduction to the Study of Fossil Organic Remains|location=[[London]]|publisher=Printed for the Author|year=1822|volume=3|page=298|chapter-url={{google books|plainurl=yes|id=lvsJAAAAIAAJ|page=298}}|url={{google books|plainurl=yes|id=lvsJAAAAIAAJ}}}}</ref><ref name=OOKMosasaurus1/><ref name=Etymology/> In 1829, [[Gideon Mantell]] added the [[Specific name (zoology)|specific epithet]] ''hoffmannii'', in honor to Hoffmann.<ref name="Mantell1829">{{cite journal|author=Gideon Mantell|title=A tabular arrangement of the organic remains of the county of Sussex|journal=[[Transactions of the Geological Society of London]]|year=1829|series=2|volume=3|pages=201–216|doi=10.1144/TRANSGSLB.3.1.201 |s2cid=84925439|url=https://upload.wikimedia.org/wikipedia/commons/d/dc/A_tabular_arrangement_of_the_organic_remains_of_the_county_of_Sussex.pdf}}</ref>{{efn|''hoffmannii'' was the original spelling used by Mantell, ending with -ii.<ref name="Mantell1829"/> Later authors began to drop the final letter and spelled it as ''hoffmanni'', as became the trend for specific epithets of similar structure in later years. Recent scientists argue that the special etymological makeup of ''hoffmannii'' cannot be subjected to [[International Code of Zoological Nomenclature]] Articles 32.5, 33.4, or 34, which would normally protect similar respellings. This makes ''hoffmannii'' the valid spelling, although ''hoffmanni'' continues to be incorrectly used by many authors.<ref name=Konishietal>{{cite journal|author1=Takuya Konishi|author2=Michael Newbrey|author3=Michael Caldwell|title=A small, exquisitely preserved specimen of ''Mosasaurus missouriensis'' (Squamata, Mosasauridae) from the upper Campanian of the Bearpaw Formation, western Canada, and the first stomach contents for the genus|year=2014|journal=Journal of Vertebrate Paleontology|volume=34|issue=4|pages=802–819|doi=10.1080/02724634.2014.838573|jstor=24523386|bibcode=2014JVPal..34..802K |s2cid=86325001}}</ref>}} Later, the second skull is designated as the new species' [[holotype]] (defining example).<ref name=LinghamSoliar /><ref name=StreetandCaldwell />

===Later discoveries and other species===
[[Image:Mosasaurus missouriensis.jpg|thumb|left|''M. missouriensis'' holotype, with the Harlan snout (MNHN 9587) attached to the Goldfuss skull (RFWUIP 1327); drawn in 1834 and 1845 respectively]]
In 1804, the [[Lewis and Clark Expedition]] discovered a now-lost fossil skeleton alongside the [[Missouri River]], which was identified as a {{convert|45|ft|m|sp=us|adj=on}} long fish.<ref name=LewisandClark>{{cite web|title=September 10, 1804|website=Journals of the Lewis & Clark Expedition|access-date=November 10, 2019|url=https://lewisandclarkjournals.unl.edu/item/lc.jrn.1804-09-10|archive-url=https://web.archive.org/web/20191111060546/https://lewisandclarkjournals.unl.edu/item/lc.jrn.1804-09-10|archive-date=November 11, 2019}}</ref><ref name=OOKMosasaurus2>{{cite web|title=The Goldfuss Mosasaur|author=Mike Everhart|date=October 21, 2013|website=Oceans of Kansas|access-date=November 10, 2019|url=http://oceansofkansas.com/Goldfuss.html|archive-url=https://web.archive.org/web/20190602090558/http://oceansofkansas.com/Goldfuss.html|archive-date=June 2, 2019}}</ref> [[Richard Ellis (biologist)|Richard Ellis]] speculated in 2003 that this may have been the earliest discovery of the second species ''M. missouriensis'',<ref name=Ellis>{{cite book|author=Richard Ellis|year=2003|title=Sea Dragons: Predators of the Prehistoric Oceans|publisher=University Press of Kansas|isbn=978-0-7006-1394-6|page=216|url=https://archive.org/details/seadragonspredat0000elli/page/216/mode/1up}}</ref> although competing speculations exist.<ref name=Meredithetal>{{cite journal|author1=Robert W. Meredith|author2=James E. Martin|author3=Paul N. Wegleitner|title=The largest mosasaur (Squamata: Mosasauridae) from the Missouri River area (Late Cretaceous; Pierre Shale Group) of South Dakota and its relationship to Lewis and Clark|journal=Geological Society of America Special Papers|year=2007|volume=427|pages=209–214|doi=10.1130/2007.2427(15)|isbn=978-0-8137-2427-0 |s2cid=11503297|url=https://www.montclair.edu/profilepages/media/5008/user/Meredith,_Martin_2007_The_largest_mosasaur_(Squamata_Mosasauridae)_from_the_Missouri_River_area_(Late_Cretaceous_Pierre_Shale_Group)_of_South_Dakota_and_its_relationship_to_Lewis.pdf}}</ref> In 1818, a fossil from [[Monmouth County, New Jersey]] became the first North American specimen to be correctly recognized as a ''Mosasaurus'' by scientists of the time.{{efn|Because the genus ''Mosasaurus'' was not coined at the time, the original identifier, [[Samuel L. Mitchill]], described the fossil as a lizard monster or saurian animal resembling the famous fossil reptile of Maestricht <nowiki>[</nowiki>''sic''<nowiki>]</nowiki>."<ref name=Dekay /> Cuvier doubted whether the two specimens were related. The congeneric relationship was eventually confirmed by [[James Ellsworth De Kay]] in 1830,<ref name=Dekay /> and the New Jersey fossil was named ''Mosasaurus dekayi'' in his honor.<ref name=Bronn>{{cite book|author=Heinrich Georg Bronn|title=Lethaea Geognostica Oder Abbildungen und Beschreibungen Der für die Gebirgs-Formationen bezeichnendsten Versteinerungen|trans-title=Lethaea Geognostica Or illustrations and descriptions of the fossils most characteristic of the mountain formations|volume=2|year=1838|pages=760|publisher=Stuttgart|language=German|doi=10.5962/bhl.title.59080|url=https://babel.hathitrust.org/cgi/pt?id=nyp.33433090769179&view=1up&seq=228}}</ref> The taxon was declared a [[nomen dubium]] in 2005,<ref name=Gallagher2005 /> and other fossils attributed to it were reidentified as ''M. hoffmannii''.<ref name=Mulder1999 /><ref name="StreetThesis"/>{{rp|108}}}}<ref name=Dekay>{{cite journal|title=On the Remains of Extinct Reptiles of the genera ''Mosasaurus'' and ''Geosaurus'' found in the secondary formation of New-Jersey; and on the occurrence of the substance recently named Coprolite by Dr. Buckland, in the same locality |author=James Ellsworth De Kay|year=1830|journal=Annals of the Lyceum of Natural History of New York|volume=3|pages=134–141|url=https://www.biodiversitylibrary.org/item/54530#page/142/mode/1up}}</ref>

The [[type (biology)|type specimen]] of ''M. missouriensis'' was first described in 1834 by [[Richard Harlan]] based on a snout fragment found along the river's [[Big Bend (Missouri River)|Big Bend]], in [[South Dakota]].<ref name=OOKMosasaurus2 />  In reference to its discovery made in the river, he coined the specific epithet and initially identified it as a species of ''[[Ichthyosaurus]]''<ref name=Harlan1834>{{cite journal|author=Richard Harlan|title=Notice of the Discovery of the Remains of the ''Ichthyosaurus'' in Missouri, N. A.|year=1834|journal=Transactions of the American Philosophical Society|volume=4|pages=405–408|doi=10.2307/1004839 |jstor=1004839|s2cid=135291243|doi-access=free}}</ref> but later as an [[amphibian]] named ''Batrachiosaurus''.<ref name=Harlan1839London>{{cite journal|author=Richard Harlan|title=Notice of the discovery of ''Basilosaurus'' and ''Batrachiosaurus''|year=1839|journal=Proceedings of the Geological Society of London|volume=3|pages=23–24|url=https://www.biodiversitylibrary.org/item/96958#page/45}}</ref> The rest of the skull had been discovered earlier by a fur-trapper, and it eventually came under the possession of prince [[Prince Maximilian of Wied-Neuwied|Maximilian of Weid-Neuwied]] between 1832 and 1834. The fossil skull, now cataloged as RFWUIP 1327,<ref name=CaldwellandBell /> was delivered to [[Georg August Goldfuss]] in [[Bonn]] for research, who published a study in 1845.<ref name="Goldfuss1845">{{cite journal|language=German|author=August Goldfuss|title=Der Schädelbau des ''Mosasaurus'', durch Beschreibung einer neuen Art dieser Gattung erläutert|trans-title=The skull structure of the ''Mosasaurus'', explained by means of a description of a new species of this genus|volume=21|journal=Nova Acta Academiae Caesareae Leopoldino-Carolinae Germanicae Naturae Curiosorum|year=1845|pages=174–200|url={{google books|plainurl=yes|id=1IZapg_k2BAC|page=1}}}}</ref> The same year, [[Christian Erich Hermann von Meyer]] suspected that the skull and Harlan's snout were part of the same individual.<ref name="Meyer1845">{{cite journal|language=German|author=Hermann von Meyer|title=(Letter directed to Professor Bronn)|journal=Neues Jahrbuch für Mineralogie, Geognosie und Geologie|year=1845|pages=308–313|url=https://www.biodiversitylibrary.org/item/110636#page/330/mode/1up}}</ref> Although the snout was noted as lost at the time,<ref name=OOKMosasaurus2 /> [[Joseph Leidy]] erected the [[Combinatio nova|new combination]] ''M. missouriensis'' after this suggestion in 1857,<ref name=Leidy1857>{{cite journal|author=Joseph Leidy|title=List of extinct Vertebrata; the remains of which have been discovered in the region of the Missouri river: with remarks on their geological age|journal=Proceedings of the Academy of Natural Sciences of Philadelphia|year=1857|volume=9|pages=89–91|url=https://www.biodiversitylibrary.org/item/30013#page/103/mode/1up}}</ref> which has since entered common use.<ref name="StreetThesis"/>{{rp|79}} The snout was finally found in 2004 in the collections of the MNHN under the catalog number MNHN 9587, thus confirming the initial suspicion expressed by von Meyer and followed by other authors.<ref name=CaldwellandBell /><ref name=Konishietal /><ref name=OOKMosasaurus2 />

The third species was described in 1881 by [[Edward Drinker Cope]] from a fragmentary fossil skeleton having been discovered in [[New Jersey]],<ref name=CopeClidastes/> now cataloged as AMNH 1380.<ref name=IkejiriandLucas /> In his description, the Cope thought that it represented a giant species of ''[[Clidastes]]'' and named it ''Clidastes conodon''.<ref name=CopeClidastes>{{cite journal|author=Edward Drinker Cope|title=A new ''Clidastes'' from New Jersey|year=1881|journal=American Naturalist|volume=15|pages=587–588|url=https://www.biodiversitylibrary.org/page/41416605#page/601/mode/1up}}</ref> In 1966, Donald Baird and Gerard R. Case reidentified it as a species of ''Mosasaurus''.<ref name=BairdandCase>{{cite journal|author1=Donald Baird|author2=Gerard R. Case|title=Rare marine reptiles from the Cretaceous of New Jersey|year=1966|journal=Journal of Paleontology|volume=40|issue=5|pages=1211–1215|jstor=1301995|s2cid=132840230|issn=0022-3360}}</ref> Although Cope did not provide the [[etymology]] for the specific epithet ''conodon'', it is suggested that it could be a [[portmanteau]] meaning "conical tooth", derived from the [[Ancient Greek]] κῶνος (''kônos'', "cone") and ὀδών (''odṓn'', "tooth"), probably in reference to conical surface teeth smooth of the species.<ref name=Etymology>{{cite web|title=Mosasauridae Translation and Pronunciation Guide|author=Ben Creisler|year=2000|website=Dinosauria On-line|url=http://www.dinosauria.com/dml/names/mosa.html|archive-url=https://web.archive.org/web/20080502054809/http://www.dinosauria.com/dml/names/mosa.html|archive-date=May 2, 2008}}</ref>

{{multiple image
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|footer = IRSNB R28, ''M. lemonnieri'' holotype skull (top), and a 1892 drawing of IRSNB 3119, one of many specimens of the species described by [[Louis Dollo]] (bottom)}}

The fourth species ''M. lemonnieri'' was described in 1889 by [[Louis Dollo]] on the basis of a relatively complete skull discovered in a quarry owned by the [[Solvay S.A.]] company in the [[Ciply Basin]] of [[Belgium]].<ref name=Dollo1889>{{cite journal|author=Louis Dollo|year=1889|title=Première note sur les mosasauriens de Mesvin|trans-title=First note on the mosasaurians of Mesvin|journal=Bulletin de la Société belge de géologie, de paléontologie et d'hydrologie|volume=3|language=French|pages=271–304|url=https://www.biodiversitylibrary.org/item/173416#page/879/mode/1up|issn=0037-8909}}</ref> This skull, since numbered as IRSNB R28,<ref name=LinghamSoliar2000 /> is one of the many fossils donated by the then director of this quarry, Alfred Lemonnier, Dollo naming the species in his honor.<ref name=Dollo1889/><ref name=Etymology/> Further mining of the quarry in subsequent years uncovered many additional well-preserved fossils, including multiple partial skeletons which collectively represented nearly the entire skeleton of the species. They were described by Dollo in later papers.<ref name=LinghamSoliar2000 /><ref name=StreetThesis />{{rp|136}} Despite being one of the best anatomically represented species, ''M. lemonnieri'' was largely ignored in scientific literature. Theagarten Lingham-Soliar suggested two reasons for this neglect. First, ''M. lemonnieri'' fossils are often known to Belgium and the Netherlands, which despite the famous discovery of the ''M. hoffmannii'' holotype attracted little attention from mosasaur paleontologists. Second, the species was overshadowed by the more famous and history-rich [[type species]].<ref name=LinghamSoliar2000 />

''M. lemonnieri'' is a controversial taxon, and there is debate on whether it is a distinct species or not.<ref name=Gonzalezetal>{{cite journal|author1=Pablo Gonzalez Ruiz|author2=Marta S. Fernandez|author3=Marianella Talevi|author4=Juan M. Leardi|author5=Marcelo A. Reguero|title=A new Plotosaurini mosasaur skull from the upper Maastrichtian of Antarctica. Plotosaurini paleogeographic occurrences|year=2019|journal=Cretaceous Research|volume=103|issue=2019|pages=104166|doi=10.1016/j.cretres.2019.06.012|bibcode=2019CrRes.10304166G |hdl=11336/125124|s2cid=198418273|url=https://rid.unrn.edu.ar/jspui/handle/20.500.12049/5094|hdl-access=free}}</ref> In 1967, [[Dale Russell]] argued that ''M. lemonnieri'' and ''M. conodon'' are the same species and designated the former as a [[junior synonym]] per the [[principle of priority]].<ref name=Russell1967 />{{rp|132–135}} In a 2000 study, Lingham-Soliar refuted this based on a comprehensive study of existing ''M. lemonnieri'' specimens,<ref name=LinghamSoliar2000 /> which was corroborated by a study on the ''M. conodon'' skull by Takehito Ikejiri and [[Spencer G. Lucas]] in 2015.<ref name=IkejiriandLucas /> In 2004, Eric Mulder, Dirk Cornelissen, and Louis Verding suggested ''M. lemonnieri'' could be a juvenile form of ''M. hoffmannii'' based on the argument that significant differences could be explained by age-based variation.<ref>{{cite book|author1=Eric W. A. Mulder|author2=Dirk Cornelissen|author3=Louis Verding|year=2004|chapter=Is ''Mosasaurus lemonnieri'' a juvenile ''Mosasaurus hoffmanni'' ? A discussion|title=First Mosasaur Meeting, Maastricht, 8-12 May 2004, Abstract book and Field guide|editor1=John W. M. Jagt|editor2=Anne S. Schulp|place=[[Maastricht]]|pages=62–66|chapter-url=https://www.researchgate.net/publication/270105741 |publisher=Natuurhistorisch Museum Maastricht}}</ref> However, the need for more research to confirm any hypotheses of synonymy was expressed.<ref name=Gonzalezetal /><ref name=Madzia>{{cite journal|author=Daniel Madzia|year=2019|title=Dental variability and distinguishability in ''Mosasaurus lemonnieri'' (Mosasauridae) from the Campanian and Maastrichtian of Belgium, and implications for taxonomic assessments of mosasaurid dentitions|journal=Historical Biology|volume=32|issue=10|pages=1–15|doi=10.1080/08912963.2019.1588892|s2cid=108526638}}</ref>

The fifth species ''M. beaugei'' was described by [[Camille Arambourg]] in 1952 from isolated teeth originating from phosphate deposits in the [[Oulad Abdoun Basin]] and the [[Ganntour Basin]] in Morocco,<ref name="Arambourg1952">{{Cite book|language=French|author=Camille Arambourg|title=Les vertébrés fossiles des gisements de phosphates (Maroc–Algérie–Tunisie)|trans-title=Fossil vertebrates from phosphate deposits (Morocco–Algeria–Tunisia)|publisher=Typographie Firmin-Didot|place=[[Paris]]|series=Notes et Mémoires du Service Géologique|year=1952|volume=92|pages=282–284|url=http://mmtk.ginras.ru/pdf/arambourg1952_vertebres_de_phosphate.pdf|archive-url=https://web.archive.org/web/20221127111754/http://mmtk.ginras.ru/pdf/arambourg1952_vertebres_de_phosphate.pdf |archive-date=2022-11-27}}</ref> the holotype tooth being cataloged as MNHN PMC 7.<ref name=Bardetetal /> The species is named in honor of Alfred Beaugé, director at the time of the [[OCP Group]], who invited Arambourg to participate in the research project and helped him to provide local fossils.<ref name=Bardetetal2010>{{cite journal|author1=Nathalie Bardet|author2=Xabier Pereda Suberbiola|author3=Stephane Jouve|author4=Estelle Bourdon|author5=Peggy Vincent|author6=Alexandra Houssaye|author7=Jean-Claude Rage|author8=Nour-Eddine Jalil|author9=Baadi Bouya|author10=Mbarek Amaghzaz|s2cid=128481560|title=Reptilian assemblages from the latest Cretaceous – Palaeogene phosphates of Morocco: from Arambourg to present time|year=2010|journal=Historical Biology|volume=22|issue=1–3|pages=186–199|url=https://www.researchgate.net/publication/233317865|doi=10.1080/08912961003754945|bibcode=2010HBio...22..186B }}</ref>

===Early depictions===
[[File:Mosasaurus Crystal Palace 2018.jpg|thumb|left|An 1854 depiction of ''Mosasaurus'' in [[Crystal Palace Dinosaurs|Crystal Palace Park]]]]
Scientists during the early and mid-1800s initially imagined ''Mosasaurus'' as an amphibious marine reptile with webbed feet and [[terrestrial locomotion|limbs for walking]]. This was based on fossils like the ''M. missouriensis'' holotype, which indicated an elastic vertebral column that Goldfuss in 1845 saw as evidence of an ability to walk and interpretations of some [[Phalanx bone|phalanges]] as claws.<ref name="Goldfuss1845"/> In 1854, [[Hermann Schlegel]] proved how ''Mosasaurus'' actually had fully aquatic flippers. He clarified that earlier interpretations of claws were erroneous and demonstrated how the phalanges show no indication of muscle or tendon attachment, which would make walking impossible. They are also broad, flat, and form a paddle.<ref name=Schlegel1854>{{cite journal|language=French|author=Hermann Schlegel|title=Note sur le mosasaure|trans-title=Note on the mosasaur|journal=Comptes rendus hebdomadaires des séances de l'Académie des sciences|year=1854|volume=34|pages=799–802|url=https://www.biodiversitylibrary.org/item/21179#page/809/mode/1up}}</ref><ref name=Mulder1986/> Schlegel's hypothesis was largely ignored by contemporary scientists but became widely accepted by the 1870s when [[Othniel Charles Marsh]] and Cope uncovered more complete mosasaur remains in North America.<ref name=Mulder /><ref name=Mulder1986>{{cite journal|title=Hermann Schlegel's investigation of the Maastricht mosasaurs|author=Eric Mulder|author2=Bert Theunissen|year=1986|journal=Archives of Natural History|volume=13|issue=1|pages=1–6|doi=10.3366/anh.1986.13.1.1|url=https://www.researchgate.net/publication/250229242}}</ref>

One of the earliest depictions of ''Mosasaurus'' in [[paleoart]] is a life-size concrete sculpture created by [[Benjamin Waterhouse Hawkins]]<ref name=Witton>{{cite web|author=Mark Witton|title=The science of the Crystal Palace Dinosaurs, part 2: ''Teleosaurus'', pterosaurs and ''Mosasaurus''|date=May 17, 2019|website=Mark Witton.com|url=http://markwitton-com.blogspot.com/2019/05/the-science-of-crystal-palace-dinosaurs.html|archive-url=https://web.archive.org/web/20190603115306/http://markwitton-com.blogspot.com/2019/05/the-science-of-crystal-palace-dinosaurs.html|archive-date=June 3, 2019}}</ref> between 1852 and 1854<ref name=NHMUKCrystalPalace>{{cite web|author=Emily Osterloff|title=The world's first dinosaur park: what the Victorians got right and wrong|url=https://www.nhm.ac.uk/discover/crystal-palace-dinosaurs.html|archive-url=https://web.archive.org/web/20210418194804/https://www.nhm.ac.uk/discover/crystal-palace-dinosaurs.html|archive-date=April 18, 2021}}</ref> as part of the [[Crystal Palace Dinosaurs|collection of sculptures of prehistoric animals]] on display at the [[Crystal Palace Park]] in [[London]]. The restoration was primarily informed by [[Richard Owen]]'s interpretation of the ''M. hoffmannii'' holotype and the anatomy of monitor lizards, so Hawkins depicted the animal as essentially a water-going monitor lizard. It was given a boxy head, nostrils at the side of the skull, large volumes of soft tissue around the eyes, [[lips]] reminiscent of monitor lizards, scales consistent with those in large monitors like the [[Komodo dragon]], and a flipper. The model was deliberately sculpted incomplete, which [[Mark Witton]] believed was likely to save time and money. Many elements of the sculpture can be considered inaccurate, even for the time. It did not take into account Golduss' 1845 study of ''M. missouriensis'' which instead called for a narrower skull, nostrils at the top of the skull, and amphibious terrestrial limbs (the latter being incorrect in modern standards<ref name=Witton />).<ref name="Goldfuss1845"/>

==Description==
[[File:Mosasaurus hoffmanni life.jpg|thumb|right|[[Paleoart|Life restoration]] of ''M. hoffmannii'']]
''Mosasaurus'' was a type of [[Primitive (phylogenetics)|derived]] mosasaur, or a latecoming member with advanced evolutionary traits such as a fully aquatic lifestyle. As such, it had a streamlined body, an elongated tail ending with a downturn supporting a two-lobed fin, and two pairs of flippers. While in the past derived mosasaurs were depicted as akin to giant flippered [[sea snake]]s, it is now understood that they were more similar in build to other large marine vertebrates such as ichthyosaurs, [[Thalattosuchia|marine crocodylomorphs]], and [[archaeoceti|archaeocete whales]] through [[convergent evolution]].<ref name=LindgrenConvergent>{{cite journal|author1=Johan Lindgren|author2= Michael W. Caldwell|author3=Takuya Konishi|author4=Luis M. Chiappe|year=2010 |title=Convergent Evolution in Aquatic Tetrapods: Insights from an Exceptional Fossil Mosasaur |journal=PLOS ONE |pmid=20711249 |volume=5 |issue=8 |pmc=2918493 |pages=e11998 |doi=10.1371/journal.pone.0011998|bibcode= 2010PLoSO...511998L|doi-access= free}}</ref><ref name=Caldwell2012>{{cite journal|author=Michael W. Caldwell|title=A challenge to categories: "What, if anything, is a mosasaur?"|year=2012|journal=Bulletin de la Société Géologique de France|volume=183|issue=1|pages=17–34|doi=10.2113/gssgfbull.183.1.7}}</ref><ref name=Lindgrenetal />

===Size===
[[File:MosaScale.svg|thumb|left|upright=1.4|Size range of ''Mosasaurus'' compared with a human]]
The type species, ''M. hoffmannii'', is one of the largest marine reptiles known,<ref name=LinghamSoliar /><ref name=Penza /> though knowledge of its skeleton remains incomplete as it is mainly known from skulls.<ref name=StreetThesis />{{rp|100}} Russell (1967) wrote that the length of the jaw equalled one tenth of the body length in the species.<ref name=Russell1967>{{cite book|author=Dale A. Russell|title=Systematics and Morphology of American Mosasaurs|year=1967|publisher=Bulletin of the Peabody Museum of Natural History|place=[[New Haven]]|volume=23|oclc=205385|url=https://www.biodiversitylibrary.org/item/88808#page/7/mode/1up}}</ref>{{rp|210}} Based on this ratio, Grigoriev (2014) used the largest lower jaw attributed to ''M. hoffmannii'' (CCMGE 10/2469, also known as the [[Penza]] specimen; measuring {{convert|171|cm|in|sp=us}} in length) to estimate a maximum length of {{convert|17.1|m|ft|sp=us}}.<ref name=Penza>{{Cite journal|author=Dimitry V. Grigoriev|title=Giant ''Mosasaurus hoffmanni'' (Squamata, Mosasauridae) from the Late Cretaceous (Maastrichtian) of Penza, Russia|journal=Proceedings of the Zoological Institute RAS|volume=318|issue=2|pages=148–167|year=2014|doi=10.31610/trudyzin/2014.318.2.148 |s2cid=53574339 |doi-access=free|url=https://www.zin.ru/journals/trudyzin/doc/vol_318_2/TZ_318_2_Grigoriev.pdf|archive-url=https://web.archive.org/web/20231003041828/https://www.zin.ru/journals/trudyzin/doc/vol_318_2/TZ_318_2_Grigoriev.pdf|archive-date=2023-10-03|url-status=live}}</ref> Using a smaller partial jaw ([[Maastricht Natural History Museum|NHMM]] 009002) measuring {{convert|90|cm|in|sp=us}} and "reliably estimated at" {{convert|160|cm|in|sp=us}} when complete, Lingham-Soliar (1995) estimated a larger maximum length of {{convert|17.6|m|ft|sp=us}} via the same ratio.{{efn|Lingham-Soliar may have misapplied the ratio. His calculations interpreted "body length" as the length of the postcranial body, not the total length of the animal as demonstrated in Russell (1967), This erroneously inflated the estimate by 10%.<ref name=Russell1967 />{{rp|210}}<ref name=LinghamSoliar />}}<ref name=LinghamSoliar>{{cite journal |author=Theagarten Lingham-Soliar |title=Anatomy and functional morphology of the largest marine reptile known, ''Mosasaurus hoffmanni'' (Mosasauridae, Reptilia) from the Upper Cretaceous, Upper Maastrichtian of The Netherlands |journal=[[Philosophical Transactions of the Royal Society B]] |volume=347 |issue=1320 |pages=155–180 |year=1995 |url=https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.1995.0019 |doi=10.1098/rstb.1995.0019 |bibcode=1995RSPTB.347..155L |jstor=55929 |s2cid=85767257 |archive-url=https://web.archive.org/web/20191026201507/https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.1995.0019 |archive-date=26 October 2019 |url-status=dead|url-access=subscription }}</ref> No explicit justification for the 1:10 ratio was provided in Russell (1967), and it has been considered to be probably overestimated by Cleary ''et al.'' (2018).<ref name=Clearyetal>{{cite journal|author1=Terri J. Cleary|author2=Roger B. J. Benson|author3=Susan E. Evans|author4=Paul M. Barrett|title=Lepidosaurian diversity in the Mesozoic–Palaeogene: the potential roles of sampling biases and environmental drivers|year=2018|journal=Royal Society Open Science|volume=5|issue=3 |pages=171830|doi=10.1098/rsos.171830|pmid=29657788 |pmc=5882712 |bibcode=2018RSOS....571830C |doi-access=free}}</ref> In 2014, Federico Fanti and colleagues alternatively argued that the total length of ''M. hoffmannii'' was more likely closer to seven times the length of the skull, which was based on a near-complete skeleton of the related species ''[[Prognathodon|Prognathodon overtoni]]''. The study estimated that an ''M. hoffmannii'' individual with a skull measuring more than {{cvt|145|cm|in}} would have been up to or more than {{convert|11|m|ft|sp=us}} in length and weighed {{convert|10|MT|ST}} in body mass.<ref name=Fantietal>{{cite journal|author1=Fedrico Fanti|author2=Andrea Cau|author3=Alessandra Negri|title=A giant mosasaur (Reptilia, Squamata) with an unusually twisted dentition from the Argille Scagliose Complex (late Campanian) of Northern Italy|year=2014|journal=Cretaceous Research|volume=49|issue=2014|pages=91–104|doi=10.1016/j.cretres.2014.01.003|bibcode=2014CrRes..49...91F |url=https://www.disva.univpm.it/sites/www.disva.univpm.it/files/disva/news_dipartimento/cretaceus%20research.pdf}}</ref> Using the same ratio, Gayford ''et al.'' (2024) calculated the total length for the Penza specimen to be {{convert|12|m|ft|sp=us}}.<ref name=Gayfordetal2024>{{cite journal|author1=Joel H. Gayford|author2=Russell K. Engelman|author3=Phillip C. Sternes|author4=Wayne M. Itano|author5=Mohamad Bazzi|author6=Alberto Collareta|author7=Rodolfo Salas-Gismondi|author8=Kenshu Shimada|year=2024|title=Cautionary tales on the use of proxies to estimate body size and form of extinct animals|journal=Ecology and Evolution|volume=14|issue=9|at=e70218|doi=10.1002/ece3.70218|pmc=11368419|pmid=39224151|doi-access=free|bibcode=2024EcoEv..1470218G }}</ref>

[[File:Нижняя челюсть.Unterkiefer..2H1A0361WI.jpg|thumb|right|The Penza specimen, one of the largest known fossils of ''Mosasaurus''<ref name=Penza />]]
Isolated bones suggest some ''M. hoffmannii'' may have exceeded the lengths of the Penza specimen. One such bone is a [[quadrate bone|quadrate]] (NHMM 003892) which is 150% larger than the average size, which Everhart and colleagues in 2016 reported can be extrapolated to scale an individual around {{convert|18|m|ft|sp=us}} in length. It was not stated whether they applied Russell's 1967 ratio,<ref name=EverhartQuadrate>{{cite conference|author1=Michael Everhart|author2=John W. M. Jagt|author3=Eric W. A. Mulder|author4=Anne S. Schulp|date=2016|title=Mosasaurs—how large did they really get?|publisher=5th Triennial Mosasaur Meeting—A Global Perspective on Mesozoic Marine Amniotes|url=https://www.researchgate.net/publication/303518964|pages=8–10}}</ref> although Gayford ''et al.'' (2024) suggested it was likely.<ref name=Gayfordetal2024 />

''M. missouriensis'' and ''M. lemonnieri'' are smaller than ''M. hoffmannii'' but are known from more complete fossils. Based on measurements of various Belgian skeletons, Dollo estimated ''M. lemonnieri'' grew to around {{convert|7|to|10|m|ft|sp=us}} in length.<ref name=Dollo1892>{{cite journal|author=Louis Dollo|title=Nouvelle note sur l'osteologie des mosasauriens|trans-title=New note on the osteology of mosasaurians|language=French|year=1892|journal=Bulletin de la Société belge de géologie, de paléontologie et d'hydrologie|volume=6|pages=219–259|url=https://www.biodiversitylibrary.org/item/159811#page/543|issn=0037-8909}}</ref><ref name=Russell1967 />{{rp|210}} He also measured the dimensions of IRSNB 3119 and recorded that the skull constituted approximately one-eleventh of the whole body.<ref name=Dollo1892 /> Polcyn ''et al.'' (2014) estimated that ''M. missouriensis'' may have measured up to {{convert|8-9|m|ft|sp=us}} in length.<ref name=Polycnetal>{{cite journal|author1=Michael J. Polcyn|author2=Louis L. Jacobs|author3=Ricardo Araújo|author4=Anne S.Schulp|author5=Octávio Mateus|title=Physical drivers of mosasaur evolution|year=2014|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=400|issue=15|pages=17–27|doi=10.1016/j.palaeo.2013.05.018|bibcode=2014PPP...400...17P|url=https://docentes.fct.unl.pt/sites/default/files/omateus/files/polcyn_et_al_2014_physical_drivers_mosasaurs.pdf}}</ref><ref name=Demicetal>{{cite journal|author1=Michael D. D'Emic |author2=Kathlyn M. Smith |author3=Zachary T. Ansley|title=Unusual histology and morphology of the ribs of mosasaurs (Squamata)|year=2015|journal=Palaeontology|volume=58|issue=3|pages=511–520|doi=10.1111/pala.12157|bibcode=2015Palgy..58..511D |s2cid=129177236 |doi-access=free}}</ref> Street (2016) noted that large ''M. missouriensis'' individuals typically had skulls exceeding lengths of {{convert|1|m|ft|sp=us}}.<ref name=StreetThesis />{{rp|288}} A particular near-complete skeleton of ''M. missouriensis'' is reportedly measured at {{convert|6.5|m|ft|sp=us}} in total length with a skull approaching {{convert|1|m|ft|sp=us}} in length.<ref name=TMP2012News /> Based on personal observations of various unpublished fossils from Morocco, Nathalie Bardet ''et al.'' (2015) estimated that ''M. beaugei'' grew to a total length of {{convert|8-10|m|ft|sp=us}}, their skulls typically measuring around {{convert|1|m|ft|sp=us}} in length.<ref name=BardetetalMorocco>{{cite journal|author1=Nathalie Bardet|author2=Alexandra Houssaye|author3=Peggy Vincent|author4=Xabier Pereda Suberbiola|author5=M'barek Amaghzaz|author6=Essaid Jourani|author7=Saïd Meslouh|year=2015|title=Mosasaurids (Squamata) from the Maastrichtian Phosphates of Morocco: Biodiversity, palaeobiogeography and palaeoecology based on tooth morphoguilds|journal=Gondwana Research|volume=27|issue=3|pages=1068–1078|doi=10.1016/j.gr.2014.08.014|bibcode=2015GondR..27.1068B|s2cid=140596842|url= https://www.researchgate.net/publication/265730987}}</ref> With a referred skull measuring {{convert|97.7|cm|in|sp=us}} in length, ''M. conodon'' has been regarded as a small to medium-sized representative of the genus.<ref name=IkejiriandLucas>{{Cite journal|url=https://www.researchgate.net/publication/271528820|doi=10.1017/njg.2014.28|title=Osteology and taxonomy of ''Mosasaurus conodon'' Cope 1881 from the Late Cretaceous of North America|journal=Netherlands Journal of Geosciences|volume=94|issue=1|pages=39–54|year=2015|author1=Takehito Ikejiri|author2=Spencer G. Lucas|s2cid=73707936|doi-access=free|bibcode=2015NJGeo..94...39I }}</ref>

===Skull===
[[File:Mosasaurus hoffmannii skull schematic.png|thumb|left|upright=1.23|Annotated schematic of a ''M. hoffmannii'' skull]]
The skull of ''Mosasaurus'' is conical and tapers off to a short [[rostrum (anatomy)|snout]] which extends a little beyond the frontmost teeth.<ref name=StreetandCaldwell>{{cite journal|author1=Hallie P. Street|author2=Michael W. Caldwell|year=2017 |title=Rediagnosis and redescription of ''Mosasaurus hoffmannii'' (Squamata: Mosasauridae) and an assessment of species assigned to the genus ''Mosasaurus''|journal=Geological Magazine|volume=154|issue=3|pages=521–557|url=https://www.researchgate.net/publication/303090121|doi=10.1017/S0016756816000236|bibcode=2017GeoM..154..521S|s2cid=88324947}}</ref><ref name=LinghamSoliar /> In ''M. hoffmannii'', this snout is blunt,<ref name=StreetandCaldwell /> while in ''M. lemonnieri'' it is pointed.<ref name=LinghamSoliar2000 /> Above the gum line in both jaws, a single row of small pits known as [[foramina]] are lined parallel to the jawline; they are used to hold the terminal branches of jaw nerves. The foramina along the snout form a pattern similar to the foramina in ''Clidastes'' skulls.<ref name=LinghamSoliar /> The upper jaws in most species are robustly built, broad, and deep except in ''M. conodon'', where they are slender.<ref name=IkejiriandLucas /> The disparity is also reflected in the [[Mandible#Other vertebrates|dentary]], the lower jawbone,<ref name=LinghamSoliar2000 /> although all species share a long and straight dentary. In ''M. hoffmannii'', the top margin of the dentary is slightly curved upwards;<ref name=StreetandCaldwell /> this is also the case with the largest specimens of ''M. lemonnieri'', although more typical skulls of the species have a near-perfectly straight jawline.<ref name=LinghamSoliar2000 /> The premaxillary bar,{{efn|Also known as the internarial bar<ref name=LinghamSoliar />}} the long portion of the premaxillary bone extending behind the premaxillary teeth, is narrow and constricts near the middle in ''M. hoffmannii''<ref name=LinghamSoliar /> and ''M. lemonnieri''<ref name=LinghamSoliar2000 /> like in typical mosasaurs.<ref name=CaldwellandBell /> In ''M. missouriensis'', the bar is robust and does not constrict.<ref name=CaldwellandBell /> The external nares ([[Nostril|nostril openings]]) are moderately sized and measure around 21–24% of the skull's length in ''M. hoffmannii''. They are placed further toward the back of the skull than in nearly all other mosasaurs (exceeded only by ''[[Goronyosaurus]]''), and begin above the fourth or fifth maxillary teeth.<ref name=LinghamSoliar /> As a result, the rear portions of the [[maxilla]] (the main tooth-bearing bone of the upper jaw) lack the dorsal concavity that would fit the nostrils in typical mosasaurs.<ref name=StreetandCaldwell />
{{multiple image
|total_width = 200
|align = right
|direction = vertical
|image1 = MOR_006_(1).jpg
|image2 = Mosasaurus_lemonnieri_456.JPG
|footer = Fossil skulls of ''M. conodon'' (top) and of ''M. lemonnieri'' (bottom)
}}
The [[palate]], which consists of the pterygoid bones, [[palatine bone]], and nearby [[Process (anatomy)|processes]] of other bones, is tightly packed to provide greater cranial stability. The [[neurocranium]] housed a brain which was narrow and relatively small compared to other mosasaurs. For example, the braincase of the mosasaur ''[[Plioplatecarpus|Plioplatecarpus marshi]]'' provided for a brain around twice the size of that in ''M. hoffmannii'' despite being only half the length of the latter. Spaces within the braincase for the [[occipital lobe]] and [[cerebral hemisphere]] are narrow and shallow, suggesting such brain parts were relatively small. The [[Pineal foramen|parietal foramen]] in ''Mosasaurus'', which is associated with the [[parietal eye]], is the smallest among [[Mosasauridae|mosasaurids]].<ref name=LinghamSoliar /> The quadrate bone, which connected the lower jaw to the rest of the skull and formed the jaw joint, is tall and somewhat rectangular in shape, differing from the rounder quadrates found in typical mosasaurs.<ref name=StreetandCaldwell /> The quadrate also housed the [[tympanum (anatomy)|hearing structures]], with the [[eardrum]] residing within a round and concave depression in the outer surface called the tympanic ala.<ref name=OOKBrain>{{cite web|author=Mike Everhart|title=Mosasaur brain|date=January 1, 2010|website=Oceans of Kansas|url=http://oceansofkansas.com/mosabrain.html|archive-url=https://web.archive.org/web/20201106140704/http://oceansofkansas.com/mosabrain.html|archive-date=November 6, 2020}}</ref> The [[trachea]] likely stretched from the [[esophagus]] to below the back end of the lower jaw's [[Coronoid process of the mandible|coronoid process]], where it split into smaller pairs of [[bronchus|bronchi]] which extended parallel to each other.<ref name=Konishietal />

===Teeth===
[[File:Mosasaurus hoffmanni first specimen teeth.jpg|thumb|left|Closeup of ''M. hoffmannii'' teeth, with a replacement tooth developing inside the root of the lower right tooth]]
The features of teeth in ''Mosasaurus'' vary across species, but unifying characteristics include a design specialized for cutting prey, highly prismatic surfaces (enamel circumference shaped by flat sides called prisms), and two opposite cutting edges.<ref name=IkejiriandLucas /><ref name=Bardetetal>{{cite journal|author1=Nathalie Bardet|author2=Xabier Pereda Suberbiola|author3=Mohamed Iarochene|author4=Fatima Bouyahyaoui|author5=Baadi Bouya|author6=Mbarek Amaghzaz|title=''Mosasaurus beaugei'' Arambourg, 1952 (Squamata, Mosasauridae) from the Late Cretaceous phosphates of Morocco|year=2004|journal=Geobios|volume=37|issue=2004|pages=315–324|doi=10.1016/j.geobios.2003.02.006|bibcode=2004Geobi..37..315B |s2cid=127441579|url=https://www.academia.edu/24312244}}</ref><ref name=Schulpetal2013>{{cite journal|author1=Anne S. Schulp|author2=Hubert B. Vonhof|author3=Jeroen van der Lubbe|author4=Renée Janssen|author5=Remy R. van Baal|title=On diving and diet: resource partitioning in type-Maastrichtian mosasaurs|year=2013|journal=Netherlands Journal of Geosciences|volume=92|issue=2–3|pages=165–170|doi=10.1017/S001677460000010X|s2cid=131884448|doi-access=free|bibcode=2013NJGeo..92..165S }}</ref><ref name=SchulpAngola>{{cite journal|author1=Anne S. Schulp|author2=Michael J. Polcyn|author3=Octavio Mateus|author4=Louis L. Jacobs|author5=Maria Lusia Morais|author6=Tatiana da Silva Tavares|title=New mosasaur material from the Maastrichtian of Angola, with notes on the phylogeny, distribution, and paleoecology of the genus ''Prognathodon''|journal=Publicaties van het Natuurhistorisch Genootschap in Limburg|volume=45|issue=1|year=2006|pages=57–67|issn=0374-955X|url=https://docentes.fct.unl.pt/sites/default/files/omateus/files/schulp_polcyn_mateus_jacobs_et_al_2006_new_mosasaur_material_from_the_maastrichtian_of_angola_with_notes_on_the_phylogeny_distribution_and_palaeoecology_of_the_genus_prognathodon.pdf}}</ref> ''Mosasaurus'' teeth are large and robust except for those in ''M. conodon'' and ''M. lemonnieri'', which instead have more slender teeth.<ref name=IkejiriandLucas /><ref name=Bardetetal /> The cutting edges of ''Mosasaurus'' differ by species. The cutting edges in ''M. hoffmannii'' and ''M. missouriensis'' are finely serrated,<ref name=StreetandCaldwell /><ref name=CaldwellandBell>{{cite journal|author=Michael W. Caldwell|author2=Gorden L. Bell Jr.|title=Of German princes and North American rivers: Harlan's lost mosasaur snout rediscovered|year=2005|journal=Netherlands Journal of Geosciences|volume=84|issue=3|pages=207–211|doi=10.1017/S0016774600020989|doi-access=free|bibcode=2005NJGeo..84..207C }}</ref> while in ''M. conodon'' and ''M. lemonnieri'' serrations do not exist.{{efn|One specimen traditionally attributed to ''M. lemonnieri'' has serration-like features in its cutting edges. Scientists believe this specimen likely belongs to a different species.<ref name=Madzia />}}<ref name=Madzia/> The cutting edges of ''M. beaugei'' are neither serrated nor smooth, but instead possess minute wrinkles known as crenulations.<ref name=Bardetetal /> The number of prisms in ''Mosasaurus'' teeth can slightly vary between tooth types and general patterns differ between species{{efn|The number of prisms in ''M. conodon'' and number of lingual prisms in ''M. lemonnieri'' are uncertain.<ref name=Bardetetal />}}{{Em dash}}''M. hoffmannii'' had two to three prisms on the labial side (the side facing lips) and no prisms on the lingual side (the side facing the tongue), ''M. missouriensis'' had four to six labial prisms and eight lingual prisms, ''M. lemonnieri'' had eight to ten labial prisms, and ''M. beaugei'' had three to five labial prisms and eight to nine lingual prisms.<ref name=Bardetetal />

[[File:Mosasaurus beaugei teeth palate.JPG|thumb|Closeup of a ''M. beaugei'' [[palate]], showing the smaller pterygoid teeth on their namesake bones]]
Like all mosasaurs, ''Mosasaurus'' had four types of teeth, classified based on the jaw bones they were located on. On the upper jaw, there were three types: the premaxillary teeth, maxillary teeth, and pterygoid teeth. On the lower jaw, only one type, the dentary teeth, were present. In each jaw row, from front to back, ''Mosasaurus'' had: two premaxillary teeth, twelve to sixteen maxillary teeth, and eight to sixteen pterygoid teeth on the upper jaw and fourteen to seventeen dentary teeth on the lower jaw. The teeth were largely consistent in size and shape with only minor differences throughout the jaws ([[heterodont|homodont]]) except for the smaller pterygoid teeth.<ref name=Konishietal /><ref name=IkejiriandLucas /><ref name=Bardetetal /><ref name=Boyd>{{cite magazine|author=Clint A. Boyd|title=A New Addition to the Cretaceous Seaway of North Dakota|year=2017|magazine=Geo News|publisher=North Dakota Geological Society|volume=44|number=1|pages=20–23|url=https://www.dmr.nd.gov/ndgs/documents/newsletter/2017Winter/A%20New%20Addition%20to%20the%20Cretaceous%20Seaway%20of%20North%20Dakota.pdf}}</ref> The number of teeth in the maxillae, pterygoids, and dentaries vary between species and sometimes even individuals{{Em dash}}''M. hoffmannii'' had fourteen to sixteen maxillary teeth, fourteen to fifteen dentary teeth, and eight pterygoid teeth;<ref name=IkejiriandLucas /><ref name=Penza /><ref name=LinghamSoliar /> ''M. missouriensis'' had fourteen to fifteen maxillary teeth, fourteen to fifteen dentary teeth, and eight to nine pterygoid teeth;<ref name=Konishietal /><ref name=Bardetetal /><ref name=OOKHorridus>{{cite web|author=Mike Everhart|title=Samuel Wilson's ''Mosasaurus horridus''|date=March 26, 2009|website=Oceans of Kansas|url=http://oceansofkansas.com/page12c.html|archive-url=https://web.archive.org/web/20210602234233/http://oceansofkansas.com/page12c.html|archive-date=June 2, 2021}}</ref> ''M. conodon'' had fourteen to fifteen maxillary teeth, sixteen to seventeen dentary teeth, and eight pterygoid teeth;<ref name=IkejiriandLucas /><ref name=Bardetetal /> ''M. lemonnieri'' had fifteen maxillary teeth, fourteen to seventeen dentary teeth, and eleven to twelve pterygoid teeth;<ref name=LinghamSoliar2000 /><ref name=IkejiriandLucas /><ref name=Bardetetal /> and ''M. beaugei'' had twelve to thirteen maxillary teeth, fourteen to sixteen dentary teeth, and six or more pterygoid teeth.<ref name=Bardetetal /> One indeterminate specimen of ''Mosasaurus'' similar to ''M. conodon'' from the [[Pembina Gorge State Recreation Area]] in [[North Dakota]] was found to have an unusual count of sixteen pterygoid teeth, far greater than in known species.<ref name=Boyd />

The dentition was [[thecodont dentition|thecodont]] (tooth roots deeply cemented within the jaw bone). Teeth were constantly shed through a process where the replacement tooth developed within the root of the original tooth and then pushed it out of the jaw.<ref name=CaldwellDentition>{{cite journal|author1=Michael W. Caldwell|title=Ontogeny, anatomy and attachment of the dentition in mosasaurs (Mosasauridae: Squamata)|year=2007|journal=Zoological Journal of the Linnean Society|volume=149|issue=4|pages=687–700|doi=10.1111/j.1096-3642.2007.00280.x|doi-access=free}}</ref> Chemical studies conducted on a ''M. hoffmannii'' maxillary tooth measured an average rate of deposition of [[odontoblast]]s, the cells responsible for the formation of [[dentin]], at {{convert|10.9|um|in|sp=us}} per day. This was by observing the [[Victor von Ebner#Career|von Ebner lines]], incremental marks in dentin that form daily. It was approximated that it took the odontoblasts 511 days and dentin 233 days to develop to the extent observed in the tooth.{{efn|This study was conducted on only one tooth and may not represent the exact durations of [[dentinogenesis]] in all ''Mosasaurus'' teeth.<ref name=Chinsamyetal />}}<ref name=Chinsamyetal>{{cite journal|author1=Anusuya Chinsamy|author2=Cemal Tunoǧlu|author3=Daniel B. Thomas|title=Dental microstructure and geochemistry of ''Mosasaurus hoffmanni'' (Squamata: Mosasauridae) from the Late Cretaceous of Turkey|year=2012|journal=Bulletin de la Société Géologique de France|volume=183|issue=2|pages=85–92|doi=10.2113/gssgfbull.183.2.85
}}</ref>

===Postcranial skeleton===
[[File:TMP 2008.036.0001.jpg|thumb|left|upright=1.25|Well-preserved fossil of ''M. missouriensis''<ref name=Konishietal />]]
One of the most complete ''Mosasaurus'' skeletons in terms of vertebral representation (''Mosasaurus'' sp.; SDSM 452)<ref name=IkejiriandLucas/> has seven [[cervical vertebrae|cervical (neck) vertebrae]], thirty-eight dorsal vertebrae (which includes [[thoracic vertebrae|thoracic]] and [[lumbar vertebrae]]) in the back, and eight pygal vertebrae (front tail vertebrae lacking [[haemal arch]]es) followed by sixty-eight [[caudal vertebrae]] in the tail. All species of ''Mosasaurus'' have seven cervical vertebrae, but other vertebral counts vary among them. Various partial skeletons of ''M. conodon'', ''M. hoffmannii'', and ''M. missouriensis'' suggest ''M. conodon'' likely had up to thirty-six dorsal vertebrae and nine pygal vertebrae; ''M. hoffmannii'' had likely up to thirty-two dorsal vertebrae and ten pygal vertebrae;{{efn|The number of caudal vertebrae is not fully certain for ''M. conodon'' and ''M. hoffmannii''. At least ten have been documented in ''M. conodon'', while the count is completely unknown in ''M. hoffmannii''.<ref name=IkejiriandLucas />}}<ref name=IkejiriandLucas /><ref name=LinghamSoliar2000 /> and ''M. missouriensis'' around thirty-three dorsal vertebrae, eleven pygal vertebrae, and at least seventy-nine caudal vertebrae. ''M. lemmonieri'' had the most vertebrae in the genus, with up to around forty dorsal vertebrae, twenty-two pygal vertebrae, and ninety caudal vertebrae.<ref name=LinghamSoliar2000 /><ref name=StreetThesis />{{rp|155}} Compared to other mosasaurs, the [[rib cage]] of ''Mosasaurus'' is unusually deep and forms an almost perfect semicircle, giving it a barrel-shaped chest. Rather than being fused together, extensive cartilage likely connected the ribs with the [[sternum]], which would have facilitated breathing movements and compression when in deeper waters.<ref name=LinghamSoliar /> The texture of the bones is virtually identical with in modern whales, which indicates ''Mosasaurus'' possessed a high range of aquatic adaptation and [[neutral buoyancy]] as seen in cetaceans.<ref name=Lindgrenetal>{{cite journal|author1=Johan Lindgren|author2=Michael J. Polcyn|author3=Bruce A. Young|title=Landlubbers to leviathans: evolution of swimming in mosasaurine mosasaurs|year=2011|journal=Paleobiology|volume=37|issue=3|pages=445–469|doi=10.1666/09023.1|jstor=23014733|bibcode=2011Pbio...37..445L |s2cid=85165085}}</ref>
[[File:Mosasaurus beaugei 34.JPG|thumb|upright=1.1|Skeletal reconstruction of ''M. beaugei'', exhibited at the [[Musée des Confluences]] in [[Lyon]], France]]
The tail structure of ''Mosasaurus'' is similar to relatives like ''Prognathodon'', in which soft tissue evidence for a two-lobed tail is known.<ref name=LindgrenBilobed>{{cite journal|author1=Johan Lindgren|author2=Hani F. Kaddumi|author3=Michael J. Polcyn|title=Soft tissue preservation in a fossil marine lizard with a bilobed tail fin|journal=Nature Communications|volume=4|issue=2423|year=2013|page=2423|doi=10.1038/ncomms3423|pmid=24022259|bibcode=2013NatCo...4.2423L|doi-access=free}}</ref> The tail vertebrae gradually shorten around the center of the tail and lengthen behind the center, suggesting rigidness around the tail center and excellent flexibility behind it. Like most advanced mosasaurs, the tail bends slightly downwards as it approached the center, but this bend is offset from the dorsal plane at a small degree. ''Mosasaurus'' also has large haemal arches located at the bottom of each caudal vertebra which bend near the middle of the tail, which contrasts with the reduction of haemal arches in other marine reptiles such as [[Ichthyosauria|ichthyosaur]]s. These and other features support a large and powerful paddle-like fluke in ''Mosasaurus''.<ref name=Lindgrenetal />

The forelimbs of ''Mosasaurus'' are wide and robust.<ref name=IkejiriandLucas /><ref name=LinghamSoliar /> The [[scapula]] and [[humerus]] are fan-shaped and wider than tall. The [[radius (bone)|radius]] and [[ulna]] are short, but the former is taller and larger than the latter.<ref name=IkejiriandLucas /> The [[ilium (bone)|ilium]] is rod-like and slender; in ''M. missouriensis'', it is around 1.5 times longer than the [[femur]]. The femur itself is about twice as long as it is wide and ends at the distal side in a pair of distinct [[articular processes|articular facets]] (of which one connects to the [[ilium (bone)|ilium]] and the other to the paddle bones) that meet at an angle of approximately 120°.<ref name=Konishietal /> Five sets of [[metacarpal]]s and phalanges (finger bones) were encased in and supported the paddles, with the fifth set being shorter and offset from the rest. The overall structure of the paddle is compressed, similar to in ''[[Plotosaurus]]'', and was well-suited for faster swimming.<ref name=IkejiriandLucas /><ref name=LinghamSoliar /> In the hindlimbs, the paddle is supported by four sets of digits.<ref name=Konishietal />

<imagemap>
Image:Mosasaurus hoffmanni.png|center|700px
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poly 3324 1682 3304 1712 2972 1720 2954 1688 2908 1670 3082 1662 3316 1660 [[Sternum]]
poly 5976 816 5984 864 6028 930 6054 956 6060 1000 6082 1006 6086 982 6144 974 6128 946 6074 910 5998 804 5974 798 [[Ilium (bone)|Ilium]]
poly 6002 1032 6022 1014 6058 986 6086 1010 6094 1060 6076 1122 6072 1172 6060 1202 6024 1182 6032 1096 6010 1072 5984 1064 5976 1046 [[Pubis (bone)|Pubis]]
poly 6172 1006 6198 1026 6192 1076 6234 1132 6220 1136 6192 1098 6182 1078 6148 1070 6138 1046 6156 1018 6156 996 [[Ischium]]
poly 6116 1140 6080 1138 6076 1116 6082 1076 6092 1058 6084 1010 6082 992 6110 974 6152 982 6158 1002 6154 1022 6138 1050 6150 1072 6186 1082 6190 1104 6168 1116 6172 1138 [[Femur]]
poly 6128 1268 6072 1262 6058 1228 6070 1206 6072 1176 6090 1134 6162 1138 6166 1160 6166 1194 6202 1218 6164 1270 6150 1262 [[Tibia]]
poly 6260 1186 6272 1154 6256 1140 6228 1148 6200 1106 6180 1108 6162 1128 6166 1148 6198 1164 6210 1202 6240 1200 [[Fibula]]
poly 6286 1154 6312 1168 6308 1214 6336 1250 6286 1272 6252 1254 6220 1288 6166 1288 6154 1272 6198 1224 6248 1192 6276 1160 [[Tarsus (skeleton)|Tarsals]]
poly 6088 1308 6112 1362 6194 1352 6190 1392 6252 1374 6260 1356 6316 1334 6366 1300 6332 1242 6368 1222 6388 1172 6348 1158 6310 1170 6310 1208 6330 1242 6290 1266 6258 1258 6226 1280 6174 1280 6154 1262 6094 1264 6074 1286 [[Metatarsal bones]]
poly 6512 1260 6436 1210 6362 1214 6328 1248 6364 1300 6320 1332 6262 1348 6250 1368 6182 1398 6186 1348 6102 1370 6102 1398 6162 1488 6236 1560 6336 1636 6476 1676 6582 1684 6660 1664 6716 1628 6686 1562 6542 1480 6434 1386 6378 1324 6370 1238 6452 1260 6512 1282 [[Phalanx bone|Phalanges]]
poly 7360 564 8488 568 9036 548 9448 612 9948 804 10480 1000 11016 1148 11292 1188 11288 1260 11052 1268 10672 1220 10152 1124 9620 968 9016 788 8856 772 8540 816 7616 840 7056 816 7060 748 7080 660 7100 556 [[Vertebra#Other_animals|Caudal vertebrae]]
poly 11116 1296 10764 1336 10260 1336 9888 1284 9404 1112 9100 956 9052 928 8968 808 8888 772 9024 784 9480 924 10176 1128 10744 1236 11076 1268 11240 1264 [[Haemal arch]]es
poly 6308 536 7092 552 7080 684 7076 752 7056 808 6652 808 5940 800 5936 740 5964 684 5932 632 5980 520 6112 524 [[Vertebra#Other_animals|Pygal vertebrae]]
</imagemap>
<div style="text-align:center;">
<br />
Interactive skeletal reconstruction of ''M. hoffmannii''
<br />
<small>(hover over or click on each skeletal component to identify the structure)</small>
</div>

==Classification==
===History of taxonomy===
{{Main|Research history of Mosasaurus#History of taxonomy}}
[[File:Mosasaurus hoffmanni 567.JPG|thumb|left|Fossil skull of the proposed new species ''{{'}}M. glycys{{'}}'']]
Because nomenclatural rules were not well-defined at the time, 19th century scientists did not give ''Mosasaurus'' a proper [[Species description|diagnosis]] during its initial descriptions, which led to ambiguity in how the genus is defined. This led ''Mosasaurus'' to become a [[wastebasket taxon]] containing as many as fifty different species. A 2017 study by Hallie Street and Michael Caldwell performed the first proper diagnosis and description of the ''M. hoffmannii'' holotype, which allowed a major taxonomic cleanup confirming five species as likely valid{{Em dash}}''M. hoffmannii'', ''M. missouriensis'', ''M. conodon'', ''M. lemonnieri'', and ''M. beaugei''. The study also held four additional species from [[Pacific Ocean|Pacific]] deposits{{Em dash}}''M. mokoroa'', ''M. hobetsuensis'', ''M. flemingi'', and ''M. prismaticus''{{Em dash}}to be possibly valid, pending a future formal reassessment.{{efn|Street & Caldwell (2017) also included ''M. dekayi'' as a potentially valid species<ref name=StreetandCaldwell /> without addressing in detail its dubious status.<ref name=Mulder1999>{{cite journal|author=Eric W. A. Mulder|s2cid=126956543|title=Transatlantic latest Cretaceous mosasaurs (Reptilia, Lacertilia) from the Maastrichtian type area and New Jersey|year=1999|journal=Geologie en Mijnbouw|volume=78|issue=3/4|pages=281–300|doi=10.1023/a:1003838929257}}</ref><ref name="StreetThesis"/>{{rp|108}}}}<ref name=StreetandCaldwell /> Street & Caldwell (2017) was derived from Street's 2016 doctoral thesis, which contained a [[Phylogenetics|phylogenetic]] study proposing the constraining of ''Mosasaurus'' into four species{{Em dash}}''M. hoffmannii'', ''M. missouriensis'', ''M. lemonnieri'', and a proposed new species ''{{'}}M. glycys{{'}}''{{Em dash}}with ''M. conodon'' and the Pacific taxa recovered as belonging to different genera and ''M. beaugei'' view as a junior synonym{{efn|Street & Caldwell (2017) revised this assessment of ''M. beaugei'' and found it to be a distinct species based on additional anatomical distinctions.<ref name=StreetandCaldwell />}} of ''M. hoffmannii''.{{efn|As the proposal remains restricted to a PhD thesis, it is defined as an unpublished work per Article 8 of the ICZN and therefore is not yet formally valid.<ref name=ICZNArticle8>{{cite web|author=International Commission on Zoological Nomenclature|work=International Code of Zoological Nomenclature|title=Article 8. What constitutes published work|year=2012|url=https://code.iczn.org/criteria-of-publication/article-8-what-constitutes-published-work/|edition=4th|access-date=July 16, 2021}}</ref><ref name=SVPOW>{{cite web|author=Mike Taylor|title=''Notes on Early Mesozoic Theropods'' and the future of zoological nomenclature|date=June 8, 2010|website=Sauropod Vertebra Picture of the Week|url=https://svpow.com/2010/06/08/notes-on-early-mesozoic-theropods-and-the-future-of-zoological-nomenclature|archive-url=https://web.archive.org/web/20210309173922/https://svpow.com/2010/06/08/notes-on-early-mesozoic-theropods-and-the-future-of-zoological-nomenclature/|archive-date=March 9, 2021}}</ref>}}<ref name=StreetThesis>{{cite thesis|author=Hallie P. Street|year=2016|title=A re-assessment of the genus Mosasaurus (Squamata: Mosasauridae)|type=PhD|publisher=University of Alberta|url=https://era.library.ualberta.ca/items/53bb82a0-7a66-4afb-a26d-b611b5f86136/view/f59a5ee9-9c57-4409-9988-5c8958216f80/Street_Hallie_P_201604_PhD.pdf|doi=10.7939/R31N7XZ1K|s2cid=92749266|doi-access=free}}</ref>{{rp|300}}

===Systematics and evolution===
{{See also|Mosasaur#Relation with snakes or monitor lizards}}
{{Featured article}}
{{multiple image
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 | alt2 = Image of a [[grass snake]]
 | footer = ''Mosasaurus'' is a [[Squamata|squamate]] like [[monitor lizard]]s and [[snake]]s, but scientists still debate which of the two is its closest living relative.
}}
As the [[type genus]] of the family Mosasauridae and the subfamily Mosasaurinae, ''Mosasaurus'' is a member of the order [[Squamata]] (which comprises [[lizard]]s and [[snake]]s). Relationships between mosasaurs and living squamates remain controversial as scientists still fiercely debate on whether the closest living relatives of mosasaurs are monitor lizards or snakes.<ref name=Caldwell2012 /><ref name=Ananjeva2019>{{cite journal|author=Natalia B. Ananjeva|s2cid=162184418|year=2019|title=Current State of the Problems in the Phylogeny of Squamate Reptiles (Squamata, Reptilia)|journal=Biology Bulletin Reviews|volume=9|issue=2|pages=119–128|doi=10.1134/s2079086419020026|bibcode=2019BioBR...9..119A }}</ref> ''Mosasaurus'', along with mosasaur genera ''[[Eremiasaurus]]'', ''Plotosaurus'',<ref name=Eremiasaurus>{{cite journal|title=A new mosasaurine from the Maastrichtian (Upper Cretaceous) phosphates of Morocco and its implications for mosasaurine systematics|author1=Aaron R. H. LeBlanc|author2=Michael W. Caldwell|author3=Nathalie Bardet|year=2012|journal=Journal of Vertebrate Paleontology|volume=32|issue=1|pages=82–104|doi=10.1080/02724634.2012.624145|jstor=41407709|bibcode=2012JVPal..32...82L |s2cid=130559113|url=https://www.researchgate.net/publication/233212853}}</ref> and ''Moanasaurus''{{efn|Some studies such as Madzia & Cau (2017) also recover ''Prognathodon'' and ''[[Plesiotylosaurus]]'' within the Mosasaurini.<ref name=MadziaandCau/>}}<ref name=StreetMosasaurini>{{cite journal|title=Reassessing Mosasaurini based on a systematic revision of ''Mosasaurus''|year=2017|author=Hallie P. Street|journal=Vertebrate Anatomy Morphology Palaeontology|volume=4|at=42|issn=2292-1389|url=https://www.researchgate.net/publication/317004037}}</ref> traditionally form a tribe within the Mosasaurinae variously called [[Mosasaurini]] or Plotosaurini.<ref name=Russell1967 />{{rp|145–148}}<ref name=Bell1997 /><ref name=Eremiasaurus/>

====Phylogeny and evolution of the genus====

One of the earliest relevant attempts at an evolutionary study of ''Mosasaurus'' was done by Russell in 1967.<ref name=Russell1967 /><ref name=Bell1997>{{cite book|author=Gorden L. Bell Jr.|title=Ancient Marine Reptiles|year=1997|chapter=A Phylogenetic Revision of North American and Adriatic Mosasauroidea|pages=293–332|publisher=Academic Press|doi=10.1016/b978-012155210-7/50017-x|isbn=978-0-12-155210-7}}</ref> He proposed that ''Mosasaurus'' evolved from a ''Clidastes''-like mosasaur, and [[Speciation|diverged]] into two lineages, one giving rise to ''M. conodon'' and another siring a [[chronospecies]] sequence which contained in order of succession ''M. ivoensis'', ''M. missouriensis'', and ''M. maximus-hoffmanni''.{{efn|''M. maximus'' is a North American taxon Russell (1967) recognized as a distinct species.<ref name=Russell1967 />{{rp|138–141}} It is now generally recognized as a junior synonym of ''M. hoffmannii'',<ref name=HarrellandMartin /> although some scientists maintain the taxon is a distinct species.<ref name=StreetandCaldwell/><ref name=StreetThesis/>{{rp|204}}}}{{efn|''maximus''-''hoffmannii'' was the wording used in Russell (1967); this is in recognition of the belief of a close relationship between the two species.<ref name=Russell1967 />{{rp|140, 202}}}}<ref name=Russell1967 />{{rp|202}} However, Russell used an early method of phylogenetics and did not use cladistics.<ref name=Bell1997 />

In 1997, Bell published the first cladistical study of North American mosasaurs. Incorporating the species ''M. missouriensis'', ''M. conodon'', ''M. maximus'', and an indeterminate specimen ([[University of Nebraska State Museum|UNSM]] 77040), some of his findings agreed with Russell (1967), such as ''Mosasaurus'' descending from an ancestral group containing ''Clidastes'' and ''M. conodon'' being the most basal of the genus. Contrary to Russell (1967), Bell also recovered ''Mosasaurus'' in a sister relationship with another group which included ''[[Globidens]]'' and ''Prognathodon'', and ''M. maximus'' as a sister species to ''Plotosaurus''. The latter rendered ''Mosasaurus'' [[paraphyly|paraphyletic]] (an unnatural grouping), but Bell (1997) nevertheless recognized ''Plotosaurus'' as a distinct genus.<ref name=Bell1997 />

Bell's study served as a precedent for later studies that mostly left the systematics of ''Mosasaurus'' unchanged,<ref name=Konishietal /><ref name=StreetThesis />{{rp|214–215}} although some later studies have recovered the sister group to ''Mosasaurus'' and ''Plotosaurus'' to instead be ''Eremiasaurus'' or ''[[Plesiotylosaurus]]'' depending on the method of data interpretation used,<ref name=Eremiasaurus/><ref name=MadziaandCau>{{cite journal|author1=Daniel Madzia|author2=Andrea Cau|title=Inferring 'weak spots' in phylogenetic trees: application to mosasauroid nomenclature|journal=PeerJ|year=2017|volume=5|pages=e3782|doi=10.7717/peerj.3782|pmid=28929018|pmc=5602675 |doi-access=free }}</ref><ref name=Simoesetal>{{cite journal|author1=Tiago R. Simões|author2=Oksana Vernygora|author3=Ilaria Paparella|author4=Paulina Jimenez-Huidobro|author5=Michael W. Caldwell|title=Mosasauroid phylogeny under multiple phylogenetic methods provides new insights on the evolution of aquatic adaptations in the group|journal=PLOS ONE|year=2017|volume=12|issue=5|pages=e0176773|doi=10.1371/journal.pone.0176773|pmid=28467456|pmc=5415187|bibcode=2017PLoSO..1276773S|doi-access=free}}</ref> with at least one study also recovering ''M. missouriensis'' to be the most basal species of the genus instead of ''M. conodon''.<ref name=Grigoriev2013>{{cite journal|author1=Dimitry V. Grigoriev|title=Redescription of ''Prognathodon lutugini'' (Squamata, Mosasauridae)|year=2013|journal=Proceedings of the Zoological Institute RAS|volume=317|issue=3|pages=246–261|doi=10.31610/trudyzin/2013.317.3.246 |s2cid=189800203 |url=http://www.zin.ru/Journals/trudyzin/doc/vol_317_3/TZ_317_3_Grigoriev.pdf}}</ref> In 2014, Konishi and colleagues expressed a number of concerns with the reliance on Bell's study. First, the genus was severely underrepresented by incorporating only the three North American species ''M. hoffmannii/M. maximus'', ''M. missouriensis'', and ''M. conodon''; by doing so, others like ''M. lemonnieri'', which is one of the most completely known species in the genus, were neglected, which affected phylogenetic results. Second, the studies relied on an unclean and shaky taxonomy of the ''Mosasaurus'' genus due to the lack of a clear holotype diagnosis, which may have been behind the genus's paraphyletic status.<ref name=StreetThesis />{{rp|214–215}}<ref name=Konishietal /> Third, there was still a lack of comparative studies of the skeletal anatomy of large mosasaurines at the time.<ref name=Konishietal /> These problems were addressed in Street's 2016 thesis in an updated phylogenetic analysis.<ref name=StreetThesis />{{rp|214–215}}
[[File:Mosasaurus missouriensis NT.png|thumb|Life restoration of ''M. missouriensis'']]
{{image flip|[[File:Mosasaurus lemonnieri.png|thumb|Life restoration of ''M. lemonnieri'']]}}
[[File:Mosasaurus beaugei1DB.jpg|thumb|Restoration of ''M. beaugei'']]
Conrad uniquely used only ''M. hoffmannii'' and ''M. lemonnieri'' in his 2008 phylogenetic analysis, which recovered ''M. hoffmannii'' as basal to a multitude of descendant clades containing (in order of most to least basal) ''Globidens'', ''M. lemonnieri'', ''Goronyosaurus'', and ''Plotosaurus''. This result indicated that ''M. hoffmannii'' and ''M. lemonnieri'' are not in the same genus.<ref name=Conrad2008>{{cite journal|author=Jack L. Conrad|s2cid=85271610|title=Phylogeny And Systematics Of Squamata (Reptilia) Based On Morphology|year=2008|journal=Bulletin of the American Museum of Natural History|volume=310|pages=1–182|doi=10.1206/310.1}}</ref>{{rp|81}} However, the study used a method unorthodox to traditional phylogenetic studies on mosasaur species because its focus was on the relationships of entire squamate groups rather than mosasaur classification. As a result, some paleontologists caution that lower-order classification results from Conrad's 2008 study such as the specific placement of ''Mosasaurus'' may contain technical problems, making them inaccurate.<ref name=Simoesetal/>

The following [[cladogram]] on the left (Topology A) is modified from a [[maximum clade credibility tree]] inferred by a [[Bayesian inference in phylogeny|Bayesian analysis]] in the most recent major phylogenetic analysis of the Mosasaurinae subfamily by Madzia & Cau (2017), which was self-described as a refinement of a larger study by Simões ''et al.'' (2017).<ref name=MadziaandCau/> The cladogram on the right (Topology B) is modified from Street's 2016 doctoral thesis proposing a revision to the Mosasaurinae, with proposed new taxa and renamings in single quotations.<ref name=StreetThesis />{{rp|267}}
{{clade gallery |width=450px |height=600px
|caption1=Topology A:
|header1=Maximum clade credibility tree by Madzia & Cau (2017)<ref name=MadziaandCau/>
|cladogram1=
{{clade| style=font-size:85%;line-height:85%
|label1x=[[Mosasaurinae]]
|1={{clade
   |1=''[[Dallasaurus turneri]]''
   |2={{clade
      |1=''[[Clidastes liodontus]]''
      |2={{clade
         |1=''[[Clidastes moorevillensis]]''
         |2={{clade
            |1=''[[Clidastes propython]]''
            |2={{clade
              |label1=[[Mosasaurini]]
              |1={{clade
                |1={{clade
                   |1={{clade
                       |1={{clade
                          |1=''[[Prognathodon overtoni]]''
                          |2=''[[Prognathodon rapax]]''
                          }}
                       |2={{clade
                          |1={{clade
                             |1={{clade
                                |1=''[[Prognathodon saturator]]''
                                |2={{clade
                                   |1=''[[Prognathodon currii]]''
                                   |2=''[[Prognathodon solvayi]]''
                                   }}
                                }}
                             }}
                          |2={{clade
                             |1=''[[Prognathodon waiparaensis]]''
                             |2={{clade
                                |1=''[[Prognathodon kianda]]''
                                |2=''[[Eremiasaurus heterodontus]]''
                                }}
                                }}
                             }}
                          }}
                       }}
                   |2={{clade|style2=background-color:#eeccFF;
                       |1=''[[Plesiotylosaurus crassidens]]''
                       |2={{clade
                           |1='''''Mosasaurus conodon'''''|style1=background-color:#ffa1a1;
                           |2={{clade
                              |1='''''Mosasaurus missouriensis'''''
                              |2={{clade
                                 |1='''''Mosasaurus hoffmannii'''''
                                 |2=''[[Plotosaurus bennisoni]]''|style2=background-color:#bbedFF;
                                 }}
                              }}
                          }}
                       }}
                 }}
              |2={{clade
                   |1=''[[Globidens alabamaensis]]''
                   |2=''[[Globidens dakotensis]]''
                 }}
}}}}}}}}}}}}

'''Positions of groups'''
{{legend|#eeccFF|''Mosasaurus'' clade|outline=gray}}
{{legend|#bbFFbb|Nominal Pacific species|outline=gray}}
'''Positions of individual taxa'''
{{legend|#ffa1a1|Species traditionally referred as ''Mosasaurus conodon''|outline=gray}}
{{legend|#bbedFF|Species traditionally referred as ''Plotosaurus bennisoni''|outline=gray}}
|caption2=Topology B:
|header2=Proposed revision by Street (2016)<ref name=StreetThesis />{{rp|267}}
|cladogram2=
{{clade| style=font-size:85%;line-height:85%
|label1x=[[Mosasaurinae]]
|1={{clade
   |1=''[[Prognathodon solvayi]]''
   |2={{clade
      |1=''[[Clidastes propython]]''
      |2={{clade
         |1={{clade
            |1=''[[Clidastes liodontus]]''
            |2=''[[Clidastes moorevillensis]]''
            }}
          |2={{clade
             |1={{clade
                |1=''[[Globidens alabamaensis]]''
                |2=''[[Globidens dakotensis]]''
                }}
             |2={{clade
                |1=''[[Prognathodon kianda]]''
                |2={{clade
                   |1=''[[Eremiasaurus heterodontus]]''
                   |2={{clade
                      |1={{clade
                         |1=''[[Prognathodon overtoni]]''
                         |2={{clade
                            |1=''[[Prognathodon saturator]]''
                            |2={{clade
                               |1=''[[Prognathodon currii]]''
                               |2=''[[Prognathodon rapax]]''
                               }}
                            }}
                         }}
                        |2={{clade
                           |1=''[[Plesiotylosaurus crassidens]]''
                           |2={{clade
                              |1=''{{'}}[[Marichimaera waiparaensis]]{{'}}''
                              |label2=[[Mosasaurini]]
                              |2={{clade
 |1={{clade
   |1=''{{'}}[[Amblyrhynchosaurus wiffeni]]{{'}}''
   |2={{clade
     |1=''{{'}}[[Moanasaurus hobetsuensis]]{{'}}''|style1=background-color:#bbFFbb;
     |2={{clade
        |1=''[[Moanasaurus|Moanasaurus mangahouange]]''
        |2=''{{'}}[[Moanasaurus longirostris]]{{'}}''
        }}}}}}
 |2={{clade|style1=background-color:#eeccFF;
    |1={{clade
       |1='''''Mosasaurus missouriensis'''''
       |2={{clade
          |1='''''Mosasaurus lemonnieri'''''
          |2={{clade
            |1='''''Mosasaurus hoffmannii'''''
            |2='''''{{'}}[[Mosasaurus glycys]]{{'}}'''''
            }}}}}}
     |2={{clade
        |1=''{{'}}[[Antipodenectes mokoroa]]{{'}}''|style1=background-color:#bbFFbb;
        |2={{clade
            |1=''{{'}}[[Umikosaurus prismaticus]]{{'}}''|style1=background-color:#bbFFbb;
            |2={{clade
                 |1=''{{'}}[[Aktisaurus conodon]]{{'}}''|style1=background-color:#ffa1a1;
                 |2=''[[Plotosaurus bennisoni]]''|style2=background-color:#bbedFF;
}}}}}}}}}}
                                 }}
                              }}
                           }}
                      }}
                   }}
                }}
             }}
         }}
      }}
   }}
}}<!-- end clade gallery-->

==Paleobiology==
===Head musculature and mechanics===
[[File:MosasaurMaastricht080910.JPG|thumb|left|alt=Closeup of a reconstructed ''M. hoffmannii'' skull|The skull of ''M. hoffmannii'' was adapted to withstand powerful bites.]]
In 1995, Lingham-Soliar studied the head musculature of ''M. hoffmannii''. Because soft tissue like muscles do not easily fossilize, reconstruction of the musculature was largely based on the structure of the skull, muscle scarring on the skull, and the musculature in extant monitor lizards.<ref name=LinghamSoliar />

In modern lizards, the mechanical build of the skull is characterized by a four-pivot geometric structure in the [[skull|cranium]] that allows flexible movement of the jaws, possibly to allow the animals to better position them and prevent prey escape when hunting. In contrast, the [[frontal bone|frontal]] and [[parietal bone]]s, which in modern lizards connect to form a flexible pivot point, overlap in the skull of ''M. hoffmannii''. This creates a rigid three-pivot geometric cranial structure. These cranial structures are united by strong interlocking sutures formed to resist compression and shear forces caused by a downward thrust of the lower jaw muscles or an upward thrust of prey. This rigid but highly shock-absorbent structure of the cranium likely allowed a powerful [[bite force]].<ref name=LinghamSoliar />

Like all mosasaurs, the lower jaws of ''Mosasaurus'' could swing forward and backward. In many mosasaurs like ''Prognathodon'' and ''M. lemonnieri'', this function mainly served to allow ratchet feeding, in which the pterygoid and jaws would "walk" captured prey into the mouth like a conveyor belt. But especially compared to those in ''M. lemonnieri'', the pterygoid teeth in ''M. hoffmannii'' are relatively small, which indicates ratchet feeding was relatively unimportant to its hunting and feeding.<ref name=LinghamSoliar /><ref name=LinghamSoliar2000>{{cite journal|author=Theagarten Lingham-Soliar|title=The mosasaur ''Mosasaurus lemonnieri'' (Lepidosauromorpha, Squamata) from the Upper Cretaceous of Belgium and The Netherlands|year=2000|journal=Paleontological Journal|issn=0031-0301|volume=34|issue=suppl. 2|pages=S225–S237}}</ref> Rather, ''M. hoffmannii'' likely employed inertial feeding (in which the animal thrusts its head and neck backward to release a held prey item and immediately thrust the head and neck forward to close the jaws around the item<ref name=InertialFeeding>{{cite web|author1=Keith A. Metzger|author2=Anthony Herrel|title=Inertial feeding in reptiles: the role of skull mass reduction|year=2002|url=https://sicb.burkclients.com/meetings/2002/schedule/abstractdetails.php3?id=429|archive-url=https://web.archive.org/web/20210610145655/https://sicb.burkclients.com/meetings/2002/schedule/abstractdetails.php3?id=429|archive-date=June 10, 2021}}</ref>) and used jaw adduction to assist in biting during prey seizure. The ''magnus adductor'' muscles, which attach to the lower jaws to the cranium and have a major role in biting function, are massive, indicating ''M. hoffmannii'' was capable of enormous bite forces. The long, narrow, and heavy nature of the lower jaws and attachment of tendons at the coronoid process would have allowed quick opening and closing of the mouth with little energy input underwater, which also contributed to the powerful bite force of ''M. hoffmannii'' and suggests it would not have needed the strong ''magnus depressor'' muscles (jaw-opening muscles) seen in some plesiosaurs.<ref name=LinghamSoliar />

===Mobility and thermoregulation===
[[File:MosasaurFrontPaddle080910.JPG|thumb|right|Reconstruction of an ''M. hoffmannii'' forelimb]]
''Mosasaurus'' swam using its tail. The swimming style was likely [[Fish locomotion#Sub-carangiform|sub-carangiform]], which is exemplified today by [[mackerel]]s.<ref name=Lindgrenetal /><ref name=LinghamSoliarLocomotion>{{cite journal|author=Theagarten Lingham-Soliar|title=Locomotion in mosasaurs|year=1991|journal=Modern Geology|volume=16|pages=229–248}}</ref> Its elongated paddle-like limbs functioned as [[hydrofoil]]s for maneuvering the animal. The paddles' steering function was enabled by large muscle attachments from the outwards-facing side of the humerus to the radius and ulna and modified joints allowed an enhanced ability of [[pronation|rotating the flippers]]. The powerful forces resulting from utilization of the paddles may have sometimes resulted in bone damage, as evidenced by a ''M. hoffmannii'' ilium with significant separation of the bone's [[Anatomical terms of bone#Gross features|head]] from the rest of the bone likely caused by frequent shearing forces at the articulation joint.<ref name=LinghamSoliar />

The tissue structure of ''Mosasaurus''{{'}} bones suggests it had a metabolic rate much higher than modern squamates and its [[basal metabolic rate|resting metabolic rate]] was between that of the [[leatherback sea turtle]] and that of ichthyosaurs and plesiosaurs.<ref name=Houssayeetal>{{cite journal|author1=Alexandra Houssaye|author2=Johan Lindgren|author3=Rodrigo Pellegrini|author4=Andrew H. Lee|author5=Damien Germain|author6=Michael J. Polcyn|title=Microanatomical and Histological Features in the Long Bones of Mosasaurine Mosasaurs (Reptilia, Squamata) – Implications for Aquatic Adaptation and Growth Rates|year=2013|journal=PLOS ONE|volume=8|issue=10|pages=e76741|doi=10.1371/journal.pone.0076741|pmid=24146919|pmc=3797777|bibcode=2013PLoSO...876741H|doi-access=free}}</ref> ''Mosasaurus'' was likely [[endotherm]]ic and maintained a constant body temperature independent of the external environment. Although there is no direct evidence specific to the genus, studies on the biochemistry of related mosasaur genera such as ''Clidastes'' suggests that endothermy was likely present in all mosasaurs.{{efn|The 2018 MS thesis of Cyrus Green disputes the notion that ''Clidastes'' was an endotherm based on the skeletochronology of the genus, finding its growth rates to be too low to be endothermic and instead similar to ectotherms. The dissertation argued that the high body temperatures calculated in pro-endotherm studies were a result of [[gigantothermy]]. However, only four specimens were studied.<ref name=GreenThesis>{{cite thesis|author=Cyrus C. Green|title=Osteohistology And Skeletochronology Of an Ontogenetic Series Of ''Clidastes'' (Squamata: Mosasauridae): Growth And Metabolism In Basal Mosasaurids|year=2018|publisher=Fort Hays State University|type=MS|doi=10.58809/MQVC1909|s2cid=91615758|doi-access=free}}</ref>}} Such a trait is unique among squamates, the only known exception being the [[Argentine black and white tegu]], which can maintain partial endothermy.<ref name=Tegu>{{cite journal|author1=Glenn J. Tattersall|author2=Cleo A. C. Leite|author3=Colin E. Sanders|author4=Viviana Cadena|author5=Denis V. Andrade|author6=Augusto S. Abe|author7=William K. Milsom|year=2016|title=Seasonal reproductive endothermy in tegu lizards|journal=Science Advances|volume=2|issue=1|pages=e1500951|doi=10.1126/sciadv.1500951|pmid=26844295|pmc=4737272|bibcode=2016SciA....2E0951T}}</ref> This adaptation would have given several advantages to ''Mosasaurus'', including increased stamina when foraging across larger areas and pursuing prey.<ref name=Harrelletal>{{cite journal|author1=T. Lynn Harrell Jr.|author2=Alberto Pérez-Huerta|author3=Celina A. Suarez|title=Endothermic mosasaurs? Possible thermoregulation of Late Cretaceous mosasaurs (Reptilia, Squamata) indicated by stable oxygen isotopes in fossil bioapatite in comparison with coeval marine fish and pelagic seabirds|year=2016|journal=Palaeontology|volume=59|issue=3|pages=351–363|doi=10.1111/pala.12240|bibcode=2016Palgy..59..351H |s2cid=130190966 |doi-access=free}}</ref> It may have also been a factor that allowed ''Mosasaurus'' to thrive in the colder climates of locations such as [[Antarctica]].<ref name=Harrelletal /><ref name=Bowmanetal>{{cite journal|author1=Vanessa C. Bowman|author2=Jane E. Francis|author3=James B. Riding|title=Late Cretaceous winter sea ice in Antarctica?|journal=Geology|year=2013|volume=41|issue=12|pages=1227–1230|doi=10.1130/G34891.1|bibcode=2013Geo....41.1227B|s2cid=128885087 |url=http://nora.nerc.ac.uk/id/eprint/504369/1/sea%20ice%20%20revised%20Jim%20edit.pdf}}</ref><ref name=Martin>{{cite journal|author=James E. Martin|s2cid=128604544|title=Biostratigraphy of the Mosasauridae (Reptilia) from the Cretaceous of Antarctica|year=2006|journal=Geological Society, London, Special Publications|volume=258|issue=1|pages=101–108|doi=10.1144/gsl.sp.2006.258.01.07|bibcode=2006GSLSP.258..101M}}</ref><ref name=AntarcticaPatagonia>{{cite journal|author1=Martin S. Fernandez|author2=Zulma Gasparini|year=2012|title=Campanian and Maastrichtian mosasaurs from Antarctic Peninsula and Patagonia, Argentina|journal=Bulletin de la Société Géologique de France|volume=183|issue=2|pages=93–102|doi=10.2113/gssgfbull.183.2.93}}</ref>

===Sensory functions===
[[File:Mosasaurus sp sclerotic ring.JPG|thumb|right|[[Sclerotic ring]] of ''Mosasaurus'']]
''Mosasaurus'' had relatively large [[orbit (anatomy)|eye sockets]]<ref name=LinghamSoliar /> with large [[sclerotic ring]]s occupying much of the sockets' diameter;<ref name=LinghamSoliar2000 /> the latter is correlated with eye size and suggests it had good vision. The eye sockets were located at the sides of the skull, which created a narrow field of [[binocular vision]] at around 28.5°<ref name=LinghamSoliar /><ref name=BFoV>{{cite journal|author1=Takuya Konishi|author2=Michael W. Caldwell|author3=Tomohiro Nishimura|author4=Kazuhiko Sakurai|author5=Kyo Tanoue|year=2015|title=A new halisaurine mosasaur (Squamata: Halisaurinae) from Japan: the first record in the western Pacific realm and the first documented insights into binocular vision in mosasaurs|journal=Journal of Systematic Palaeontology|volume=14|issue=10|pages=809–839|doi=10.1080/14772019.2015.1113447|s2cid=130644927|url=https://www.researchgate.net/publication/286118827}}</ref> but alternatively allowed excellent processing of a two-dimensional environment, such as the near-surface waters inhabited by ''Mosasaurus''.<ref name=LinghamSoliar />

[[endocast|Brain casts]] made from fossils of ''Mosasaurus'' show that the [[olfactory bulb]] and [[vomeronasal organ]], which both control the function of smell, are poorly developed and lack some structures in ''M. hoffmannii''; this indicates the species had a poor sense of smell. In ''M. lemonnieri'', these olfactory organs, although still small, are better developed and have some components lacking in ''M. hoffmannii''. The lack of a strong sense of smell suggests that olfaction was not particularly important in ''Mosasaurus''; instead, other senses like vision may have been more useful.<ref name=LinghamSoliar />

===Feeding===
[[File:Mosasaurus 21copy.jpg|thumb|left|Restoration of ''M. hoffmannii'' preying on a sea turtle]]
Paleontologists generally agree that ''Mosasaurus'' was likely an active predator of a variety of marine animals.<ref name=LinghamSoliar /><ref name=Schulpetal2013 /> Fauna likely preyed upon by the genus include bony fish, sharks, cephalopods, birds, and marine reptiles such as other mosasaurs<ref name=Schulpetal2013 /> and turtles.<ref name=LinghamSoliar /> It is unlikely ''Mosasaurus'' was a scavenger as it had a poor sense of smell. ''Mosasaurus'' was among the largest marine animals of its time,<ref name=LinghamSoliar /> and with its large, robust cutting teeth, scientists believe larger members of the genus would have been able to handle virtually any animal.<ref name=Schulpetal2013 /> Lingham-Soliar (1995) suggested that ''Mosasaurus'' had a rather "savage" feeding behavior as demonstrated by large tooth marks on scutes of the giant sea turtle ''[[Allopleuron|Allopleuron hoffmanni]]'' and fossils of re-healed fractured jaws in ''M. hoffmannii''.<ref name=LinghamSoliar /> The species likely hunted near the ocean surface as an ambush predator, using its large two-dimensionally adapted eyes to more effectively spot and capture prey.<ref name=LinghamSoliar /> Chemical and structural data in the fossils of ''M. lemonnieri'' and ''M. conodon'' suggests they may have also hunted in deeper waters.<ref name=Robbins2010>{{cite thesis|author=John A. Robbins|year=2010|title=Investigating Holocene climate change on the northern Channel Islands and Cretaceous mosasaur ecology using stable isotopes|type=PhD|publisher=Southern Methodist University|isbn=978-1-124-43286-1|url=https://www.proquest.com/openview/e4bfb5f68e4265760c93e86c08288fe8/1?pq-origsite=gscholar&cbl=18750|archive-url=https://web.archive.org/web/20210621023621/https://www.proquest.com/openview/e4bfb5f68e4265760c93e86c08288fe8/1?pq-origsite=gscholar&cbl=18750|archive-date=June 21, 2021}}</ref>{{rp|152}}

Carbon isotope studies on fossils of multiple ''M. hoffmannii'' individuals have found extremely low values of [[δ13C|δ<sup>13</sup>C]], the lowest in all mosasaurs for the largest individuals. Mosasaurs with lower δ<sup>13</sup>C values tended to occupy higher trophic levels, and one factor for this was dietary: a diet of prey rich in lipids such as sea turtles and other large marine reptiles can lower δ<sup>13</sup>C values. ''M. hoffmannii'''s low δ<sup>13</sup>C levels reinforces its likely position as an apex predator.<ref name=Schulpetal2013 /> [[Dental microwear]] has likewise shown that ''M. hoffmannii'' had a [[Generalist and specialist species|generalist]] diet,<ref>{{Cite journal|author1=Femke M. Holwerda|author2=Jordan Bestwick|author3=Mark A. Purnell|author4=John W. M. Jagt|author5=Anne S. Schulp|year=2023|title=Three-dimensional dental microwear in type-Maastrichtian mosasaur teeth (Reptilia, Squamata)|journal=Scientific Reports|volume=13|issue=1|at=18720|doi=10.1038/s41598-023-42369-7|doi-access=free|issn=2045-2322|pmid=37945619|pmc=10636054}}</ref> which also appears to be the case with ''M. missouriensis''.<ref>{{Cite journal|author1=Femke M. Holwerda|author2=Mark T. Mitchell|author3=Madelon van de Kerk|author4=Anne S. Schulp|year=2025|title=Mosasaur Feeding Ecology from the Campanian Bearpaw Formation, Alberta, Canada: A Preliminary Multi-Proxy Approach|journal=Diversity|volume=17|issue=3|at=205|doi=10.3390/d17030205|doi-access=free|bibcode=2025Diver..17..205H|issn=1424-2818|s2cid=277013757}}</ref>

Currently, there are only two known examples of a ''Mosasaurus'' preserved with stomach contents. The first is a well-preserved partial skeleton of a small ''M. missouriensis'' dated about 75 million years old with dismembered and punctured remains of a {{convert|1|m|ft|sp=us}} long fish in its gut. This fish was much longer than the length of the mosasaur's skull, which measured {{convert|66|cm|in|sp=us}} in length, confirming that ''M. missouriensis'' [[macrophage (ecology)|consumed prey larger than its head]] by dismembering and consuming bits at a time. Due to coexistence with other large mosasaurs like ''Prognathodon'', which specialized in robust prey, ''M. missouriensis'' likely specialized more on prey best consumed using cutting-adapted teeth in an example of [[Niche differentiation|niche partitioning]].<ref name=Konishietal /> The second is from a subadult ''M. hoffmannii'' partial skeleton recovered from the same locality as the species' holotype near Maastricht, nicknamed "Lars". This specimen was reported in a 2024 conference to contain acid-corroded bones of a juvenile turtle reminiscent of  ''[[Ctenochelys]]'' and of other distinct but unidentified animals in the gut region. Bones of an indeterminate bird or [[Avialae|bird-like dinosaur]] were also found in association with the mosasaur, which could have also represented gut contents.<ref name=LarsGut>{{cite conference|author1=Lars P.J. Barten|author2=John W.M. Jagt|author3=Eric W.A. Mulder|year=2024|title=Gut contents of a subadult individual of ''Mosasaurus hoffmannii'' Mantell, 1829 from the Maastrichtian type area (the Netherlands) hint at the species' dietary preferences|publisher=7th Triennial Mosasaur Meeting—A Global Perspective on Mesozoic Marine Amniotes|url=https://www.nhmmaastricht.nl/wp-content/uploads/2024/10/NHM_Abstract-Maastrichtian-Mosasaurus-Meet.pdf|pages=23–26}}</ref>

''Mosasaurus'' may have taught their offspring how to hunt, as supported by a fossil [[nautiloid]] ''Argonautilus catarinae'' with bite marks from two conspecific mosasaurs, one being from a juvenile and the other being from an adult. Analysis of the tooth marks by a 2004 study by Kauffman concluded that the mosasaurs were either ''Mosasaurus'' or ''Platecarpus''. The positioning of both bite marks are at the direction the nautiloid's head would have been facing, indicating it was incapable of escaping and was thus already sick or dead during the attacks; it is possible this phenomenon was from a parent mosasaur teaching its offspring about cephalopods as an alternate source of prey and how to hunt one. An alternate explanation postulates the bite marks as from one individual mosasaur that lightly bit the nautiloid at first, then proceeded to bite again with greater force. However, there are differences in tooth spacing between both bites which indicate different jaw sizes.<ref name=Kauffman>{{cite journal|author=Erle G. Kauffman|title=Mosasaur Predation on Upper Cretaceous Nautiloids and Ammonites from the United States Pacific Coast|year=2004|journal=PALAIOS|volume=19|issue=1|pages=96–100|doi=10.1669/0883-1351(2004)019<0096:MPOUCN>2.0.CO;2|bibcode=2004Palai..19...96K|s2cid=130690035 |url=http://doc.rero.ch/record/14992/files/PAL_E2143.pdf }}</ref>

===Behavior and paleopathology===
====Intraspecific combat====
[[File:Royal Tyrrell Mosasaurus missouriensis.jpg|thumb|222x222px|''M. missouriensis'' skull with another individual's tooth embedded in the rear lower jaw, likely via head grappling]]
There is fossil evidence that ''Mosasaurus'' engaged in aggressive and lethal combat with others of its kind. One partial skeleton of ''M. conodon'' bears multiple cuts, breaks, and punctures on various bones, particularly in the rear portions of the skull and neck, and a tooth from another ''M. conodon'' piercing through the quadrate bone. No injuries on the fossil show signs of healing, suggesting that the mosasaur was killed by its attacker by a fatal blow in the skull.<ref name="BellandMartin">{{cite journal|author1=Gorden L. Bell Jr.|author2=James E. Martin|title=Direct evidence of aggressive intraspecific competition in ''Mosasaurus conodon'' (Mosasauridae:Squamata)|year=1995|journal=Journal of Vertebrate Paleontology|volume=15|issue=suppl. to 3|pages=18A|doi=10.1080/02724634.1995.10011277}}</ref> Likewise, an ''M. missouriensis'' skeleton has a tooth from another ''M. missouriensis'' embedded in the lower jaw underneath the eye. In this case, there were signs of healing around the wound, implying survival of the incident.<ref name="TMP2012News">{{cite news|author=Carolyn Gramling|title=Ancient sea monster battle revealed in unusual fossil|date=October 26, 2016|journal=Science|url=https://www.science.org/content/article/ancient-sea-monster-battle-revealed-unusual-fossil|doi=10.1126/science.aal0310}}</ref> Takuya Konishi suggested an alternative cause of this example being head-biting behavior during [[Courtship display|courtship]] as seen in modern lizards.<ref name="TMP2012News" /><ref name="KonishiWebsite">{{cite web|author=Takuya Konishi|title=Anything Mosasaur|website=Takuya Konishi, PhD|url=https://konishta.wixsite.com/vpauc/copy-of-research|archive-url=https://web.archive.org/web/20210324182350/https://konishta.wixsite.com/vpauc/copy-of-research|archive-date=March 24, 2021}}</ref> Attacks by another ''Mosasaurus'' are a possible cause of physical [[paleopathology|pathologies]] in other skulls, but they could have instead arisen from other incidents like attempted biting on hard turtle shells. In 2004, Lingham-Soliar observed that if these injuries were indeed the result of an intraspecific attack, then there is a pattern of them concentrating in the skull region. Modern crocodiles commonly attack each other by grappling an opponent's head using their jaws, and Lingham-Soliar hypothesized that ''Mosasaurus'' employed similar head-grappling behavior during intraspecific combat. Many of the fossils with injuries possibly attributable to intraspecific combat are of juvenile or sub-adult ''Mosasaurus'', leading to the possibility that attacks on smaller, weaker individuals may have been more common.<ref name=LinghamSoliar2004>{{cite journal|author=Theagarten Lingham-Soliar|title=Palaeopathology and injury in the extinct mosasaurs (Lepidosauromorpha, Squamata) and implications for modern reptiles|year=2004|journal=Lethaia|volume=37|issue=3|pages=255–262|doi=10.1080/00241160410006519|bibcode=2004Letha..37..255L |url=http://doc.rero.ch/record/16193/files/PAL_E3402.pdf }}</ref> However, the attacking mosasaurs of the ''M. conodon'' and ''M. missouriensis'' specimens were likely similar in size to the victims.<ref name=TMP2012News /><ref name=BellandMartin /> In 2006, Schulp and colleagues speculated that ''Mosasaurus'' may have occasionally engaged in [[cannibalism]] as a result of intraspecific aggression.<ref name=Schulpetal>{{cite journal|author1=Anne S. Schulp|author2=Geert H. I. M. Walenkamp|author3=Paul A. M. Hofman|author4=Yvonne Stuip|author5=Bruce M. Rothschild|title=Chronic bone infection in the jaw of ''Mosasaurus hoffmanni'' (Squamata)|year=2006|journal=Oryctos|volume=6|issue=2006|pages=41–52|issn=1290-4805|url=http://www.dinosauria.org/documents/2009/oryctos_v.6_41-52_schulp_et_al.pdf}}</ref>

====Diseases====
[[File:Mosasaurus hoffmanni jaws.JPG|thumb|left|''M. hoffmannii'' specimen IRSNB R25, with an infected fracture to the left dentary (seen between the two middle tooth crowns in the back)]]
There are some ''M. hoffmannii'' jaws with evidence of infectious diseases as a result of physical injuries. Two examples include IRSNB R25 and IRSNB R27, both having fractures and other pathologies in their dentaries. IRSNB R25 preserves a [[Bone fracture#Fragments|complete fracture]] near the sixth [[dental alveolus|tooth socket]]. Extensive amounts of bony [[callus]] almost overgrowing the tooth socket are present around the fracture along with various [[Osteolysis|osteolytic cavities]], [[abscess]] canals, damages to the [[trigeminal nerve]], and inflamed erosions signifying severe bacterial infection. There are two finely ulcerated scratches on the bone callus, which may have developed as part of the healing process. IRSNB R27 has two fractures: one had almost fully healed and the other is an open fracture with nearby teeth broken off as a result. The fracture is covered with a [[nonunion]] formation of [[Fibrocartilage callus|bony callus]] with shallow scratch marks and a large pit connected to an abscess canal. Lingham-Soliar described this pit as resembling a tooth mark from a possible attacking mosasaur. Both specimens show signs of deep bacterial infection alongside the fractures; some bacteria may have spread to nearby damaged teeth and caused [[tooth decay]], which may have entered deeper tissue from prior post-traumatic or secondary infections. The dentaries ahead of the fractures in both specimens are in good condition, suggesting that the arteries and trigeminal nerves had not been damaged; if they were, those areas would have [[Necrosis|necrotized]] due to lack of blood. The dentaries' condition suggests that the species may have had an efficient process of immobilizing the fracture during healing, which helped prevent damage to vital blood vessels and nerves. This, along with signs of healing, indicates that the fractures were not imminently fatal.<ref name=LinghamSoliar2004 />

In 2006, Schulp and colleagues published a study describing a quadrate of ''M. hoffmannii'' with multiple unnatural openings and an estimated {{convert|0.5|L|gal|sp=us}} of tissue destroyed. This was likely a severe [[osteomyelitis|bone infection]] initiated by [[septic arthritis]], which progressed to the point where a large portion of the quadrate was reduced to abscess. Extensive amounts of bone reparative tissue were also present, suggesting the infection and subsequent healing process may have progressed for a few months. This level of bone infection would have been tremendously painful and severely hampered the mosasaur's ability to use its jaws. The location of the infection may have also interfered with breathing. Considering how the individual was able to survive such conditions for an extended period of time, Schulp and colleagues speculated it switched to a foraging-type diet of soft-bodied prey like squid that could be swallowed whole to minimize jaw use. The cause of the infection remains unknown, but if it were a result of an intraspecific attack then it is possible one of the openings on the quadrate may have been the point of entry for an attacker's tooth from which the infection entered.<ref name=Schulpetal />

[[Avascular necrosis]] has been reported by many studies to be present in every examined specimen of ''M. lemonnieri'' and ''M. conodon''.<ref name=Schulpetal2013 /><ref name=Ascending>{{cite journal|author1=Bruce M. Rothschild |author2=Larry D. Martin|title=Mosasaur ascending: the phytogeny of bends|journal=Netherlands Journal of Geosciences|year=2005|volume=84|issue=Special Issue 3|pages=341–344|doi=10.1017/S0016774600021120|doi-access=free|bibcode=2005NJGeo..84..341R }}</ref><ref name=Carlsen>{{cite journal|author=Agnete Weinreich Carlsen|s2cid=23194069|title=Frequency of decompression illness among recent and extinct mammals and "reptiles": a review|year=2017|journal=The Science of Nature|volume=104|issue=7–8|pages=56|doi=10.1007/s00114-017-1477-1|pmid=28656350|bibcode=2017SciNa.104...56C}}</ref> In examinations of ''M. conodon'' fossils from Alabama and New Jersey and ''M. lemonnieri'' fossils from Belgium, Rothschild and Martin in 2005 observed that the condition affected between 3-17% of the vertebrae in the mosasaurs' spines.<ref name=Ascending /> Avascular necrosis is a common result of [[decompression illness]]; it involves bone damage caused by the formation of nitrogen bubbles from inhaled air decompressed during frequent deep-diving trips, or by intervals of repetitive diving and short breathing. This indicates that both ''Mosasaurus'' species may have either been habitual deep-divers or repetitive divers. Agnete Weinreich Carlsen considered it the simplest explanation that such conditions were a product of inadequate anatomical adaptation. Nevertheless, fossils of other mosasaurs with invariable avascular necrosis still exhibit substantial adaptations like eardrums that were well-protected from rapid changes in pressure.<ref name=Carlsen />

Unnatural fusion of tail vertebrae has been documented in ''Mosasaurus'', which occurs when the bones [[ossification|remodel]] themselves after damage from trauma or disease. A 2015 study by Rothschild and Everhart surveyed 15 ''Mosasaurus'' specimens from North America and Belgium and found cases of fused tail vertebrae in three of them.{{efn|Two of the 15 surveyed fossils were reported from the [[Niobrara Formation]],<ref name=RothschildandEverhart /> a deposit that ''Mosasaurus'' was previously thought<ref name=Russell1967 />{{rp|136}} but is no longer recognized to be present in.<ref name=StreetThesis />{{rp|164, 294}}}} Two of these cases displayed irregular surface deformities around the fusion site caused by drainage of the [[Sinus (anatomy)|vertebral sinuses]], which is indicative of a bone infection. The causes of such infections are uncertain, but records of fused vertebrae in other mosasaurs suggest attacks by sharks and other predators as a possible candidate. The third case was determined to be caused by a form of [[spondyloarthropathy|arthritis]] based on the formation of smooth bridging between fused vertebrae.<ref name=RothschildandEverhart>{{cite journal|author1=Bruce Rothschild|author2=Michael J. Everhart|year=2015|title=Co-Ossification of Vertebrae in Mosasaurs (Squamata, Mosasauridae); Evidence of Habitat Interactions and Susceptibility to Bone Disease|journal=Transactions of the Kansas Academy of Science|volume=118|issue=3–4|pages=265–275|doi=10.1660/062.118.0309|s2cid=83690496|url=https://www.researchgate.net/publication/283849396}}</ref>

===Life history===
[[File:MosasaurusJuvenile.JPG|thumb|upright=1.4|Fragmentary skull of a juvenile ''Mosasaurus'' (NHMM 200793) from [[Geulhem]], Netherlands]]
It is likely that ''Mosasaurus'' was [[viviparity|viviparous]] (giving live birth) like most modern mammals today. There is no evidence for live birth in ''Mosasaurus'' itself, but it is known in a number of other mosasaurs;<ref name=Fieldetal>{{cite journal|author1=Daniel J. Field |author2=Aaron LeBlanc|author3=Adrienne Gau|author4=Adam D. Behlke|title=Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs|year=2015|journal=Palaeontology|volume=58|issue=3|pages=401–407|doi=10.1111/pala.12165|bibcode=2015Palgy..58..401F |s2cid=4660322 |doi-access=free}}</ref> examples include a skeleton of a pregnant ''[[Carsosaurus]]'',<ref name=Fieldetal /> a ''Plioplatecarpus'' fossil associated with fossils of two mosasaur embryos,<ref name=Belletal>{{cite journal|author1=Gorden L. Bell Jr.|author2=M. Amy Sheldon|author3=James P. Lamb|author4=James E. Martin|title=The first direct evidence of live birth in Mosasauridae (Squamata): Exceptional preservation in Cretaceous Pierre Shale of South Dakota|year=1996|journal=Journal of Vertebrate Paleontology|volume=16|issue=suppl. to 3|pages=21A|doi=10.1080/02724634.1996.10011371}}</ref> and fossils of newborn ''Clidastes'' from [[Pelagic zone|pelagic]] (open ocean) deposits.<ref name=Fieldetal /> Such fossil records, along with a total absence of any evidence suggesting external egg-based reproduction, indicates the likeliness of viviparity in ''Mosasaurus''.<ref name=Fieldetal /><ref name=Belletal /> Microanatomical studies on bones of juvenile ''Mosasaurus'' and related genera have found that their bone structures are comparable to adults. They do not exhibit the bone mass increase found in juvenile primitive mosasauroids to support buoyancy associated with a lifestyle in shallow water, implying that ''Mosasaurus'' was [[precociality|precocial]]: They were already efficient swimmers and lived fully functional lifestyles in open water at a very young age and did not require nursery areas to raise their young.<ref name=HoussayeandTofforeau>{{cite journal|author1=Alexandra Houssaye|author2=Paul Tafforeau|s2cid=84662320|title=What vertebral microanatomy reveals about the ecology of juvenile mosasaurs (Reptilia, Squamata)|year=2012|journal=Journal of Vertebrate Paleontology|volume=32|issue=5|pages=1042–1048|doi=10.1080/02724634.2012.680999|bibcode=2012JVPal..32.1042H }}</ref><ref name=Fieldetal /> Some areas in Europe and South Dakota have yielded concentrated assemblages of juvenile ''M. hoffmannii'', ''M. missouriensis'' and/or ''M. lemonnieri''. These localities are all shallow ocean deposits, suggesting that juvenile ''Mosasaurus'' may still have lived in shallow waters.<ref name=Martin2002>{{cite journal|author=James E. Martin|title=Juvenile marine reptiles from the Late Cretaceous of the Antarctic peninsula and their relationships to other such occurrences in central South Dakota and Belgium|year=2002|journal=Proceedings of the South Dakota Academy of Science|volume=81|pages=53–57|url=https://sdaos.org/wp-content/uploads/pdfs/2002/53-57.pdf|archive-url=https://web.archive.org/web/20231115193804/https://sdaos.org/wp-content/uploads/pdfs/2002/53-57.pdf|archive-date=November 15, 2023}}</ref>

==Paleoecology==
===Distribution, ecosystem, and ecological impact===
[[File:LateCretaceousMap.jpg|thumb|alt=Map of the earth during the [[Late Cretaceous]]|''Mosasaurus'' inhabited the [[Western Interior Seaway]] of North America and [[Tethys Ocean|Mediterranean Tethys]] of Europe and Africa.]]
''Mosasaurus'' had a transatlantic distribution, with its fossils having been found in marine deposits on both sides of the Atlantic Ocean. These localities include the [[Midwestern United States|Midwest]] and [[East Coast of the United States|East Coast]] of the United States, Canada, Europe, Turkey, Russia, the [[Levant]], the African coastline from Morocco<ref>{{Cite journal|last1=Rempert|first1=Trevor|last2=Vinkeles Melchers|first2=Alexander P.M.|last3=Rempert|first3=Ashley N.|last4=Haque|first4=Muhammad R.|last5=Armstrong|first5=Andrew|title=Occurrence of Mosasaurus hoffmannii Mantell, 1829 (Squamata, Mosasauridae) in the Maastrichtian Phosphates of Morocco|journal=Journal of Paleontological Sciences|url=https://www.researchgate.net/publication/357836567}}</ref> to South Africa, Brazil, Argentina, and Antarctica.<ref name=StreetandCaldwell /><ref name=Martin /><ref name=Bardet2012 /> During the Late Cretaceous, these regions made up the three seaways inhabited by ''Mosasaurus'': the Atlantic Ocean, the [[Western Interior Seaway]], and the [[Tethys Ocean|Mediterranean Tethys]].<ref name=Bardet2012>{{cite journal|author=Nathalie Bardet|title=Maastrichtian marine reptiles of the Mediterranean Tethys: a palaeobiogeographical approach|year=2012|journal=Bulletin de la Société Géologique de France|volume=183|issue=6|pages=573–596|doi=10.2113/gssgfbull.183.6.573}}</ref> Multiple oceanic [[climate classification|climate zones]] encompassed the seaways, including [[tropical]], [[subtropical]], [[temperate]], and [[subarctic|subpolar]] climates.<ref name=Bardet2012 /><ref name=NichollsandRussel>{{cite journal|author=Elizabeth L. Nicholls and Anthony P. Russell|title=Paleobiogeography of the Cretaceous Western Interior Seaway of North America: the vertebrate evidence|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|volume=79|issue=1–2|pages=149–169|year=1990|doi=10.1016/0031-0182(90)90110-S|bibcode=1990PPP....79..149N}}</ref><ref name=Kempetal>{{cite journal|author1=David B. Kemp|author2=Stuart A. Robinson|author3=J. Alistair Crame|author4=Jane E. Francis|author5=Jon Ineson|author6=Rowan J. Whittle|author7=Vanessa Bowman|author8=Charlotte O'Brien|title=A cool temperate climate on the Antarctic Peninsula through the latest Cretaceous to early Paleogene|year=2014|journal=Geology|volume=42|issue=7|pages=583–586|doi=10.1130/g35512.1|bibcode=2014Geo....42..583K|doi-access=free|hdl=2164/4380|hdl-access=free}}</ref> The wide range of oceanic climates yielded a large diversity of fauna that coexisted with ''Mosasaurus''.

====Mediterranean Tethys====
The Mediterranean Tethys during the [[Maastrichtian]] [[Stage (stratigraphy)|stage]] was located in what is now Europe, Africa, and the Middle East. In recent studies, the confirmation of paleogeographical affinities extended this range to areas across the Atlantic including Brazil and the East Coast state of New Jersey. It is geographically subdivided into two [[List of biogeographic provinces|biogeographic provinces]] that respectively include the northern and southern Tethyan margins. The two mosasaurs ''Mosasaurus'' and ''Prognathodon'' appear to have been the dominant taxa, being widespread and ecologically diversified throughout the seaway.<ref name=Bardet2012 />

The northern Tethyan margin was located around the paleolatitudes of [[30th parallel north|30]]–[[40th parallel north|40°N]], consisting of what is now the European continent, Turkey, and New Jersey. At the time, Europe was a scattering of islands with most of the modern continental landmass being underwater. The margin provided a warm-temperate climate with habitats dominated by mosasaurs and sea turtles. ''M. hoffmannii'' and ''Prognathodon sectorius'' were the dominant species in the northern province.<ref name=Bardet2012 /> In certain areas such as Belgium, other ''Mosasaurus'' species like ''M. lemonnieri'' were instead the dominant species, where its occurrences greatly outnumber those of other large mosasaurs.<ref name=LinghamSoliar2000 /> Other mosasaurs found in the European side of the northern Tethyan margin include smaller genera such as ''[[Halisaurus]]'', ''Plioplatecarpus'', and ''Platecarpus''; the shell-crusher ''[[Carinodens]]''; and larger mosasaurs of similar trophic levels including ''[[Hainosaurus|Tylosaurus bernardi]]'' and four other species of ''Prognathodon''. Sea turtles such as ''Allopleuron hoffmanni'' and ''[[Glyptochelone|Glyptochelone suickerbuycki]]'' were also prevalent in the area and other marine reptiles including indeterminate [[Elasmosauridae|elasmosaurs]] have been occasionally found. Marine reptile assemblages in the New Jersey region of the province are generally equivalent with those in Europe; the mosasaur faunae are quite similar but exclude ''M. lemonnieri'', ''Carinodens'', ''Tylosaurus'', and certain species of ''Halisaurus'' and ''Prognathodon''. In addition, they exclusively feature ''M. conodon'', ''Halisaurus platyspondylus'' and ''Prognathodon rapax''.<ref name=Bardet2012 /> Many types of sharks such as ''[[Squalicorax]]'', ''[[Cretalamna]]'', ''[[Serratolamna]]'', and [[sand sharks]],<ref name=SpainShark>{{cite journal|author1=Jose-Carmelo Corral|author2=Ana Berreteaga|author3=Henri Cappetta|title=Upper Maastrichtian shallow marine environments and neoselachian assemblages in North Iberian palaeomargin (Castilian Ramp, Spain)|year=2016| journal=Cretaceous Research|volume=57|pages=639–661|doi=10.1016/j.cretres.2015.08.001|bibcode=2016CrRes..57..639C }}</ref> as well as bony fish such as ''[[Cimolichthys]]'', the saber-toothed herring ''[[Enchodus]]'', and the swordfish-like ''[[Protosphyraena]]'' are represented in the northern Tethyan margin.<ref name=Bardet2012 /><ref name=Friedman>{{cite journal|author=Matt Friedman|title=Ray-finned fishes (Osteichthyes, Actinopterygii) from the type Maastrichtian, the Netherlands and Belgium|year=2012|journal=Scripta Geologica|issue=8|pages=113–142|url=https://www.researchgate.net/publication/267841275}}</ref>

[[File:Tellus Mosasaurus.jpg|thumb|Skeleton of ''M. beaugei'', which is known from Morocco and Brazil]]

The southern Tethyan margin was located along the equator between [[20th parallel north|20°N]] and [[20th parallel south|20°S]], resulting in warmer tropical climates. Seabeds bordering the [[craton]]s in Africa and Arabia and extending to the Levant and Brazil provided vast shallow marine environments. These environments were dominated by mosasaurs and [[Bothremydidae|marine side-necked turtles]]. Of the mosasaurs, ''Globidens phosphaticus'' is the characteristic species of the southern province; in the African and Arabian domain, ''Halisaurus arambourgi'' and ''{{'}}Platecarpus ptychodon{{'}}''{{efn|A dubious taxon that may represent various mosasaurs such as ''[[Gavialimimus]]'' or ''Platecarpus somenensis''<ref name=Strongetal>{{cite journal|author1=Catherine R. C. Strong|author2=Michael W. Caldwell|author3=Takuya Konishi|author4=Alessandro Palci|title=A new species of longirostrine plioplatecarpine mosasaur (Squamata: Mosasauridae) from the Late Cretaceous of Morocco, with a re-evaluation of the problematic taxon'' 'Platecarpus' ptychodon''|year=2020|journal=Journal of Systematic Palaeontology|volume=18|issue=21|pages=1769–1804|doi=10.1080/14772019.2020.1818322|doi-access=free|bibcode=2020JSPal..18.1769S }}</ref>}}<ref name=Bardet2012 /> were also common mosasaurs alongside ''Globidens''.<ref name="Bardet2012" /> ''Mosasaurus'' was not well-represented: the distribution of ''M. beaugei'' was restricted to Morocco and Brazil and isolated teeth from Syria suggested a possible presence of ''M. lemonnieri'', although ''M. hoffmannii'' also had some presence throughout the province.<ref name="StreetandCaldwell" /><ref name="Bardet2012" /> Other mosasaurs from the southern Tethyan margin include the enigmatic ''Goronyosaurus'', the shell-crushers ''[[Igdamanosaurus]]'' and ''Carinodens'', ''Eremiasaurus'', four other species of ''Prognathodon'', and various other species of ''Halisaurus''. Other marine reptiles such as the marine monitor lizard ''[[Pachyvaranus]]'' and the sea snake ''[[Palaeophis]]'' are known there. Aside from ''[[Zarafasaura]]'' in Morocco, plesiosaurs were scarce. As a tropical area, bony fish such as ''Enchodus'' and ''[[Stratodus]]'' and various sharks were common throughout the southern Tethyan margin.<ref name=Bardet2012 />

====Western Interior Seaway====
[[File:Xiphactinus and Platecarpus.jpg|thumb|left|alt=Mounted skeletons of ''[[Platecarpus]]'', ''[[Protostega]]'', and ''[[Xiphactinus]]''|''Mosasaurus'' coexisted with bony fish such as ''[[Xiphactinus]]'', sea turtles like ''[[Protostega]]'' and [[Plioplatecarpinae|plioplatecarpine]] mosasaurs in North America.]]
Many of the earliest fossils of ''Mosasaurus'' were found in [[Campanian]] stage deposits in North America, including the Western Interior Seaway, an inland sea which once flowed through what is now the central United States and Canada, and connected the [[Arctic Ocean]] to the modern-day [[Gulf of Mexico]]. The region was shallow for a seaway, reaching a maximum depth of about {{convert|800-900|m|ft|sp=us}}.<ref name=Stanley>{{cite book |author=Steven M. Stanley |title=Earth System History |location=New York |publisher=W.H. Freeman and Company |year=1999 |isbn=978-0-7167-2882-5 |pages=487–489}}</ref> Extensive drainage from the neighboring continents, [[Appalachia (Mesozoic)|Appalachia]] and [[Laramidia]], brought in vast amounts of sediment. Together with the formation of a nutrient-rich deepwater mass from the mixing of continental freshwater, Arctic waters from the north, and warmer saline Tethyan waters from the south, this created a warm and productive seaway that supported a rich diversity of marine life.<ref name=OOKBook2017>{{cite book|author=Michael J. Everhart|title=Oceans of Kansas|year=2017|publisher=Indiana University Press|pages=24–263|isbn=978-0-253-02632-3}}</ref><ref name=Heetal>{{cite journal|author1=Shaoneng He|author2=T. Kurtis Kyser|author3=William G. E. Caldwell|title=Paleoenvironment of the Western Interior Seaway inferred from δ18O and δ13C values of molluscs from the Cretaceous Bearpaw marine cyclothem|year=2005|journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=217|issue=1–2|pages=67–85|doi=10.1016/j.palaeo.2004.11.016|bibcode=2005PPP...217...67H|s2cid=129666891|url=https://www.researchgate.net/publication/256821697}}</ref><ref name=FisherandArthur>{{cite journal|author1=Cynthia G. Fisher|author2=Michael A. Arthur|title=Water mass characteristics in the Cenomanian US Western Interior seaway as indicated by stable isotopes of calcareous organisms|year=2002|journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=188 |issue=3–4|pages=89–213|doi=10.1016/S0031-0182(02)00552-7|bibcode=2002PPP...188..189F}}</ref>

The biogeography of the region has been subdivided into two Interior Subprovinces characterized by different climates and faunal structures, and their borders are separated in modern-day [[Kansas]]. The oceanic climate of the Northern Interior Subprovince was likely a cool temperate one, while the Southern Interior Subprovince had warm temperate to subtropical climates.<ref name="NichollsandRussel" /> The fossil assemblages throughout these regions suggest a complete faunal turnover when ''M. missouriensis'' and ''M. conodon'' appeared at 79.5 Ma, indicating that the presence of ''Mosasaurus'' in the Western Interior Seaway had a profound impact on the restructuring of marine ecosystems.<ref name=Kiernan>{{cite journal|author=Caitlin R. Kiernan|title=Stratigraphic distribution and habitat segregation of mosasaurs in the Upper Cretaceous of western and central Alabama, with a historical review of Alabama mosasaur discoveries|year=2002|journal=Journal of Vertebrate Paleontology|volume=22|issue=1|pages=91–103|doi=10.1671/0272-4634(2002)022[0091:sdahso]2.0.co;2|s2cid=130280406|url=https://www.researchgate.net/publication/254314199}}</ref> The faunal structure of both provinces was generally much more diverse prior to the appearance of ''Mosasaurus'', during a [[Stage (stratigraphy)|faunal stage]] known as the [[Niobrara Formation|Niobraran Age]], than it was during the following [[Navesink Formation|Navesinkan Age]].<ref name=Kiernan /><ref name=NichollsandRussel /><ref name=DemopolisAge>{{cite journal|author1=Andrew D. Gentry|author2=James F. Parham|author3=Dana J. Ehret|author4=Jun A. Ebersole|title=A new species of ''Peritresius'' Leidy, 1856 (Testudines: Pan-Cheloniidae) from the Late Cretaceous (Campanian) of Alabama, USA, and the occurrence of the genus within the Mississippi Embayment of North America|year=2018|journal=PLOS ONE|volume=13|issue=4|pages=e0195651|doi=10.1371/journal.pone.0195651|pmid=29668704|pmc=5906092|bibcode=2018PLoSO..1395651G|doi-access=free}}</ref>

In what is now Alabama within the Southern Interior Subprovince, most of the key genera including sharks like ''[[Cretoxyrhina]]'' and the mosasaurs ''Clidastes'', ''Tylosaurus'', ''Globidens'', ''Halisaurus'', and ''Platecarpus'' disappeared and were replaced by ''Mosasaurus''.<ref name=Kiernan /><ref name=AlabamaOverview>{{cite journal|author1=Jun Ebersole|author2=Takehito Ikejiri |author3=Harry Lyon Blewitt |author4=Sandy Ebersole|title=An Overview of Late Cretaceous Vertebrates from Alabama|journal=Bulletin of the Alabama Museum of Natural History|year=2013|volume=31|issue=1|pages=46–70|url=https://www.researchgate.net/publication/272508435}}</ref> During the Navesinkan Age, ''Mosasaurus'' dominated the whole region, accounting for around two-thirds of all mosasaur diversity with ''Plioplatecarpus'' and ''Prognathodon'' sharing the remaining third. The Northern Interior Subprovince also saw a restructuring of mosasaur assemblages, characterized by the disappearance of mosasaurs like ''Platecarpus'' and their replacement by ''Mosasaurus'' and ''Plioplatecarpus''.<ref name=Kiernan /> Some Niobraran genera such as ''Tylosaurus'',<ref name=TylosaurusHypothesis>{{cite journal|author1=Paulina Jiménez-Huidobro|author2=Michael W. Caldwell|title=A New Hypothesis of the Phylogenetic Relationships of the Tylosaurinae (Squamata: Mosasauroidea)|journal=Frontiers in Earth Science|volume=7|issue=47|year=2019|page=47|doi=10.3389/feart.2019.00047|bibcode=2019FrEaS...7...47J|doi-access=free}}</ref> ''Cretoxyrhina'',<ref name = Cooketal2017>{{cite journal|author1=Todd Cook |author2=Eric Brown |author3=Patricia E. Ralrick |author4=Takuya Konishi|title=A late Campanian euselachian assemblage from the Bearpaw Formation of Alberta, Canada: some notable range extensions|year=2017|journal=Canadian Journal of Earth Sciences|volume=54|issue=9|pages=973–980|doi=10.1139/cjes-2016-0233|hdl=1807/77762|bibcode=2017CaJES..54..973C|hdl-access=free}}</ref> hesperornithids,<ref name=TokarykandHarington>{{cite journal|author1=Tim T. Tokaryk|author2=C. R. Harington|title=''Baptornis'' sp. (Aves: Hesperornithiformes) from the Judith River Formation (Campanian) of Saskatchewan, Canada|journal=Journal of Paleontology|year=1992|volume=66|issue=6|pages=1010–1012|doi=10.1017/S002233600002093X|bibcode=1992JPal...66.1010T |s2cid=130444236 }}</ref> and plesiosaurs including elasmosaurs such as ''[[Terminonatator]]''<ref name=Sato2003>{{cite journal|author=Tamaki Sato|title=''Terminonatator ponteixensis'', a new elasmosaur (Reptilia; Sauropterygia) from the Upper Cretaceous of Saskatchewan|journal=Journal of Vertebrate Paleontology|year=2003|volume=23|issue=1|pages=89–103|doi=10.1671/0272-4634(2003)23[89:tpanes]2.0.co;2|s2cid=130373116 }}</ref> and polycotylids like ''[[Dolichorhynchops]]''<ref name=Sato2005>{{cite journal|author=Tamaki Sato|title=A new polycotylid plesiosaur (Reptilia: Sauropterygia) from the Upper Cretaceous Bearpaw Formation in Saskatchewan, Canada|journal=Journal of Paleontology|volume=79|issue=5|pages=969–980|year=2005|doi=10.1666/0022-3360(2005)079[0969:anpprs]2.0.co;2|s2cid=131128997 }}</ref> maintained their presence until around the end of the Campanian, during which the entire Western Interior Seaway started receding from the north.<ref name=OOKBook2017 /> ''Mosasaurus'' continued to be the dominant genus in the seaway until the end of the Navesinkan Age at the end of the Cretaceous.<ref name=Kiernan /> Contemporaneous fauna included sea turtles such as ''[[Protostega]]''<ref name=AlabamaOverview /> and ''[[Archelon]]'';<ref name=Hoganson>{{cite journal|author1=John W. Hoganson|author2=Brett Woodward|year=2004|title=Skeleton of the Rare Giant Sea Turtle, ''Archelon'', Recovered from the Cretaceous DeGrey Member of the Pierre Shale near Cooperstown, Griggs County, North Dakota|journal=North Dakota Geological Society Newsletter|volume=32|issue=1|pages=1–4|url=https://www.dmr.nd.gov/ndfossil/education/pdf/Archelon.pdf|archive-url=https://web.archive.org/web/20200603210943/https://www.dmr.nd.gov/ndfossil/education/pdf/Archelon.pdf|archive-date=June 3, 2020}}</ref> many species of sea birds including ''[[Baptornis]]'',<ref name=TokarykandHarington /> ''[[Ichthyornis]]'', and ''[[Halimornis]]''; sharks such as the mackerel sharks ''Cretalamna'', ''Squalicorax'', ''[[Pseudocorax]]'', and ''Serratolamna'', the goblin shark ''[[Scapanorhynchus]]'', the sand tiger ''[[Odontaspis]]'', and the sawfish-like ''[[Ischyrhiza]]''; and bony fish such as ''Enchodus'', ''Protosphyraena'', ''Stratodus'', and the [[Ichthyodectidae|ichthyodectids]] ''[[Xiphactinus]]'' and ''[[Saurodon]]''.<ref name=AlabamaOverview /><ref name=Cicimurrietal>{{cite journal|author1=David J. Cicimurri|author2=Gorden L. Bell, Jr.|author3=Philip W. Stoffer|title=Vertebrate Paleontology of the Pierre Shale and Fox Hills Formations (Late Campanian-Late Maastrichtian) of Badlands National Park, South Dakota|journal=National Park Service Paleontological Research|volume=4|year=1999|pages=1–7|url=https://doc.rero.ch/record/31563/files/PAL_E647.pdf#page=9}}</ref>

====Antarctica====
[[File:Geologic map of Seymour Island, Antarctica.png|thumb|right|alt=Geological map of the [[Seymour Island]]|''Mosasaurus'' fossils were found in the [[Seymour Island]] of Antarctica, which once provided cool temperate waters.]]
''Mosasaurus'' is known from late Maastrichtian deposits in the [[Antarctic Peninsula]], specifically the [[López de Bertodano Formation]] in [[Seymour Island]].<ref name=Martin /> Located within the [[polar circle]] at around 65°S,<ref name=Kempetal /> temperatures at medium to large water depths would have been around {{convert|6|C|F|sp=us}} on average, while sea surface temperatures may have dropped below freezing and sea ice may have formed at times.<ref name=Bowmanetal /><ref name=Tobinetal>{{cite journal|author1=Thomas S. Tobin|author2=Peter D. Ward|author3=Eric J. Steig|author4=Eduardo B. Olivero|author5=Isaac A. Hilburn|author6=Ross N. Mitchell|author7=Matthew R. Diamond|author8=Timothy D. Raub|author9=Joseph L. Kirschvink|title=Extinction patterns, δ18 O trends, and magnetostratigraphy from a southern high-latitude Cretaceous–Paleogene section: Links with Deccan volcanism|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|year=2012|volume=350–352|pages=180–188|doi=10.1016/j.palaeo.2012.06.029|bibcode=2012PPP...350..180T|url=https://authors.library.caltech.edu/35186/2/mmc1.doc}}</ref> ''Mosasaurus'' appears to be the most diverse mosasaur in the Maastrichtian Antarctica. At least two species of ''Mosasaurus'' have been described, but the true number of species is unknown as remains are often fragmentary and specimens are described in [[open nomenclature]]. These species include one comparable with ''M. lemonnieri'', and another that appears to be closely related to ''M. hoffmannii''.<ref name="Martin" /> ''M. sp.'' has also been described. However, it is possible that such specimens may actually represent ''Moanasaurus'', although this depends on the outcome of a pending revision of the genus.<ref name=Gonzalezetal /> At least four other mosasaur genera have been reported in Antarctica, including ''Plioplatecarpus'', the mosasaurines ''Moanasaurus'' and ''[[Liodon]]'',<ref name="Martin" /> and ''[[Kaikaifilu]]''. The validity of some of these genera is disputed as they are primarily based on isolated teeth.<ref name=Kaikaifilu>{{cite journal|author1=Rodrigo A. Otero|author2= Sergio Soto-Acuña|author3=David Rubilar-Rogers|author4=Carolina S. Gutstein| title = ''Kaikaifilu hervei'' gen. et sp. nov., a new large mosasaur (Squamata, Mosasauridae) from the upper Maastrichtian of Antarctica | journal = Cretaceous Research |year= 2017| volume = 70 | pages = 209–225 | doi = 10.1016/j.cretres.2016.11.002|bibcode= 2017CrRes..70..209O}}</ref> ''Prognathodon'' and ''Globidens'' are also expected to be present based on distribution trends of both genera, although conclusive fossils have yet to be found.<ref name=Martin /> Other Antarctic marine reptiles included elasmosaurid plesiosaurs like ''[[Aristonectes]]'' and other indeterminate elasmosaurids.<ref name=OGormanetal>{{cite journal|author1=José P. O'Gorman|author2=Karen M. Panzeri|author3=Marta S. Fernández|author4=Sergio Santillana|author5=Juan J. Moly|author6=Marcelo Reguero|s2cid=134265841|title=A new elasmosaurid from the upper Maastrichtian López de Bertodano Formation: new data on weddellonectian diversity|year=2018|journal=Alcheringa: An Australasian Journal of Palaeontology|volume=42|issue=4|pages=575–586|doi=10.1080/03115518.2017.1339233|bibcode=2018Alch...42..575O|hdl=11336/49635|url=http://sedici.unlp.edu.ar/handle/10915/148051 |hdl-access=free}}</ref> The fish assemblage of the López de Bertodano Formation was dominated by ''Enchodus'' and ichthyodectiformes.<ref name=Cioneetal2018>{{cite journal|author1=Alberto L. Cione|author2=Sergio Santillana|author3=Soledad Gouiric-Cavalli|author4=Carolina Acosta Hospitaleche|author5=Javier N. Gelfo|author6=Guillermo M. Lopez|author7=Marcelo Reguero|title=Before and after the K/Pg extinction in West Antarctica: New marine fish records from Marambio (Seymour) Island|year=2018|journal=Cretaceous Research|volume=85|pages=250–265|doi=10.1016/j.cretres.2018.01.004|bibcode=2018CrRes..85..250C |s2cid=133767014|hdl=10915/147537|hdl-access=free}}</ref>

===Habitat preference===
[[File:Mosasaurus hoffmannii.JPG|left|thumb|alt=Restoration of ''M. hoffmannii''|''Mosasaurus'' inhabited offshore ocean habitats of various depths.]]
Known fossils of ''Mosasaurus'' have typically been recovered from deposits representing nearshore habitats during the Cretaceous period, with some fossils coming from deeper-water deposits.<ref name=Robbins2010 />{{rp|152, 178}}<ref name=REE>{{cite journal|author1=T. Lynn Harrell Jr.|author2=Alberto Pérez-Huerta|title=Habitat preference of mosasaurs indicated by rare earth element (REE) content of fossils from the Upper Cretaceous marine deposits of Alabama, New Jersey, and South Dakota (USA)|year=2014|journal=Netherlands Journal of Geosciences|volume=94|issue=1|pages=145–154|doi=10.1017/njg.2014.29|s2cid=128587386|doi-access=free}}</ref> Lingham-Soliar (1995) elaborated on this, finding that Maastrichtian deposits in the Netherlands with ''M. hoffmannii'' occurrences represented nearshore waters around {{convert|40-50|m|ft|sp=us}} deep. Changing temperatures and an abundance in marine life were characteristic of these localities. The morphological build of ''M. hoffmannii'', nevertheless, was best adapted for a pelagic surface lifestyle.<ref name=LinghamSoliar />

δ<sup>13</sup>C is also correlated with a marine animal's feeding habitat as isotope levels deplete when habitat is farther from the shoreline, so some scientists interpreted isotope levels as a proxy for habitat preference. Separate studies involving multiple ''Mosasaurus'' specimens have yielded consistently low δ<sup>13</sup>C levels of tooth enamel, indicating that ''Mosasaurus'' fed in more offshore or open waters. It has been pointed out how δ<sup>13</sup>C can be influenced by other factors in an animal's lifestyle, such as diet and diving behavior.<ref name=Robbins2010 />{{rp|122}}<ref name="REE" /> To account for this, a 2014 study by T. Lynn Harrell Jr. and Alberto Perez-Huerta examined the concentration ratios of [[neodymium]], [[gadolinium]], and [[ytterbium]] in ''M. hoffmannii'' and ''Mosasaurus'' sp. fossils from Alabama, the [[Demopolis Chalk]], and the [[Hornerstown Formation]]. Previous studies demonstrated that ratios of these three elements can act as a proxy for relative ocean depth of a fossil during early [[diagenesis]] without interference from biological processes, with each of the three elements signifying either shallow, deep, or fresh waters. The rare earth element ratios were very consistent throughout most of the examined ''Mosasaurus'' fossils, indicating consistent habitat preference, and clustered towards a ratio representing offshore habitats with ocean depths deeper than {{convert|50|m|ft|sp=us}}.<ref name=REE />

===Interspecific competition===
{{image flip|[[File:Prognathodon saturator DB.jpg|thumb|alt=Restoration of ''[[Prognathodon|Prognathodon saturator]]''|''Mosasaurus'' was able to coexist with other large predatory mosasaurs like ''Prognathodon'' through [[niche partitioning]].]]}}
''Mosasaurus'' lived alongside other large predatory mosasaurs also considered apex predators, most prominent among them being the [[tylosaurine]]s and ''[[Prognathodon]]''.<ref name=LinghamSoliar /><ref name=Schulpetal2013 /> ''Tylosaurus bernardi'', the only surviving species of the genus during the Maastrichtian, measured up to {{convert|12.2|m|ft|sp=us}} in length<ref name=Hainosaurus>{{cite journal|author=Johan Lindgren|year=2005|title=The first record of ''Hainosaurus'' (Reptilia: Mosasauridae) from Sweden|journal=Journal of Paleontology|volume=79|issue=6|pages=1157–1165|doi=10.1111/j.1502-3931.1998.tb00520.x|bibcode=1998Letha..31..308L |s2cid=128711108 |url=http://doc.rero.ch/record/16192/files/PAL_E3401.pdf }}</ref> while the largest coexisting species of ''Prognathodon'' like ''P. saturator'' exceeded {{convert|12|m|ft|sp=us}}.<ref name=Schulpetal2013 /> These three mosasaurs preyed on similar animals such as marine reptiles.<ref name=Konishietal /><ref name=LinghamSoliar /><ref name=Schulpetal2013 />

A study published in 2013 by Schulp and colleagues specifically tested how mosasaurs such as ''M. hoffmannii'' and ''P. saturator'' were able to coexist in the same localities through δ<sup>13</sup>C analysis. The scientists utilized an interpretation that differences in isotope values can help explain the level of resource partitioning because it is influenced by multiple environmental factors such as lifestyle, diet, and habitat preference. Comparisons between the δ<sup>13</sup>C levels in multiple teeth of ''M. hoffmannii'' and ''P. saturator'' from the Maastrichtian-age [[Maastricht Formation]] showed that while there was some convergence between certain specimens, the average δ<sup>13</sup>C values between the two species were on average different. This is one indication of niche partitioning, where the two mosasaur genera likely foraged in different habitats or had different specific diets to coexist without direct competitive conflict. The teeth of ''P. saturator'' are much more robust than those of ''M. hoffmannii'' and were specifically equipped for preying on robust prey like turtles. While ''M. hoffmannii'' also preyed on turtles, its teeth were built to handle a wider range of prey less suited for ''P. saturator''.<ref name=Schulpetal2013 />

Another case of presumed niche partitioning between ''Mosasaurus'' and ''Prognathodon'' from the [[Bearpaw Formation]] in [[Alberta]] was documented in a 2014 study by Konishi and colleagues. The study found a dietary divide between ''M. missouriensis'' and ''P. overtoni'' based on stomach contents. Stomach contents of ''P. overtoni'' included turtles and ammonites, providing another example of a diet specialized for harder prey. In contrast, ''M. missouriensis'' had stomach contents consisting of fish, indicative of a diet specialized in softer prey. It was hypothesized that these adaptations helped maintain resource partitioning between the two mosasaurs.<ref name=Konishietal />

Nevertheless, competitive engagement evidently could not be entirely avoided. There is also evidence of aggressive interspecific combat between ''Mosasaurus'' and other large mosasaur species. This is shown from a fossil skull of a subadult ''M. hoffmannii'' with fractures caused by a massive concentrated blow to the braincase; Lingham-Soliar (1998) argued that this blow was dealt by a ramming attack by ''T. bernardi'', as the formation of the fractures were characteristic of a coordinated strike (and not an accident or fossilization damage), and ''T. bernardi'' was the only known coexisting animal likely capable of causing such damage, using its robust arrow-like elongated snout. This sort of attack has been compared to the defensive behavior of [[bottlenose dolphin]]s using their beaks to kill or repel [[lemon shark]]s, and it has been speculated that ''T. bernardi'' dealt the offensive attack via an ambush on an unsuspecting ''Mosasaurus''.<ref name=LinghamSoliar1998>{{cite journal|author=Theagarten Lingham-Soliar|title=Unusual death of a Cretaceous giant|journal=Lethaia|year=1998|volume=31|issue=4|pages=308–310|doi=10.1111/j.1502-3931.1998.tb00520.x|bibcode=1998Letha..31..308L |s2cid=128711108 |url=http://doc.rero.ch/record/16192/files/PAL_E3401.pdf }}</ref>

==Extinction==
[[File:KT boundary 054.jpg|thumb|left|alt=An exposure of the [[Cretaceous–Paleogene boundary]]|''Mosasaurus'' went extinct as a result of the [[Cretaceous–Paleogene extinction event|K-Pg extinction event]]; its last fossils were found at or close to the [[Cretaceous–Paleogene boundary|boundary]], which is represented by the thick dark band separating the lighter and darker layers of this cliff.]]
By the end of the Cretaceous, mosasaurs were at the height of their [[Adaptive radiation|evolutionary radiation]], and their extinction was a sudden event.<ref name=LinghamSoliar /> During the late Maastrichtian, global sea levels dropped, draining the continents of their nutrient-rich seaways and altering circulation and nutrient patterns, and reducing the number of available habitats for ''Mosasaurus''. The genus adapted by accessing new habitats in more open waters.<ref name=Gallagheretal2005>{{cite journal|author1=William B. Gallagher|author2=Carl E. Campbell|author3=John W. M. Jagt|author4=Eric W. A. Mulder|title=Mosasaur (Reptilia, Squamata) material from the Cretaceous-Tertiary boundary interval in Missouri|year=2005|journal=Journal of Vertebrate Paleontology|volume=25|issue=22|pages=473–475|doi=10.1671/0272-4634(2005)025[0473:mrsmft]2.0.co;2|s2cid=130130952|url=https://www.researchgate.net/publication/232667934}}</ref><ref name=Lecture38>{{cite web|author=Thomas R. Holtz| title=GEOL 104 Lecture 38: The Cretaceous-Tertiary Extinction III: Not With a Bang, But a Whimper|website=University of Maryland Department of Geology|year=2006|url=https://www.geol.umd.edu/~tholtz/G104/10438kt3.htm|archive-url=https://web.archive.org/web/20120313102328/https://www.geol.umd.edu/~tholtz/G104/10438kt3.htm|archive-date=March 13, 2012}}</ref> The last fossils of ''Mosasaurus'', which include those of ''M. hoffmannii'' and indeterminate species, occur up to the [[Cretaceous-Paleogene boundary]] (K-Pg boundary). The demise of the genus was likely a result of the [[Cretaceous-Paleogene extinction event]] which also wiped out the non-avian dinosaurs. ''Mosasaurus'' fossils have been found less than {{convert|15|m|ft|sp=us}} below the boundary in the Maastricht Formation, the [[Davutlar Formation]] in Turkey, the [[Jagüel Formation]] in Argentina, [[Stevns Klint]] in Denmark, Seymour Island, and Missouri.<ref name=Jagtetal2008>{{cite journal|author1=John W. Jagt|author2=Dirk Cornelissen|author3=Eric W. Mulder|author4=Anne S. Schulp|author5=Jacques Severinjns|author6=Louis Verding|title= The youngest ''in situ'' record to date of ''Mosasaurus hoffmannii'' (Squamata, Mosasauridae) from the Maastrichtian type area, The Netherlands|year=2008|journal=Proceedings of the Second Mosasaur Meeting|pages=73–80|url=https://www.researchgate.net/publication/270103474}}</ref>

''M. hoffmannii'' fossils have been found within the K-Pg boundary itself in southeastern Missouri between the [[Paleocene]] [[Clayton Formation]] and Cretaceous [[Owl Creek Formation]]. Fossil vertebrae from the layer were found with fractures formed after death. The layer was likely deposited as a [[Tsunami deposit|tsunamite]], alternatively nicknamed the "Cretaceous cocktail deposit". This formed through a combination of catastrophic seismic and geological disturbances, mega-hurricanes, and giant tsunamis caused by the impact of the [[Chicxulub impactor|Chicxulub asteroid]] that catalyzed the K-Pg extinction event.<ref name=Gallagheretal2005 /> As well as physical destruction, the impact also blocked out sunlight<ref name=kaiho2016>{{cite journal |author1=Kunio Kaiho|author2=Naga Oshima|author3=Kouji Adachi|author4=Yukimasa Adachi|author5=Takuya Mizukami|author6=Megumu Fujibayashi|author7=Ryosuke Saito|year=2016 |title=Global climate change driven by soot at the K-Pg boundary as the cause of the mass extinction |journal=Scientific Reports |volume=6 |issue=1 |pages=1–13 |doi=10.1038/srep28427|pmid=27414998 |pmc=4944614 |bibcode=2016NatSR...628427K}}</ref> leading to a collapse of marine food webs.<ref name=Gallagheretal2005 /> Any ''Mosasaurus'' surviving the immediate cataclysms by taking refuge in deeper waters would have died out due to starvation from a loss of prey.<ref name=Gallagheretal2005 />

One enigmatic occurrence of ''Mosasaurus'' sp. fossils is in the Hornerstown Formation, a deposit typically dated to be from the Paleocene [[Danian]] age, which was immediately after the Maastrichtian age. The fossils were found in association with fossils of ''Squalicorax'', ''Enchodus'', and various ammonites within a uniquely fossil-rich bed at the base of the Hornerstown Formation known as the Main Fossiliferous Layer. This does not mean ''Mosasaurus'' and its associated fauna survived the K-Pg extinction. According to one hypothesis, the fossils may have originated from an earlier Cretaceous deposit and were [[Fossil#Derived, or reworked|reworked]] into the Paleocene formation during its early deposition. Evidence of reworking typically comes from fossils worn down due to further erosion during their exposure at the time of redeposition. Many of the ''Mosasaurus'' fossils from the Main Fossiliferous Layer consist of isolated bones commonly abraded and worn, but the layer also yielded better-preserved ''Mosasaurus'' remains. Another explanation suggests the Main Fossiliferous Layer is a Maastrichtian time-averaged [[zombie taxon|remanié]] deposit, which means it originated from a Cretaceous deposit with [[Winnowing (sedimentology)|winnowed]] low-sediment conditions. A third hypothesis proposes that the layer is a [[lag deposit]] of Cretaceous sediments forced out by a strong impact by a tsunami, and what remained was subsequently refilled with Cenozoic fossils.<ref name=Gallagher2005 />

== See also ==
{{Portal|Paleontology}}
* ''[[Plotosaurus]]''
* ''[[Eremiasaurus]]''
* ''[[Moanasaurus]]''

==Notes==
{{notelist}}

==References==
{{Reflist}}

==External links==
{{Commons category|Mosasaurus}}
* {{Commons category-inline|Mosasaurus|''Mosasaurus''}}
* {{Wikispecies-inline|Mosasaurus|''Mosasaurus''}}
*[http://www.oceansofkansas.com Oceans of Kansas]

{{Mosasauridae}}
{{Taxonbar|from=Q312131}}

[[Category:Mosasaurines]]
[[Category:Mosasaurs of North America]]
[[Category:Mosasaurs of Europe]]
[[Category:Fossil taxa described in 1822]]
[[Category:Taxa named by William Conybeare]]
[[Category:Demopolis Chalk]]
[[Category:Mooreville Chalk]]
[[Category:Apex predators]]
[[Category:Fossils of the Netherlands]]
[[Category:Fossils of Belgium]]