View source for Segnosaurus

{{short description|Extinct genus of therizinosaurid dinosaur from late Cretaceous}}
{{Use American English|date=January 2020}}
{{Use mdy dates|date=January 2020}}
{{speciesbox
| fossil_range = [[Late Cretaceous]], {{fossilrange|90|earliest=102|latest=86}}
| image = Segnosaurus skeletal.png
| image_caption = Diagram showing known remains
| genus = Segnosaurus
| parent_authority = [[Altangerel Perle|Perle]], [[1979 in paleontology|1979]]
| species = galbinensis
| authority = Perle, 1979
}}

'''''Segnosaurus''''' is a [[genus]] of [[therizinosaurid]] [[dinosaur]] that lived in what is now southeastern Mongolia during the [[Late Cretaceous]], about 102–86&nbsp;million years ago. Multiple incomplete but well-preserved specimens were discovered in the [[Gobi Desert]] in the 1970s, and in 1979 the genus and species '''''Segnosaurus galbinensis''''' were named. The generic name ''Segnosaurus'' means "slow lizard" and the specific name ''galbinensis'' refers to the Galbin region. The known material of this dinosaur includes the lower jaw, neck and tail vertebrae, the pelvis, shoulder girdle, and limb bones. Parts of the specimens have gone missing or become damaged since they were collected.

''Segnosaurus'' was a large-bodied [[therizinosaur]] that is estimated to have been about {{convert|6–7|m|ft|abbr=on}} long and to have weighed about {{convert|1.3|MT|ST|abbr=on}}. It would have been bipedal, with the trunk of its body tilted upwards. The head was small with a beak at the tip of the jaws, and the neck was long and slender. The lower jaw was down-turned at the front and the teeth were distinct in having additional {{Dinogloss|denticles}} as well as third cutting edges in some of the hindmost teeth. The forelimbs were robust and had three fingers which bore large claws, and the feet had four toes supporting the foot—apart from therizinosaurs, all [[theropods]] had three-toed feet. The front of the pelvis was adapted to support the enlarged belly. The [[pubic bone]] was turned backwards, a feature that is only seen in birds and the dinosaurs most closely related to them.

The affinities of ''Segnosaurus'' were originally obscure and it received its own theropod family, Segnosauridae, and later when related genera were identified, an [[infraorder]], Segnosauria. Alternative classification schemes were proposed until more complete relatives were described in the 1990s, which confirmed them as theropods. The new fossils also showed Segnosauridae was a [[junior synonym]] of the earlier named family Therizinosauridae. ''Segnosaurus'' and its relatives are thought to have been slow-moving animals that, as indicated by their unusual features, were mainly herbivorous, whereas most other theropod groups were carnivorous. Therizinosaurs probably used their long forelimbs, long necks, and beaks when [[Browsing (herbivory)|browsing]], and large guts for processing food. ''Segnosaurus'' is known from the [[Bayan Shireh Formation]], where it lived alongside the fellow therizinosaurs ''[[Erlikosaurus]]'' and ''[[Enigmosaurus]]''; these related genera were probably [[niche partitioned]].

==History of discovery==
[[File:Cretaceous-aged dinosaur fossil localities of Mongolia.PNG|thumb|left|upright=1.4|[[Cretaceous]]-aged dinosaur fossil localities of Mongolia; ''Segnosaurus'' was found by areas C and D (right, Amtgay and Khara-Khutul localities).]]

In 1973, a joint [[Soviet]]–Mongolian expedition investigating the [[Bayan Shireh Formation]] at the Amtgay locality in the [[Gobi Desert]] of southeastern Mongolia discovered fossils that included the partial skeleton of an unknown [[dinosaur]]. Through 1974 and 1975, more remains were uncovered at the Amtgay and Khara-Khutul localities; though the skeletons were incomplete, the recovered bones were well-preserved. Other localities listed in the literature include Bayshin-Tsav and Urilbe-Khuduk. These fossils were scientifically [[species description|described]] in 1979 by the paleontologist [[Altangerel Perle]], who named the new [[genus]] and species ''Segnosaurus galbiensis''. The generic name is derived from the Latin word ''segnis'' ("slow") and the [[Ancient Greek]] ''sauros'' ("lizard"). The [[specific name (zoology)|specific name]] refers to the Galbin region of the Gobi Desert.<ref name="Perle1979">{{Cite journal |last=Perle |first=A. |author-link=Altangerel Perle |year=1979 |title=Segnosauridae – novoe semeistvo teropod is posdnego mela Mongolii |trans-title=Segnosauridae – a new family of Theropoda from the Lower Cretaceous of Mongolia |url=http://paleoglot.org/files/Perle%2079.pdf |url-status=live |journal=Trudy – Sovmestnaya Sovetsko-Mongol'skaya Paleontologicheskaya Ekspeditsiya |language=Russian |volume=8 |pages=45–55 |archive-url=https://web.archive.org/web/20121128221548/http://paleoglot.org/files/Perle%2079.pdf |archive-date=2012-11-28 |translator-last=Siskron |translator-first=C. |translator2-last=Welles |translator2-first=S. P.}}</ref><ref name="DFG97b">{{Cite book |title=Dinosaurs: The Encyclopedia |last=Glut |first=D. F. |publisher=McFarland & Co |year=1997 |isbn=978-0-89950-917-4 |location=Jefferson |pages=806–807}}</ref><ref name="barsbold1983"/>

The [[holotype specimen]] from the Amtgay locality is housed at the [[Mongolian Academy of Sciences]] under the specimen number IGM 100/80 (Mongolian Institute of Geology, formerly GIN). It includes the {{Dinogloss|mandible}} (lower jaws), an incomplete {{Dinogloss|humerus}}, a complete {{Dinogloss|radius}} and {{Dinogloss|ulna}} (lower arm bones), {{Dinogloss|phalanges}} of the fingers, a forelimb {{Dinogloss|ungual}} (claw bone), an almost-complete {{Dinogloss|pelvis}}, an incomplete right {{Dinogloss|femur}}, six {{Dinogloss|sacral vertebrae}}, ten {{Dinogloss|caudal vertebrae}} from the front of the tail, fifteen from the hindmost part of the tail, the first {{Dinogloss|gastral rib}}, and fragments of the dorsal ribs. Two more specimens were designated as [[paratype specimens]]; specimen IGM 100/82 from the Khara Khutul locality includes a femur, {{Dinogloss|tibia}} and {{Dinogloss|fibula}} (leg bones), {{Dinogloss|tarsals}} and {{Dinogloss|metatarsals}}, five toe phalanges including a foot ungual, rib fragments, complete {{Dinogloss|ilia}}, the upper portion of an {{Dinogloss|ischium}}, and the lower portion of a {{Dinogloss|pubis}}. Specimen IGM 100/83 includes a left {{Dinogloss|scapulocoracoid}} (shoulder girdle), a radius, an ulna, forelimb unguals, and a fragment of a {{Dinogloss|cervical}} (neck) vertebra.<ref name="Perle1979" /><ref name=Zanno2010/> In 1980, Perle and the paleontologist [[Rinchen Barsbold]] assigned another specimen to ''Segnosaurus''; IGM 100/81 from the Amtgay locality included a left tibia and fibula.<ref name="perle&barsbold80">{{Cite journal |last1=Barsbold |first1=R. |last2=Perle |first2=A. |author2-link=Altangerel Perle |year=1980 |title=Segnosauria, a new infraorder of carnivorous dinosaurs |url=https://www.app.pan.pl/article/item/app25-187.html |url-status=live |journal=Acta Palaeontologica Polonica |volume=25 |issue=2 |pages=187–195 |archive-url=https://web.archive.org/web/20190921004254/https://www.app.pan.pl/article/item/app25-187.html |archive-date=2019-09-21}}</ref><ref name=Zanno2010/>

In 1983, Barsbold listed additional specimens GIN 100/87 and 100/88. In 2010, however, the paleontologist [[Lindsay E. Zanno]] suggested these may refer to paratypes IGM 100/82 and IGM 100/83 (which had already been listed in 1979) because the Russian-to-English translation of Barsbold's article has several typographical errors in regard to specimen numbers. Zanno also noted that by the time of her study, there were numerous problems with the ''Segnosaurus'' IGM specimens, including damage caused since collection, the disappearance of elements of the holotype, incorrect identification of assigned elements, and more than one individual bearing the same specimen number. Holotype elements Zanno was able to access in 2010 included a severely damaged ilium, a sacrum missing the left sacral ribs with damage so it could not conjoin well with the rest of the ilium, and a pubic bone and ischium missing their upper portions. More bones bearing the specimen number IGM 100/82 were located but were not mentioned in Perle's description, while the whereabouts of some paratype elements was unknown.<ref name="barsbold1983">{{Cite journal |last=Barsbold |first=R. |year=1983 |title=Khishchnye dinosavry mela Mongoliy|trans-title=Carnivorous dinosaurs from the Cretaceous of Mongolia |url=https://paleoglot.org/files/Barsbold_83.pdf |url-status=live |journal=Transactions of the Joint Soviet-Mongolian Paleontological Expedition |volume=19 |pages=51–52, 76, 108–109 |archive-url=https://web.archive.org/web/20160313031010/http://paleoglot.org/files/barsbold_83.pdf |archive-date=2016-03-13 |access-date=2019-09-21|language=Russian|translator-last=Siskron |translator-first=C. |translator2-last=Welles |translator2-first=S. P.}}</ref><ref name=Zanno2010/> In a 2016 re-description of the holotype mandible, which had been little studied since 1979, Zanno and colleagues reported the majority of the tooth crowns had been damaged after collection, and most of them were missing their tips. Of the two {{Dinogloss|hemimandibles}} (halves of the lower jaw), the right is nearly complete; only the hindmost part and the upper front of its [[mandibular symphysis]] (the area where the halves of the mandible meet) was missing. The left hemimandible is fragmented and preserves the front part with some displacement of bone due to crushing.<ref name="Zanno2016"/>

==Description==
[[File:Segnosaurus Scale.svg|thumb|Size compared to a human]]
''Segnosaurus'' was a large-bodied [[therizinosaur]] that is estimated to have been about {{convert|6–7|m|ft|abbr=on}} long and to have weighed about {{convert|1.3|MT|ST|abbr=on}}.<ref name="paul2016"/><ref name="holtz">{{cite book|last=Holtz|first=T. R. Jr.|year=2012|title=Dinosaurs: The Most Complete, Up-to-date Encyclopedia for Dinosaur Lovers of All Ages|publisher=Random House|page=[https://archive.org/details/dinosaursmostcom00holt/page/382 382]|isbn=978-0-375-82419-7|location=New York|url=https://archive.org/details/dinosaursmostcom00holt/page/382}}</ref><ref name="Zanno2016"/> Campione & Evans in 2020, however, calculated its body mass at {{convert|4.17|MT|ST}}.<ref>{{Cite journal|last1=Campione|first1=Nicolás E.|last2=Evans|first2=David C.|date=2020|title=The accuracy and precision of body mass estimation in non-avian dinosaurs|journal=Biological Reviews|language=en|volume=95|issue=6|pages=1759–1797|doi=10.1111/brv.12638|pmid=32869488|s2cid=221404013|issn=1469-185X|doi-access=free}}</ref> ''Segnosaurus'' is incompletely known, but as a [[therizinosaurid]], it would have been bipedal and robustly built with the trunk of the body tilted upwards compared to other [[theropods]]. The head would have been small with a {{Dinogloss|rhamphotheca}} (horny beak) at the tip of the jaws, and a long, slender neck. The fingers were not particularly long, but bore large claws. The front of the pelvis was adapted to support the enlarged belly.<ref name="paul2016">{{Cite book|title=The Princeton Field Guide to Dinosaurs |last=Paul |first=G.S. |publisher=Princeton University Press |year=2016 |isbn=978-0-691-16766-4 |edition=2 |location=Princeton |pages=162, 166–167}}</ref><ref name="Zanno2016"/><ref name="Perle1979" /> Therizinosaurs are known to have had simple, [[primitive feathers]] as evidenced by fossils of the [[Basal (phylogenetics)|basal]] (or "primitive") genera ''[[Beipiaosaurus]]''—the second-known non-bird dinosaur preserved with such [[integuments]] after ''[[Sinosauropteryx]]''—and ''[[Jianchangosaurus]]''.<ref name="Beipiaosaurus">{{Cite journal |last1=Xu |first1=X. |last2=Tang |first2=Z. |last3=Wang |first3=X. |date=1999 |title=A therizinosauroid dinosaur with integumentary structures from China |journal=Nature |volume=399 |issue=6734 |pages=350–354 |bibcode=1999Natur.399..350X |doi=10.1038/20670|s2cid=204993327 }}</ref><ref name="Hendrickx">{{cite journal | last1 =  Hendrickx | first1 =  C. | last2 =  Hartman | first2 =  S. A. | last3 =  Mateus | first3 =  O. | title =  An overview on non-avian theropod discoveries and classification | journal =  PalArch's Journal of Vertebrate Palaeontology | volume =  12 | issue =  1 | url =  http://www.palarch.nl/2015/08/christophe-hendrickx-scott-a-hartman-octavio-mateus-2015-an-overview-of-non-avian-theropod-discoveries-and-classification-palarchs-journal-of-vertebrate-palaeontology-12-1-2015/ | pages =  27–29 | year =  2015 | archive-url =  https://web.archive.org/web/20180622060300/http://www.palarch.nl/2015/08/christophe-hendrickx-scott-a-hartman-octavio-mateus-2015-an-overview-of-non-avian-theropod-discoveries-and-classification-palarchs-journal-of-vertebrate-palaeontology-12-1-2015/ | archive-date =  2018-06-22 | url-status =  live}}</ref> Since most therizinosaurs are incompletely known, it is uncertain how many of the anatomical features that are used to distinguish ''Segnosaurus'' are widespread among the group; many genera cannot be directly compared because the equivalent bones are not preserved.<ref name=Zanno2010/><ref name="Zanno2016"/>

===Mandible and lower dentition===
The mandible of ''Segnosaurus'' was low and elongated, yet relatively robust and shapeless compared to that of ''[[Erlikosaurus]]'', which was more [[Gracility|gracile]]. The nearly complete right hemimandible (half of the mandible) is {{convert|379|mm|in|abbr=on}} long from front to back, {{convert|55.5|mm|in|abbr=on}} at the highest point, and {{convert|24.5|mm|in|abbr=on}} at the lowest. The {{Dinogloss|dentary bone}}, the tooth-bearing bone forming most of the mandible's front part, was complex in shape compared to those of early therizinosaurs. The tooth-bearing part was almost rectangular and sloped downwards in side view with a pronounced arc throughout the upper length of the front end—more extreme than what is known in other therizinosaurs. The front-most part of the dentary was strongly deflected downwards at about a 30-degree angle, a unique feature for this genus. When each hemimandible is articulated with the other, they form a broadly U-shaped, toothless mandibular symphysis that projects upwards towards the front as in ''Erlikosaurus'' and ''[[Neimongosaurus]]''. The expansive, toothless front region of the dentary spans {{convert|25.5|mm|in|abbr=on}} on the right hemimandible of the holotype. Proportionally, the toothless part of the dentary is 20% of its tooth row, which is {{convert|150.3|mm|in|abbr=on}} long. By comparison, the toothless region of ''Erlikosaurus'' was about 12% of the tooth row's length and was almost absent in ''Jianchangosaurus''. The height of the dentary diminished towards the hindmost extend of the tooth row, whereafter it sharply fanned out to contact the {{Dinogloss|surangular}} bone behind it; by contrast, the hind part of the dentary in ''Erlikosaurus'' gradually approached the surangular in a gentle arc.<ref name="Zanno2016">{{cite journal|last1=Zanno|first1=L. E.|last2=Tsogtbaatar|first2=K.|last3=Chinzorig|first3=T.|last4=Gates|first4=T. A.|author1-link=Lindsay Zanno|year=2016|title=Specializations of the mandibular anatomy and dentition of ''Segnosaurus galbinensis'' (Theropoda: Therizinosauria)|journal=PeerJ|volume=4|at=e1885|doi=10.7717/peerj.1885|pmid=27069815|pmc=4824891 |doi-access=free }}</ref><ref name="Perle1979" />

[[File:Segnosaurus hemimandible.jpg|thumb|left|Right half of the holotype mandible in outer and inner view, with component bones marked by different colors; the {{Dinogloss|dentary bone}} (green) bore the teeth.]]

''Segnosaurus'' was distinct among therizinosaurs in that the hindmost part of the dentary was toothless. The teeth were restricted to the front two-thirds of the dentary, which bore 24 {{Dinogloss|alveoli}} (tooth sockets) in a manner similar to ''Jianchangosaurus'' but different from ''Erlikosaurus'', in which nearly the entire dentary was toothed, bearing 31 alveoli. The tooth row of ''Segnosaurus'' was inset and demarcated by a shelf on the outer side as it was in all derived (or "advanced") therizinosaurs. Unlike in other related taxa, the shelf was restricted to the hind part of the dentary and the raised rim that defined it was not as pronounced. ''Segnosaurus'' was unique in having a low ridge rising between the fifth and fourteenth alveoli that divided the dentary into two almost-equally sized front and hind parts. Just above this ridge, the dentary was pierced by a row of {{Dinogloss|foramina}} as in ''Jianchangosaurus'' and ''[[Alxasaurus]]'', which became less regular by the region around the mandibular symphysis, where the two halves of the mandible met at the front. This row was instead directly in line with and on the side of the ridge in ''Erlikosaurus''. The {{Dinogloss|Meckelian groove}} that ran along the inner side of the mandible, was placed further down than in ''Erlikosaurus'' and had a consistent depth until the thirteenth tooth position, whereafter it widened. The lower jaw elements behind the dentary (the {{Dinogloss|splenial}}, surangular, {{Dinogloss|angular}}, and {{Dinogloss|prearticular}} bones) were distinct from those of other therizinosaurs, being gracile and linear, and contributing to the hind part of the hemimandible being elongate and almost rectangular.<ref name="Zanno2016"/> The surangular was long and sword-shaped, the angular was wing-like in shape, the prearticular was narrow and curved, and the splenial was thin and triangular in outline. The external mandibular fenestra, an opening at the outer side of the mandible, was larger than that of ''Erlikosaurus'' because the surangular was shallow from top to bottom.<ref name="Perle1979" />

[[File:Mesial dentary teeth of Segnosaurus.png|thumb|upright|Frontmost dentary teeth, showing folded {{Dinogloss|carinae}} (lf) and accessory {{Dinogloss|denticles}} (ad)]]
''Segnosaurus'' had the fewest teeth in the dentary; 24 in each half determined from the number of sockets, as well as the largest teeth known among therizinosaurs. The dentary teeth were foliodont (leaf-shaped) and bore enlarged, relatively tall, sideways compressed crowns with a slight recurvature at the upper margin of the tips. By comparison, the teeth of ''Erlikosaurus'' were smaller, symmetrical, and simpler. The bases of the crowns increased slightly in size hindwards across the tooth row, which reflected a decrease in sideways compression. The front surfaces of the crowns and outward-facing sides were convex while the inward-facing sides were concave. A thickened ridge ran along the longitude of the inward-facing side near the upper half of the crown, which was flanked by weak grooves near the front and back edges of the teeth, reaching almost to the cervix (neck; the transition between the crown and root) of the teeth. In general, the 18 front-most teeth were relatively homodont (of the same type), though the crown of the second tooth was relatively shorter and more tapered; this may also have been true for the first tooth, but it was not preserved. The teeth further back in the row also decreased in relative height hindwards. By comparison, the front four to five dentary teeth of ''Erlikosaurus'' were conidont (cone-shaped) with a gradual transition to foliodont teeth.<ref name="Zanno2016"/><ref name="Perle1979" />

The dentary teeth were tightly packed, but not pressed closely together, with the tooth crowns approaching each other at mid-length. The {{Dinogloss|denticles}} (serrations) were large and bulbous, diminishing slightly in size towards the tooth tips, with about 5–6 denticles per {{convert|3|mm|in|abbr=on}}. The front {{Dinogloss|carinae}} (cutting edges) folded upwards to overlap the inner surface of the crowns on the third to eighteenth teeth, but such folds were absent on the second and probably first crowns. The denticles were roughly perpendicular with the tip of the tooth crowns but parallel to the crown height on the front side fold and triangular facet on the hind side. There was a series of accessory denticles (in addition to those on the carinae) that projected from the front surfaces of the carinal folds, which made the front edges of the crowns more broadly roughened. The carinae of the hind edges were also very modified, and bifurcated (split in two) near the cervix, where they formed a flattened triangular, raised facet, which projected from the tooth crown and contacted or approached the folded carinae on the front edge of the crowns behind them (this arrangement is present in teeth 2–12). Such split carinae are known from other [[tetanuran]] theropods, where they are considered abnormalities caused by trauma, aberrant [[tooth replacement]], or genetic factors. Though the condition in ''Segnosaurus'' was similar, it was uniformly expressed across the teeth of both dentaries, and does not appear to have been an abnormality, but served to roughen the contacts between tooth bases.<ref name="Zanno2016"/>

The 22nd and 23rd dentary teeth of ''Segnosaurus'' were significantly smaller than the rest, almost conidont, and had an additional third carina with denticles on their inner sides. Most of the other hindmost tooth crowns are damaged so their complete features are unknown. The additional carina on tooth 23 appears to have been fully denticulated while the denticles were restricted to the basal side of the crown in tooth 22. ''Segnosaurus'' was unique among all known theropods in possessing triple carinae. The 14th alveolus on the right dentary of the holotype is walled over by seemingly [[pathological]] (due to injury or disease) bone growth but the teeth in that part of the dentary are damaged so it is not possible to determine how the teeth were affected by this. The teeth in the same area of the left dentary bear triple carinae, though this dentary has no external indications of pathology that could have led to this condition, thus it cannot be concluded nor ruled out that this feature is the result of a pathology. ''Segnosaurus'' [[Polyphyodont|replaced its teeth]] in waves running from back to front of the jaws, that encompassed two to three erupting crowns. Some of the fully erupted teeth have wear on the carinae of their hind sides, unlike what is seen in other therizinosaurs. The texture of the {{Dinogloss|enamel}} appears to have been broadly irregular and the roots of the teeth were almost circular.<ref name="Zanno2016"/>

===Postcranial skeleton===
[[File:Segno.jpg|thumb|[[paleoart|Life restoration]]]]

The {{Dinogloss|scapula}} (shoulder blade) of ''Segnosaurus'' was straight and flat at the upper end, and was fused to the coracoid bone, forming the scapulocoracoid. The coracoid was very wide, rectangular in outline and thick at the middle. The massive humerus was {{convert|560|mm|in|abbr=on}} in length; it had an almost-cylindrical shaft and well-defined condyles for articulation with the radius and ulna of the lower arm. The deltopectoral crest, where the [[deltoid muscle]] was attached to the upper front of the humerus, was well-developed.<ref name=Perle1979/> The humerus was distinct from those of other therizinosaurs, being straight rather than [[sigmoid shaped]] and not expanded or deflected forwards at its upper end. The humerus was also not expanded at the middle, and the [[entepicondyle]] was not well-developed. The lack of these features was more similar to [[ornithomimosaurs]] and [[troodontids]] than to other therizinosaurs.<ref name=Zanno2010/> The radius was also massive—about 60 percent of the humerus—with a straight shaft. The ulna was thicker than the radius and slightly longer—about 70 percent of the humerus—and slightly twisted along its middle axis. The hand was [[tridactyl]] (three-fingered). The {{Dinogloss|phalanx bones}} of the fingers were flattened from top to bottom and the articular depressions on their sides were not very developed. The first phalanx of the first finger was long and thin while the first and second phalanxes of the second finger were short. The ungual of the third finger was somewhat longer than the second phalanx and quite flat from top to bottom, which may have been a unique feature of ''Segnosaurus''. This ungual was sharpy curved, very pointed, and compressed from side to side. The lower tubercle, where the flexor tendons attached to the ungual, was thick and robust.<ref name=Perle1979/><ref name=Zanno2010/><ref name="perle&barsbold80" /><ref name="Dinosauria2">{{cite book| last1 = Clark | first1 = J. M. |last2=Maryańska |first2=T.|last3=Barsbold |first3=R. |year= 2004 | pages =151–164 |title= The Dinosauria |publisher= University of California Press |isbn= 978-0-520-24209-8|location= Berkeley|chapter=Therizinosauroidea|edition= 2 | editor1 =Weishampel, D. B.| editor2 =Dodson, P.| editor3 =Osmolska, H.}}</ref>

[[File:Segnosaurus holotype.png|left|thumb|Reconstructed [[holotype]] pelvis in left side view and {{Dinogloss|metatarsus}} in top view]]
The pelvis of ''Segnosaurus'' was robust and had sharply sideways-directed lobes at the front. The pelvis was shortened at the front, a feature found among bird-like theropods but uncommon among theropods as a whole.<ref name=Perle1979/> The pubic bone was directed backwards and down in parallel with the ischium; this backwards orientation of the pubic bone is known as the {{Dinogloss|opisthopubic}} condition. This feature is only known from birds and their closest [[coelurosaurian]] relatives while other theropod dinosaurs had forwards-directed pubic bones.<ref name="Opisthopubic">{{cite journal |last1=Barsbold |first1=R. |title=Opisthopubic pelvis in the carnivorous dinosaurs |journal=Nature |date=1979 |volume=279 |issue=5716 |pages=792–793 |doi=10.1038/279792a0|bibcode=1979Natur.279..792B |s2cid=4348297 }}</ref><ref name="Dinosauria2"/> The pubic bone was elongated, flattened sideways, and had an ellipsoid projection or "boot" at the front of its lower end.<ref name="perle&barsbold80" /><ref name="Dinosauria2"/> The pelvis was distinct from those of other therizinosaurs in that the upper margin of the ilium had a pronounced overhang on the lower side and that the hindwards projecting process of the ischium was extensive, almost 50 percent of the front-to-back length of the {{Dinogloss|obturator process}}. Some features of the pelvis were similar to that of ''[[Nothronychus]]'', particularly the ischia, but it is uncertain whether these similarities were due to them having a common ancestor to the exclusion of other derived therizinosaurids, or because they retained basal features since lost in other relatives. The ischium of ''Segnosaurus'' was distinct from that of ''Nothronychus'' in that it had an almost-rectangular obturator process and an almost-circular obturator foramen. The pelvis was distinct from that of ''[[Enigmosaurus]]'' by its deep obturator process not fusing with its counterpart at the middle, by its unfused pubic boot, and because the lower part of the pubic shaft was wide from front to back. ''Segnosaurus'' was distinct from both ''Nothronychus'' and ''Enigmosaurus'' in having a deep {{Dinogloss|brevis fossa}} (a groove where the [[caudofemoralis brevis muscle]] of the tail originated) and because its pubic boot lacked a well developed hindwards projection.<ref name=Zanno2010/>

The femur was straight with an oval cross-section and was {{convert|840|mm|in|abbr=on}} in length. The head of the femur was placed on a long "neck" and the lower condyles were well-defined. The tibia was straight, slightly shorter than the femur, and twisted along its axis. The fibula was long and narrowed towards its lower end. The metatarsus of the foot was short, massive, and consisted of five bones—four of which functioned as support elements and terminated in four toes. Functionally tetradactyl (four-toed) feet were unique to derived therizinosaurs; basal therizinosaurs and all other theropods had tridactyl feet in which the first toe was short and did not reach the ground. Externally, the metatarsus was similar to, though proportionally larger than, those of [[prosauropods]], an early [[evolutionary grade]] of [[sauropodomorphs]]. The epiphyses on the upper metatarsals were hypertrophied (enlarged), a distinctive feature of the genus. The first toe was shorter than the others but was of equal functional importance; the second and third toes were equally long while the fourth was thinnest. The toe ungual was robust, sharply curved, flat at the side, and more pointed than those of prosauropods. The lower tubercle where the flexor ligaments attached was robust. While the lack of strong compression of the toe unguals distinguished ''Segnosaurus'' from ''Erlikosaurus'' from the same formation, the lack of compression was common among therizinosaurs and therefore not unique to ''Segnosaurus''.<ref name=Perle1979/><ref name=Zanno2010/><ref name="perle&barsbold80" /> The cervical vertebrae were platycoelous and had large, massive centra (bodies) and low neural arches. The sacrum consisted of six, firmly fused vertebrae; the centra of these vertebrae were broadened and relatively elongated, and each centrum was slightly longer than their width. The neural spines here were not very long but surpassed the level of the ilia. The caudal (tail) vertebrae closest to the body were massive, high, and somewhat compressed from side to side. The neural arch was low with a small neural canal. The caudal vertebrae closer to the tip of the tail were platycoelous and had short, massive centra. The transverse processes of the caudal vertebrae and the ribs were robust and elongated.<ref name=Perle1979/><ref name="perle&barsbold80" />

==Classification==
[[File:Erlikosaurus skull and foot.jpg|thumb|Skull and foot bones of ''[[Erlikosaurus]]'', which together with ''Segnosaurus'' (both from Mongolia) became the basis of the new [[infraorder]] Segnosauria; this group is now a [[junior synonym]] of [[Therizinosauria]].]]

''Segnosaurus'' and its relatives, which are now recognized as therizinosaurs ("scythe reptiles"), were long considered an enigmatic group. Their mosaic of features resembling those of different dinosaur groups and the scarcity of their fossils led to controversy over their evolutionary relationships for decades after their initial discovery (the forelimb elements of ''[[Therizinosaurus]]'' itself were originally identified as belonging to a giant turtle when described in 1954).<ref name="Zanno2006">{{cite journal |last=Zanno |first=L. E. |author-link=Lindsay Zanno |title=The pectoral girdle and forelimb of the primitive therizinosauroid ''Falcarius utahensis'' (Theropoda, Maniraptora): analyzing evolutionary trends within Therizinosauroidea |journal=Journal of Vertebrate Paleontology |date=2006 |volume=26 |issue=3 |page=636 |doi=10.1671/0272-4634(2006)26[636:TPGAFO]2.0.CO;2|s2cid=86166623 }}</ref><ref>{{cite book|last1=Holtz|first1=T. R.|chapter=Theropods|editor-last1=Farlow|editor-first1=J. O.|editor-last2=Brett-Surman|editor-first2=M. K.|title=The Complete Dinosaur|date=2000|publisher=Indiana University Press|location=Bloomington|isbn=978-0-253-21313-6|page=404}}</ref><ref name="Dinosauria2"/> In 1979, Perle noted the ''Segnosaurus'' fossils were possibly representative of a new family of dinosaurs, which he named Segnosauridae, ''Segnosaurus'' being the [[type genus]] and sole member. He tentatively classified Segnosauridae as theropods, traditionally thought of as the "meat-eating" dinosaurs, pointing to similarities in the mandible and its front teeth. Using features of their humeri and hand claws, he distinguished Segnosauridae from the theropod families [[Deinocheiridae]] and Therizinosauridae, which were then only known from the genera ''[[Deinocheirus]]'' and ''Therizinosaurus'', respectively, mainly represented by large forelimbs found in Mongolia.<ref name=Perle1979/> Later in 1979, Barsbold and Perle found the pelvic features of segnosaurids and dromaeosaurids were so different from those of "true" theropods that they should be separated into three taxa of the same rank, possibly at the level of [[infraorder]] within [[Saurischia]], one of the two main divisions of dinosaurs—the other being [[Ornithischia]].<ref name="Barsbold&Perle1979" />

In 1980, Barsbold and Perle named the new theropod infraorder Segnosauria, containing only Segnosauridae. In the same article, they named the new genus ''Erlikosaurus'' (known from a well-preserved skull and partial skeleton)—which they tentatively considered a segnosaurid—and reported a partial pelvis of an undetermined segnosaurian, both from the same formation as ''Segnosaurus''. The specimens provided relatively complete data on this group; they were united by their opisthopubic pelvis, slender mandible, and the toothless front of their jaws. Barsbold and Perle stated that, though some of their features resembled those of ornithischians and sauropods, these similarities were superficial and distinct when examined in detail. While they were essentially different from other theropods—perhaps due to diverging from them relatively early—and warranted a new infraorder, they did show similarities with the theropods. Because the ''Erlikosaurus'' specimen lacked a pelvis, the authors were unsure that the undetermined segnosaurian could belong to the same genus, in which case they would consider it part of a separate family.<ref name="perle&barsbold80" /> Though ''Erlikosaurus'' was difficult to compare directly to ''Segnosaurus'' because its remains were incomplete, Perle stated in 1981 there was no justification for separating it into another family.<ref name=Perle1981>{{cite journal | last=Perle | first=A. |author-link=Altangerel Perle |year=1981 |title=Noviy segnozavrid iz verchnego mela Mongolii |journal=Trudy – Sovmestnaya Sovetsko-Mongol'skaya Paleontologicheskaya Ekspeditsiya |volume=15 |pages=50–59|language=Russian|trans-title=New Segnosauridae from the Upper Cretaceous of Mongolia|translator-last=Siskron |translator-first=C. |translator2-last=Welles |translator2-first=S. P. |url=http://paleoglot.org/files/Perle_82.pdf}}</ref>
[[File:Therizinosaurus claw.jpg|thumb|left|''[[Therizinosaurus]]'', the first known therizinosaur, was originally known only from forelimb bones from Mongolia (cast shown here, in [[Aathal Dinosaur Museum]]), which created confusion about its affinities with other [[theropods]].]] 
In 1982, Perle reported the discovery of hindlimb fragments similar to those of ''Segnosaurus'' and assigned them to ''Therizinosaurus'', whose forelimbs had been found in almost the same location. He concluded that the Therizinosauridae, Deinocheiridae, and Segnosauridae, which all had enlarged forelimbs, represented the same taxonomic group. ''Segnosaurus'' and ''Therizinosaurus'' were particularly similar, leading Perle to suggest they belonged in a family to the exclusion of Deinocheiridae (today, ''Deinocheirus'' is recognized as an [[ornithomimosaur]]).<ref name="Perle1982">{{Cite journal |last=Perle |first=A. |author-link=Altangerel Perle |year=1982 |title=A hind limb of ''Therizinosaurus'' from the Upper Cretaceous of Mongolia |journal=Problems in Mongolian Geology |language=Russian |volume=5 |pages=94–98 |translator-last=Welsh |translator-first=W. R. |url=http://paleoglot.org/files/Perle_82.pdf |archive-date=October 1, 2020 |access-date=January 20, 2020 |archive-url=https://web.archive.org/web/20201001060111/http://paleoglot.org/files/Perle_82.pdf |url-status=live }}</ref><ref name="nature13874">{{cite journal | title=Resolving the long-standing enigmas of a giant ornithomimosaur ''Deinocheirus mirificus'' | last1=Lee | first1=Y. N. | last2=Barsbold | first2=R. | last3=Currie | first3=P. J. | last4=Kobayashi | first4=Y. | last5=Lee | first5=H. J. | last6=Godefroit | first6=P. | last7=Escuillié | first7=F. O. | last8=Chinzorig | first8=T. | journal=Nature | year=2014 | volume=515 | issue=7526 | pages=257–260 | doi=10.1038/nature13874| pmid=25337880 | bibcode=2014Natur.515..257L | s2cid=2986017 }}</ref> Barsbold retained ''Segnosaurus'' and ''Erlikosaurus'' in the family Segnosauridae in 1983 and named the new genus ''Enigmosaurus'' based on the previously undetermined segnosaurian pelvis. The structure of the pelvis of ''Erlikosaurus'' was unknown but Barsbold considered it unlikely the ''Enigmosaurus'' pelvis belonged to it because ''Erlikosaurus'' and ''Segnosaurus'' were so similar in other respects while the pelvis of ''Enigmosaurus'' was very different from that of ''Segnosaurus''. Barsbold found that segnosaurids were so peculiar compared to more typical theropods that they were either a very significant deviation in theropod evolution, or were possibly outside the group; he nevertheless retained them within Theropoda.<ref name=barsbold1983/> Later in 1983, Barsbold stated the segnosaurian pelvis deviated significantly from the theropod norm and found the configuration of their ilia generally similar to those of [[sauropods]].<ref name="Barsbold1983B">{{Cite journal |last=Barsbold |first=R. |year=1983 |title=O ptich'ikh chertakh v stroyenii khishchnykh dinozavrov |trans-title=“Avian” features in the morphology of predatory dinosaurs |journal=Transactions of the Joint Soviet Mongolian Paleontological Expedition |volume=24 |pages=96–103 |language=Russian |translator-last=Welsh |translator-first=W. R. |url=https://paleoglot.org/files/barsbold_83b.pdf |archive-date=September 29, 2020 |access-date=January 20, 2020 |archive-url=https://web.archive.org/web/20200929110800/http://paleoglot.org/files/Barsbold_83b.pdf |url-status=live }}</ref>

[[Image:Erlikosaurus.jpg|thumb|Outdated restoration of a [[prosauropod]]-like, quadrupedal ''Erlikosaurus''. "Segnosaurs" were often depicted this way until they were definitively identified as theropods.]]
[[Gregory S. Paul]] concluded in 1984 that segnosaurs had no theropodan features but were derived, late-surviving [[Cretaceous]] prosauropods with adaptations similar to those of ornithischians. He found segnosaurs to be similar to prosauropods in the morphology of their snout, mandible, and hindfoot; to ornithischians in their cheek, palate, pubis, and ankle; and to early dinosaurs in other respects. He proposed that ornithischians were descended from prosauropods and that the segnosaurs were an intermediate relic of this transition, which supposedly took place during the [[Triassic period]]. In this way, he considered segnosaurians to have a comparable position to herbivorous dinosaurs in general, as [[monotremes]] have to mammals. He found it unlikely but did not rule out that segnosaurs could have derived from theropods or that segnosaurs, prosauropods and ornithischians were each independently derived from early dinosaurs.<ref name="Paul1984">{{cite journal |last1=Paul |first1=G. S. |title=The segnosaurian dinosaurs: relics of the prosauropod-ornithischian transition? |journal=Journal of Vertebrate Paleontology |date=1984 |volume=4 |issue=4 |pages=507–515 |jstor= 4523011 |issn=0272-4634|doi=10.1080/02724634.1984.10012026 |bibcode=1984JVPal...4..507P }}</ref> [[David B. Norman]] considered Paul's idea contentious and "bound to provoke much argument" in 1985.<ref name="DBN85">{{cite book |last=Norman |first=D. B.|title=The Illustrated Encyclopedia of Dinosaurs: An Original and Compelling Insight into Life in the Dinosaur Kingdom |chapter=Mischellaneous theropods |year=1985 |publisher=Crescent Books |location=New York |pages=52–53 |isbn=978-0-517-46890-6 }}</ref> In 1988, Paul maintained that segnosaurs were late-surviving, ornithischian-like prosauropods and proposed a segnosaurian identity for ''Therizinosaurus''. He also placed segnosauria within [[Phytodinosauria]], a [[superorder]] [[Robert Bakker]] had created in 1985 to contain all plant-eating dinosaurs.<ref name=GSP88a>{{cite book |last=Paul |first=G. S. |title=Predatory Dinosaurs of the World |year=1988 |publisher=Simon & Schuster |pages=185, 283 |location=New York |isbn=978-0-671-61946-6 }}</ref> In a 1986 study of the inter-relationships of saurischian dinosaurs, [[Jacques Gauthier]] concluded segnosaurs were prosauropods. While he conceded they had similarities with ornithischians and theropods, he proposed these features had evolved independently.<ref name="Gauthier">{{cite journal |last1=Gauthier |first1=J. |title=Saurischian monophyly and the origin of birds |journal=Memoirs of the California Academy of Sciences |date=1986 |volume=8 |page=45 |url=https://www.biodiversitylibrary.org/page/15651737#page/331/mode/1up |archive-url=https://web.archive.org/web/20190816230537/https://www.biodiversitylibrary.org/page/15651737#page/331/mode/1up |archive-date=2019-08-16 |url-status=live }}</ref> In a 1989 conference abstract about sauropodomorph inter-relationships, [[Paul Sereno]] also considered segnosaurs to be prosauropods, based on skull features.<ref name="Sereno">{{cite conference |last1=Sereno |first1=P. |title=Prosauropod monophyly and basal sauropodomorph phylogeny |book-title=Abstract of Papers. Forty-Ninth Annual Meeting Society of Vertebrate Paleontology |journal=Journal of Vertebrate Paleontology |date=1989 |volume=9 |issue=3 Supplement|page=39A |jstor= 4523276 |issn=0272-4634}}</ref>

[[File:Alxasaurus Tyrrell.jpg|thumb|left|Reconstructed skeleton of ''[[Alxasaurus]]'' from China in [[Royal Tyrrell Museum]], the completeness of which confirmed therizinosaurs as theropods]]
In a 1990 [[review article]], Barsbold and [[Teresa Maryańska]] found Segnosauria to be a rare and aberrant group of saurischians in an unresolved position among sauropodomorphs and theropods, and probably closer to the former. Accordingly, they listed them as Saurischia ''sedis mutabilis'' (position subject to change). They agreed the hindlimbs assigned to ''Therizinosaurus'' in 1982 were segnosaurian but did not consider this a sufficient justification for ''Therizinosaurus'' itself being a segnosaur because it was only known from forelimbs.<ref name=Dinosauria1>{{cite book |last1=Barsbold |first1=R. |editor1-last = Weishampel |editor1-first = D. B. |editor2-last=Osmolska |editor2-first=H. |editor3-last = Dodson | editor3-first = P. |last2=Maryańska |first2=T.|title=The Dinosauria |edition=1st |year=1990 |publisher=University of California Press |location=Berkeley |isbn=978-0-520-06727-1|pages=408–415 |chapter=Saurischia Sedis Mutabilis: Segnosauria}}</ref> In 1993, [[Dale A. Russell]] and [[Dong Zhi-Ming]] described the new genus ''Alxasaurus'' from China; at the time this was the most complete large theropod from its time and place. While ''Alxasaurus'' was similar in some respects to prosauropods, the detailed morphology of its limbs linked it to ''Therizinosaurus'' and segnosaurs. Because its fore and hindlimbs were preserved, ''Alxasaurus'' showed that Perle's assignment of segnosaurian hindlimbs to ''Therizinosaurus'' was probably correct. Russell and Dong, therefore, proposed that Segnosauridae was a [[junior synonym]] of the older name Therizinosauridae, and that ''Alxasaurus'' was the most completely known representative so far. They also named the new higher taxonomic rank [[Therizinosauroidea]] to contain ''Alxasaurus'' and Therizinosauridae because the new genus was somewhat different from its relatives. They concluded that therizinosaurs were tetanuran theropods, most closely related to ornithomimids, troodontids, and oviraptorids, which they placed together in the group [[Oviraptorosauria]] (because they found [[Maniraptora]]—the conventional grouping of these—invalid, and the higher-level taxonomy of theropods was in flux).<ref name="Alxasaurus">{{cite journal |last1=Russell |first1=D. A. |last2=Dong |first2=Z.-M. |title=The affinities of a new theropod from the Alxa Desert, Inner Mongolia, People's Republic of China |journal=Canadian Journal of Earth Sciences |date=1993 |volume=30 |issue=10 |pages=2107–2127 |doi=10.1139/e93-183|bibcode=1993CaJES..30.2107R }}</ref><ref name=Russell1997/>

[[File:Beipiaosaurus-Paleozoological Museum of China.jpg|thumb|Partial forelimb of the [[Basal (phylogenetics)|basal]] therizinosaur ''[[Beipiaosaurus]]'' with impressions of [[feather-like structures]], [[Paleozoological Museum of China]]]]

Perle and his co-authors of a 1994 redescription of ''Erlikosaurus''{{'}}s skull accepted the synonymy of Segnosauridae with Therizinosauridae and they considered therizinosaurs to have been maniraptoran theropods, the group that also includes modern birds (because they did find Maniraptora to be valid through their analysis). They also discussed the alternative previous hypotheses for therizinosaur affinities and demonstrated faults with them.<ref>{{cite journal |last1=Clark |first1=J. M. |last2=Altangerel |first2=P. |last3=Norell |first3=M. |title=The skull of ''Erlicosaurus andrewsi'', a late Cretaceous "segnosaur" (Theropoda, Therizinosauridae) from Mongolia|issue=3115 |date=1994 |journal=American Museum Novitates|hdl=2246/3712 |pages=2, 32–36}}</ref> In 1995, Lev A. Nessov rejected the idea therizinosaurs were theropods; he considered them a distinct group within Saurischia.<ref name="Nessov" /> In 1996, [[Thomas R. Holtz Jr.]] found therizinosaurs to group with oviraptorosaurs in a [[phylogenetic analysis]] of coelurosaurian theropods.<ref name="Holtz96">{{cite journal |last1=Holtz |first1=T. R. |title=Phylogenetic taxonomy of the Coelurosauria (Dinosauria: Theropoda) |journal=Journal of Paleontology |date=1996 |volume=70 |issue=3 |pages=536–538 |issn=0022-3360|jstor=1306452 |doi=10.1017/S0022336000038506 |bibcode=1996JPal...70..536H |s2cid=87599102 }}</ref> Russell coined the name [[Therizinosauria]] for the wider group in 1997.<ref name=Russell1997/> In 1999, [[Xu Xing (paleontologist)|Xing Xu]] and colleagues described ''Beipiaosaurus'', a small, basal therizinosaur from China, which confirmed the group belonged among the coelurosaurian theropods, and that similarities with prosauropods had evolved independently. They published the first [[cladogram]] showing the evolutionary relationships of Therizinosauria and demonstrated ''Beipiaosaurus'' retained features of more basal theropods and coelurosaurs, which linked them with therizinosaurs. The preservation of feather-like structures in ''Beipiaosaurus'' also suggested this feature was more widely distributed among theropods than previously thought.<ref name="Beipiaosaurus" />

By the early 21st century, many more therizinosaur taxa had been discovered—including some outside Asia—the first being ''Nothronychus'' from North America in 2001. Basal taxa that helped illuminate the early evolution of the group, such as ''Falcarius'' in 2005, had also been discovered. Therizinosaurs were no longer considered as rare or aberrant but more diverse in features—including size—than previously thought and their classification as maniraptoran theropods was generally accepted.<ref name="Nothronychus">{{cite journal |last1=Kirkland |first1=James I. |last2=Wolfe |first2=Douglas G. |title=First definitive therizinosaurid (Dinosauria; Theropoda) from North America |journal=Journal of Vertebrate Paleontology |date=2001 |volume=21 |issue=3 |pages=410–414 |issn=0272-4634|jstor=20061971 |doi=10.1671/0272-4634(2001)021[0410:FDTDTF]2.0.CO;2 |s2cid=85705529 }}</ref><ref name=":0">{{Cite journal|last1=Zhang|first1=X.-H.|last2=Xu|first2=X.|last3=Zhao|first3=Z.-J.|last4=Sereno|first4=P.|last5=Kuang|first5=X.-W.|last6=Tan|first6=L.|date=2001|title=A long-necked therizinosauroid dinosaur from the Upper Cretaceous Iren Dabasu Formation of Nei Mongol, People's Republic of China|url=http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/200901/W020090813372120151973.pdf|journal=Vertebrata PalAsiatica|volume=39|issue=4|pages=282–290|archive-url=https://web.archive.org/web/20160304111348/http://www.ivpp.cas.cn/cbw/gjzdwxb/xbwzxz/200901/W020090813372120151973.pdf|archive-date=2016-03-04|url-status=live}}</ref><ref name="Falcarius">{{cite journal |last1=Kirkland |first1=J. I. |last2=Zanno |first2=L. E. |last3=Sampson |first3=S. D. |last4=Clark |first4=J. M. |last5=DeBlieux |first5=D. D. |author2-link=Lindsay Zanno |title=A primitive therizinosauroid dinosaur from the Early Cretaceous of Utah |journal=Nature |date=2005 |volume=435 |issue=7038 |pages=84–87 |doi=10.1038/nature03468|pmid=15875020 |bibcode=2005Natur.435...84K |s2cid=4428196 |url=http://doc.rero.ch/record/15297/files/PAL_E2596.pdf }}</ref> The placement of Therizinosauria within Maniraptora continued to be unclear; in 2017, Alan H. Turner and colleagues found them to group with oviraptorosaurs while in 2009 Zanno and colleagues found them to be the most basal clade within Maniraptora, bracketed by Ornithomimosauria and [[Alvarezsauridae]].<ref>{{cite journal |last1=Turner |first1=A. H. |last2=P. |first2=D. |last3=Clarke |first3=J. A. |last4=Erickson |first4=G. M. |last5=Norell |first5=M. A. |title=A basal dromaeosaurid and size evolution preceding avian flight |journal=Science |date=2007 |volume=317 |issue=5843 |pages=1378–1381 |doi=10.1126/science.1144066|pmid=17823350 |bibcode=2007Sci...317.1378T |doi-access=free }}</ref><ref name="Zanno2009"/> Despite the additional fossil material, the interrelations within the group were also still uncertain by 2010, when Zanno conducted the most detailed phylogenetic analysis of the Therizinosauria to that point. She cited the inaccessibility, damage, potential loss of holotype specimens, scarcity of cranial remains, and fragmentary specimens with few overlapping elements as the most significant obstacles to resolving the evolutionary relationships within the group. The position of ''Segnosaurus'' and those of some other Asian therizinosaurids was affected by these factors; Zanno stated more well-preserved specimens and the rediscovery of missing elements would be necessary. Zanno also revised Therizinosauroidea to exclude ''Falcarius'' and retained it in the wider clade Therizinosauria, which became the senior synonym of Segnosauria.<ref name=Zanno2010>{{Cite journal|last=Zanno |first=L. E. |author-link=Lindsay Zanno |year=2010 |title=A taxonomic and phylogenetic re-evaluation of Therizinosauria (Dinosauria: Maniraptora) |journal=Journal of Systematic Palaeontology |volume=8 |issue=4 |pages=507–508, 512–515, 522–524, 527, 533, 538–539 |doi=10.1080/14772019.2010.488045 |bibcode=2010JSPal...8..503Z |s2cid=53405097 }}</ref> By 2015, ''Segnosaurus'' remained one of the best known therizinosaurs, according to Christophe Hendrickx and colleagues.<ref name="Hendrickx"/>

The following cladogram shows the relationships within Therizinosauria according to a 2013 study by Hanyong Pu and colleagues, which was based on Zanno's 2010 analysis, with the addition of the basal genus ''Jianchangosaurus'':<ref name=os>{{Cite journal | last1 = Pu | first1 = H. | last2 = Kobayashi | first2 = Y. | last3 = Lü | first3 = J. | last4 = Xu | first4 = L. | last5 = Wu | first5 = Y. | last6 = Chang | first6 = H. | last7 = Zhang | first7 = J. | last8 = Jia | first8 = S. | editor1-last = Claessens | editor1-first = Leon | doi = 10.1371/journal.pone.0063423 | title = An unusual basal therizinosaur dinosaur with an ornithischian dental arrangement from Northeastern China | journal = PLOS ONE | volume = 8 | issue = 5 | pages = e63423 | year = 2013 | pmid =  23734177| pmc = 3667168| bibcode = 2013PLoSO...863423P | doi-access = free }}</ref>
[[File:Therizinosaur skeletons.jpg|thumb|upright=1.8|Known elements of various therizinosaurs shown to scale, with ''Segnosaurus'' in the upper middle]]
{{clade| style=font-size:90%;line-height:85%
|label1=[[Therizinosauria]]
|1={{clade
   |1=''[[Falcarius]]''
   |label2=<span style="color:white;">unnamed</span>
   |2={{clade
      |1=''[[Jianchangosaurus]]''
      |label2=[[Therizinosauroidea]]
      |2={{clade
         |1=''[[Beipiaosaurus]]''
         |label2=<span style="color:white;">unnamed</span>
            |2={{clade
               |1=''[[Alxasaurus]]''
               |label2=[[Therizinosauridae]]
               |2={{clade
                  |1=''[[Erliansaurus]]''
                  |2=''[[Nanshiungosaurus]]''
                  |3=''[[Neimongosaurus]]''
                  |4={{clade
                     |1='''''Segnosaurus'''''
                     |2=''[[Erlikosaurus]]''
                     |3=''[[Suzhousaurus]]''
                     |4=''[[Enigmosaurus]]''
                     |5=''[[Therizinosaurus]]''
                     |6=''[[Nothronychus]]'' }} }} }} }} }} }} }}
The basalmost definite therizinosaur is ''Falcarius'' from the [[Early Cretaceous]] of North America; it showed the pelvis and dentition were the first features that were modified away from the more general maniraptoran plan in therizinosaurs, probably reflecting their transition from carnivory to herbivory.<ref name="Falcarius"/><ref name="Osteology">{{cite journal |last=Zanno |first=L. E. |author-link=Lindsay Zanno |title=Osteology of ''Falcarius utahensis'' (Dinosauria: Theropoda): characterizing the anatomy of basal therizinosaurs |journal=Zoological Journal of the Linnean Society |date=2010 |volume=158 |issue=1 |pages=225 |doi=10.1111/j.1096-3642.2009.00464.x|doi-access=free }}</ref> Therizinosaurs are mainly known from the Cretaceous of Asia and North America, and possible remains from other ages and places are controversial. Since therizinosaurs are known to have lived across the [[supercontinent]] [[Laurasia]] (which consisted of what are now North America, Europe, and Asia), Zanno suggested two scenarios for their [[paleobiogeographic]] distribution in 2010. One possibility is they dispersed through [[vicariance]], whereby therizinosaurs were present in the areas that became Asia and North America before the [[rifting]] that divided these areas in the [[Late Triassic]]. The other possibility is that basal therizinosaurs dispersed between Asia and North America via Europe after the rifting event but before the middle [[Barremian]]; between 132 and 138&nbsp;million years ago, a temporary [[land bridge]] connected North America and Europe, whereafter the landmasses were again isolated from each other, explaining why the basal therizinosaurs ''Beipiaosaurus'' from Asia and ''Falcarius'' from North America were so morphologically divergent from each other, though coeval. The presence of the derived therizinosaurid ''Nothronychus'', which was most-closely related to Asian genera, in North America during the [[Turonian]] stage of the early [[Late Cretaceous]] also shows there would have been a [[faunal interchange]] between North America and Asia via a late-Early Cretaceous land bridge before that (during the [[Aptian]]/[[Albian]]), which is also seen in some other dinosaur groups.<ref name=Zanno2010/>

==Paleobiology==
[[File:Therizinosaurs.jpg|thumb|left|Reconstructed skeletons of the North American therizinosaurs ''[[Nothronychus]]'' (large) and ''[[Falcarius]]'', in modern, bipedal postures, [[Natural History Museum of Utah]]]]

In 1979 and 1981, Barsbold and Perle said the short, massive metatarsus and unusually large, splayed toes indicated that ''Segnosaurus'' and its relatives were not adapted for rapid locomotion, perhaps because it was not required by their lifestyle; Barsbold and Perle suggested they could have been [[Semiaquatic|amphibious]].<ref name="Barsbold&Perle1979">{{Cite journal |last1=Barsbold |first1=R. |last2=Perle |first2=A. |author2-link=Altangerel Perle |year=1979 |title=Modiphikatsiy tasa sayrisziy i parallelinoe rasvitie zishchnich dinosavrov |trans-title=Modification in the saurischian pelvis and the parallel development of predatory dinosaurs |journal=Transactions of the Joint Soviet Mongolian Paleontological Expedition |volume=8 |pages=39–44 |language=Russian |translator-last=Welsh |translator-first=W. R. |url=http://paleoglot.org/files/Bars%26Perle_79a.pdf |archive-date=September 18, 2020 |access-date=January 20, 2020 |archive-url=https://web.archive.org/web/20200918131952/http://paleoglot.org/files/Bars%26Perle_79a.pdf |url-status=live }}</ref><ref name=Perle1981/> Barsbold and Maryańska agreed in 1990 the short, broad feet and bulky trunks of the group indicated they were slow-moving animals.<ref name=Dinosauria1/> Paul depicted a prosauropod-like "segnosaur" skeleton (a composite of various genera) in a quadrupedal posture in 1988.<ref name=GSP88a/> Based on the more complete remains of ''Alxasaurus'' and the articulation of its vertebral column, Russell concluded in 1993 that Paul's skeletal restoration was inaccurate and that the arms of therizinosaurs were held clear off the ground.<ref name="NatGeo" /> In 1995, Nessov suggested the elongated claws of therizinosaurs were used for defense against predators and that their young could have used their claws for [[arboreal locomotion]] along trunks and in tree crowns in a similar manner to [[sloths]].<ref name="Nessov">{{Cite book |url=https://paleoglot.org/files/Nessov%2095.pdf |title=Dinosaurs of northern Eurasia: new data about assemblages, ecology and paleobiogeography |last=Nessov |first=L. A. |publisher=Saint Petersburg State University |year=1995 |translator-last=Platonova |translator-first=T. |location=Saint Petersburg |pages=13, 18, 45–46 |archive-date=November 5, 2020 |access-date=December 27, 2019 |archive-url=https://web.archive.org/web/20201105174915/https://paleoglot.org/files/Nessov%2095.pdf |url-status=live }}</ref>

In a 2012 study of the {{Dinogloss|endocranial}} anatomy of ''Erlikosaurus'' and other therizinosaurs that preserve {{Dinogloss|braincase|braincases}}, Stephan Lautenschlager and colleagues found these dinosaurs had well-developed senses of smell, hearing, and [[Sense of balance|balance]]. The former two senses may have played a role in foraging, predator evasion, and social behavior. These senses were also well-developed in earlier coelurosaurs, so therizinosaurs may have inherited these traits from their carnivorous ancestors and used them for different dietary purposes.<ref name="Endocranial">{{cite journal |last1=Lautenschlager |first1=S. |last2=Rayfield |first2=E. J. |last3=Altangerel |first3=P. |last4=Zanno |first4=L. E. |last5=Witmer |first5=L. M. |last6=Butler |first6=R. J. |author4-link=Lindsay Zanno |title=The endocranial anatomy of Therizinosauria and its implications for sensory and cognitive function |journal=PLOS ONE |date=2012 |volume=7 |issue=12 |pages=e52289 |doi=10.1371/journal.pone.0052289|pmid=23284972 |pmc=3526574 |bibcode=2012PLoSO...752289L |doi-access=free }}</ref> In a 2014 study of the function of therizinosaur hand claws, Lautenschlager found that these would not have been used for digging, which would have been done with the foot claws because, since as in other maniraptorans, feathers on the forelimbs would have interfered with this function. He could neither confirm nor disregard that the hand claws could have been used for defense, combat, stabilization by grasping tree trunks during high [[Browsing (herbivory)|browsing]], sexual display, or gripping mates during copulation. He largely ruled out that they dug burrows, due to their size.<ref name="Claws">{{cite journal |last1=Lautenschlager |first1=S. |title=Morphological and functional diversity in therizinosaur claws and the implications for theropod claw evolution |journal=Proceedings of the Royal Society B: Biological Sciences |date=2014 |volume=281 |issue=1785 |pages=20140497 |doi=10.1098/rspb.2014.0497|pmid=24807260 |pmc=4024305 }}</ref>

[[File:Segnosaurus nest 2.jpg|thumb|Nest attributed to therizinosaurs, [[Dinosaurland Fossil Museum]]]]

[[Dinosaur eggs]] with embryos of the [[Dendroolithidae]] type from the [[Nanchao Formation]] of China were identified as belonging to therizinosaurs and described by Martin Kundrát and colleagues in 2007. The development of the embryos and the fact that no adults were found in association with the nests indicate that therizinosaur hatchlings were [[precocial]] (capable of locomotion from birth) and able to leave their nests to feed alone, independently of their parents.<ref name="ChinaEggs">{{cite journal |last1=Kundrát |first1=M. |last2=Cruickshank |first2=A. R. I. |last3=Manning |first3=T. W. |last4=Nudds |first4=J. |title=Embryos of therizinosauroid theropods from the Upper Cretaceous of China: diagnosis and analysis of ossification patterns |journal=Acta Zoologica |date=2007 |volume=89 |issue=3 |pages=231–251 |doi=10.1111/j.1463-6395.2007.00311.x}}</ref><ref name="Dinosauria2"/> In a 2013 conference abstract, Yoshitsugu Kobayashi and colleagues reported a nesting ground of theropod dinosaurs from the [[Javkhlant Formation]] of Mongolia that contained at least 17 [[egg clutch]]es within an area {{Convert|22|x|52|m|feet|sp=us}}. Each clutch contained eight spherical eggs with rough surfaces; the eggs were in contact with each other and arranged in a circular structure with no central opening. The researchers identified the eggs as dendroolithid, and therefore therizinosaurian. Though therizinosaurs are not known from the formation, it overlies the Bayan Shireh Formation where ''Segnosaurus'', ''Erlikosaurus'', and ''Enigmosaurus'' were found. The multiple clutches indicate some therizinosaurs were colonial nesters like [[hadrosaurs]], prosauropods, [[titanosaurs]], and birds. The eggs were found in a single [[stratigraphic layer]], suggesting the dinosaurs nested at the site on a single occasion and did not exhibit [[site fidelity]] (always returning to the same site to breed).<ref name="MongoliaEggs">{{cite journal |title=First record of a dinosaur nesting colony from Mongolia reveals nesting behavior of therizinosauroids |journal=Hokkaido University |url=https://www.global.hokudai.ac.jp/blog/first-record-of-a-dinosaur-nesting-colony-from-mongolia-reveals-nesting-behavior-of-therizinosauroids/ |access-date=2019-12-21 |archive-url=https://web.archive.org/web/20191221194650/https://www.global.hokudai.ac.jp/blog/first-record-of-a-dinosaur-nesting-colony-from-mongolia-reveals-nesting-behavior-of-therizinosauroids/ |archive-date=2019-12-21 |url-status=live |date=2013}}</ref>

===Diet and feeding===
[[File:Therizinosaurian mandibular morphology.png|thumb|left|Right half of the holotype mandible shown from the inner side (A), the top (C), and bottom (D), and the left half shown from the inner side (B), compared with the mandibles of other therizinosaurs (to scale in lower right)]]

The unusual features of therizinosaurs have led to several interpretations of their feeding behavior; there is no direct evidence of their diet, such as stomach contents and feeding traces. In 1970, [[Anatoly K. Rozhdestvensky]] suggested ''Therizinosaurus''—the only member of the group known at the time—used its large claws to open [[termite mounds]] or gather fruits from trees.<ref name="Dinosauria2"/> Barsbold and Perle pointed out in 1979 and 1980 that their peculiar features probably reflected a different evolutionary direction than those of more typical theropods, many of which were considered effective, active predators. Their delicate jaws, small, weak teeth and beaks, and short, compact feet indicated they would not have used the armaments of other theropods to procure food but could have preyed on fish.<ref name="Barsbold&Perle1979" /><ref name="perle&barsbold80" /> In 1983, Barsbold said the horny beak at the front of the jaws and weakened teeth at the back were common features among herbivorous dinosaurs but not of carnivorous theropods, and speculated this might indicate segnosaurs had shifted to herbivory.<ref name=barsbold1983/> In 1984, Paul suggested they were herbivorous due to the similarities of their skulls to those of prosauropod and ornithischian dinosaurs, which include horny beaks, inset rows of teeth, and a shelf at the side of the jaws that indicated the presence of cheeks. Like ornithischians, they could, therefore, crop, manipulate, and chew plants in a sophisticated manner. He also suggested the ilia of the pelvis had sideways-flaring blades at the front similar to those of sauropods to support a large gut that was used to ferment and process food.<ref name="Paul1984"/> Norman stated in 1985 the possibility ''Segnosaurus'' was an aquatic fish-eater could explain its small, pointed teeth and broad and perhaps webbed feet, but found it mysterious why it should have a horny beak.<ref name="DBN85"/>

[[File:Segnosaurus Restoration.jpg|thumb|Restoration showing large area behind the legs caused by the backwards directed [[pubic bone]]; therizinosaurs may have "sat" on their pelvises during feeding]]
In 1993, Russell and Dong considered the small size of the head, blunt beaks and large body weights of therizinosaurs consistent with herbivory.<ref name="Alxasaurus"/> In 1993 and 1997, Russell suggested therizinosaurs would have "sat" on their pelvises and supported their bodies on their hind limbs while using their long arms, claws, and flexible necks to reach leaves from trees and bushes with their beaks. They could have reached even higher while standing and browsing bipedally. This parallels the way some herbivorous mammals use their forelimbs to manipulate vegetation; Russell considered the extinct [[chalicotheres]] and [[ground sloth]]s, as well as [[gorilla]]s, adaptively convergent with therizinosaurs. Because therizinosaur remains are often found in [[sediments]] deposited in river and lake environments, Russell said they may have browsed on [[riparian]] bushes and trees.<ref name=Russell1997>{{cite book |last=Russell |first=D. A. |year=1997 |chapter=Therizinosauria |editor=[[Phil Currie|Currie, Philip J.]] |editor2=Padian, Kevin |title=Encyclopedia of Dinosaurs |publisher= Academic Press |location=San Diego |pages=729–730 |isbn=978-0-12-226810-6}}</ref><ref name="NatGeo">{{Cite journal |last1=Russell |first1=D. A. |last2=Russell |first2=D. E. |year=1993 |title=Mammal-dinosaur convergence |journal=National Geographic Research |volume=9 |pages=70–79 |issn=8755-724X}}</ref> Based on the assemblage of fossils in the [[Bissekty Formation]] of Uzbekistan, Nessov suggested in 1995 that therizinosaurs could have been part of its nutrient-rich aquatic ecosystems, though perhaps indirectly, by feeding on wasps which had themselves fed on carrion of aquatic vertebrates. He found this consistent with Rozhdestvensky's suggestion that therizinosaurs may have fed on [[social insects]].<ref name="Nessov" /> In a 2006 conference abstract, Sara Burch presented the inferred range of motion in the arms of the therizinosaur ''Neimongosaurus'' and concluded the overall motion at the {{Dinogloss|glenoid}}-humeral joint at the shoulder was roughly circular, and directed sideways and slightly downwards, which diverged from the more oval, backwards-and-downwards-directed ranges of other theropods. This ability to extend their arms considerably forwards may have helped therizinosars reach and grasp for foliage.<ref name="burch2006">{{Cite conference |last=Burch |first=S. |year=2006 |title=The range of motion of the glenohumeral joint of the therizinosaur ''Neimongosaurus yangi'' (Dinosauria: Theropoda) |conference=Chicago Biological Investigator |volume=3 |issue=2 |pages=20}}</ref>

In 2009, Zanno and colleagues stated therizinosaurs were the most-widely regarded candidates for herbivory among theropods and listed features associated with this diet. These included small, densely packed, coarse serrations; lanceolate (lance-shaped) teeth with a low replacement rate; a beak at the front of the jaws; an inset tooth row that suggests fleshy cheeks; an elongated neck; a small skull; a very large gut capacity as indicated by the rib circumference at the trunk and the outwards flaring processes of the ilia; and the loss of [[cursorial]] (related to running) adaptations in the hind limbs, including development of functionally tetradactyl feet. Zanno and colleagues found the clades at the base of Maniraptora—Ornithomimosauria, Therizinosauria, and Oviraptorosauria—had either direct or morphological evidence for herbivory, which would mean either this diet evolved independently multiple times in coelurosaurian theropods or that the primitive condition of the group was at least facultative herbivory with carnivory only emerging in more derived maniraptorans.<ref name="Zanno2009">{{cite journal |last1=Zanno |first1=L. E. |last2=Gillette |first2=D. D. |last3=Albright |first3=L. B. |last4=Titus |first4=A. L. |author1-link=Lindsay Zanno |title=A new North American therizinosaurid and the role of herbivory in 'predatory' dinosaur evolution |journal=Proceedings: Biological Sciences |date=2009 |volume=276 |issue=1672 |pages=3505–3511 |issn=0962-8452|jstor=30244145 |doi=10.1098/rspb.2009.1029 |pmid=19605396 |pmc=2817200 }}</ref> Zanno and Peter J. Makovicky found, in 2011, therizinosaurs and some other groups of herbivorous dinosaurs that had beaks and retained teeth were unable to lose their teeth completely because they lacked [[gastric mill]]s (gizzards) and needed the teeth to process food, and that the high-fiber [[folivorous]] (leaf-based) diet of therizinosaurs and other [[archosaurs]] may also have precluded the evolution of a complete beak.<ref>{{cite journal |last1=Zanno |first1=L. E. |last2=Makovicky |first2=P. J. |author1-link=Lindsay Zanno |title=Herbivorous ecomorphology and specialization patterns in theropod dinosaur evolution |journal=Proceedings of the National Academy of Sciences |date=2011 |volume=108 |issue=1 |pages=232–237 |doi=10.1073/pnas.1011924108|pmid=21173263 |pmc=3017133 |bibcode=2011PNAS..108..232Z |doi-access=free }}</ref> Lautenschlager found in 2014 the hands of therizinosaurs would have had to be able to extend the range of the animal to a point that could not be reached by the head if they were used for browsing and pulling down vegetation. In genera where both neck and forelimb elements are preserved, however, the necks were equal in length or longer than the forelimbs, so pulling vegetation would only make sense if lower parts of long branches were pulled down to access out-of-reach parts of trees.<ref name="Claws"/>

[[File:Dentary teeth of Segnosaurus.png|thumb|upright|Hindmost dentary teeth showing the third cutting edge (lc), unique to ''Segnosaurus'' among theropods]]
Zanno and colleagues stated in 2016 that therizinosaurs were generally accepted to fall within the spectrum of [[omnivory]] and herbivory, with a trend towards intensified herbivory. While various anatomical features have been used to support this idea, tooth morphology had been considered relatively simplistic and with few unique specializations compared to other herbivorous dinosaurs. The few modifications include the increased symmetry in the teeth of ''Erlikosaurus'' and the enlargement of denticles in ''Segnosaurus''. Zanno and colleagues identified novel, complex features in the dentary teeth of ''Segnosaurus'', including the presence of additional carinae and folded carinae with denticulated front edges, which indicate ''Segnosaurus'' had a higher degree of oral food processing than other therizinosaurs. These traits together created a roughened, shredding surface near the base of the tooth crowns that was unique to ''Segnosaurus'' and suggest it consumed unique food resources or used highly specialized feeding strategies. Because multiple geological formations show evidence of [[sympatric]] therizinosaur species—related species that lived in the same area at the same time—it is possible [[niche partitioning]] between them could have played a role in the group's evolutionary success. This is supported by ''Segnosaurus'' with its highly specialized dentition being a contemporary of ''Erlikosaurus'' with its relatively indistinct teeth, indicating their partitioning in food acquisition, processing, or resources. This conclusion is also strengthened by the large difference in estimated body masses of sympatric therizinosaurs in the Bayan Shireh Formation, which was up to 500%.<ref name="Zanno2016"/>

In a 2017 study of niche partitioning in therizinosaurs through digital simulations, Lautenschlager found the dentaries of ''Segnosaurus'' experienced one of the lowest stress magnitudes during extrinsic feeding scenarios. ''Segnosaurus'' and ''Erlikosaurus'' were aided by the down-turned tip of the lower jaws and symphyseal regions, and probably also by beaks, which are known to mitigate stress and strain. By contrast, the straighter and more elongated dentaries of basal therizinosaurs—typical of their coelurosaurian ancestors—had the highest magnitudes of stress and strain. A downwards-pulling motion of the head while gripping vegetation was more likely than a sideways or upwards movement, though such behavior would be more likely in ''Segnosaurus'' and ''Erlikosaurus'' with their stress-mitigating jaws. Difference in relative bite force between the sympatric ''Segnosaurus'' and ''Erlikosaurus'' show the former would have been able to feed on tougher vegetation while the overall robustness of the latter suggests greater flexibility in its manner of feeding, because stress levels stayed low across feeding simulations. Lautenschlager agreed the two taxa were adapted to different modes of feeding and food selection; ''Segnosaurus'' was more adapted to using its dentition to procure or process food while ''Erlikosaurus'' mostly used its beak for cropping and its neck musculature while foraging. The difference in size between ''Segnosaurus'' and ''Erlikosaurus'' (the former of which is estimated to have weighed more than the latter) indicates these effects were increased and that there were further mechanisms partitioning their resources, such as different heights. Because other therizinosaur taxa were more divided in time and space, other factors than competition within their group may also have contributed to their variation, such as adaptations to different flora and competition with other kinds of herbivores.<ref name="Niche">{{cite journal |last1=Lautenschlager |first1=S. |title=Functional niche partitioning in Therizinosauria provides new insights into the evolution of theropod herbivory |journal=Palaeontology |date=2017 |volume=60 |issue=3 |pages=375–387 |doi=10.1111/pala.12289 |bibcode=2017Palgy..60..375L |s2cid=90965431 |url=http://pure-oai.bham.ac.uk/ws/files/39402933/FUNCTIONAL_NICHE_PARTITIONING_IN_THERIZINOSAURIA_REVISED2.pdf |archive-date=January 28, 2021 |access-date=December 23, 2019 |archive-url=https://web.archive.org/web/20210128051751/http://pure-oai.bham.ac.uk/ws/files/39402933/FUNCTIONAL_NICHE_PARTITIONING_IN_THERIZINOSAURIA_REVISED2.pdf |url-status=live }}</ref>

In 2018, Loredana Macaluso and colleagues pointed out that the hips of therizinosaurs were peculiar because the shaft of the pubic bone was rotated backwards whereas the pubic boot was strongly projected forwards. While the larger gut associated with herbivory was able to push the shaft backwards, they suggested the pubic boot was restrained by [[ventilatory muscles]] that were crucial for cuirassal ventilation—breathing with extra [[air sacs]]—which shows the importance of this mode of respiration.<ref name="Macaluso">{{cite journal |last1=Macaluso |first1=L. |last2=Tschopp |first2=E. |last3=Mannion |first3=P. |title=Evolutionary changes in pubic orientation in dinosaurs are more strongly correlated with the ventilation system than with herbivory |journal=Palaeontology |date=2018 |volume=61 |issue=5 |pages=703–719 |doi=10.1111/pala.12362|bibcode=2018Palgy..61..703M |s2cid=133643430 }}</ref> In a 2019 study of jaw musculature, Ali Nabavizadeh concluded therizinosaurs were mainly orthal feeders—moving their jaws up and down—and raised their jaws isognathously whereby the upper and lower teeth of each side [[Occlusion (dentistry)|occluded]] (contacted each other) at once. The origin and insertion sites of their jaw muscles also added strength to their jaw closure.<ref name="Nabavizadeh">{{cite journal |last1=Nabavizadeh |first1=A. |title=Cranial musculature in herbivorous dinosaurs: a survey of reconstructed anatomical diversity and feeding mechanisms |journal=The Anatomical Record |date=2019 |volume=303 |issue=4 |pages=1104–1145 |doi=10.1002/ar.24283|pmid=31675182 |s2cid=207815224 |doi-access=free }}</ref> David J. Button and Zanno found in 2019 herbivorous dinosaurs mainly followed two distinct modes of feeding, either processing food in the gut—characterized by gracile skulls and low bite forces—or the mouth, characterized by features associated with extensive processing. ''Segnosaurus'', along with [[diplodocoid]] and titanosaur sauropods, deinocheirid and ornithomimid ornithomimosaurs, and [[caenagnathids]], was found to be in the former category, whereas ''Erlikosaurus'' was more similar to some sauropodomorph and ornithischian taxa, indicating these two therizinosaurs were functionally separated and occupied different niches.<ref name="DivergentHerbivory">{{cite journal |last1=Button |first1=D. J. |last2=Zanno |first2=L. E. |author2-link=Lindsay Zanno |title=Repeated evolution of divergent modes of herbivory in non-avian dinosaurs |journal=Current Biology |volume=30 |issue=1 |pages=158–168.e4|date=2019 |doi=10.1016/j.cub.2019.10.050|pmid=31813611 |s2cid=208652510 |doi-access=free }}</ref>

==Paleoenvironment==
[[File:Bayan Shireh Formation Fauna.png|thumb|upright=1.5|Dinosaurs from the [[Bayan Shireh Formation]] compared in size (''Segnosaurus'' fifth from right, in dark red)]]
Fossils of ''Segnosaurus'' have been recovered from the Bayan Shireh Formation in Mongolia, which has been dated to about 102–86&nbsp;million years ago during the [[Cenomanian]] to Turonian stages of the Late Cretaceous period, based on [[paleomagnetic]] analysis and [[calcite]] [[U–Pb measurements]].<ref name="Magnetic"/><ref name="Formation">{{cite journal |last1=Kurumada |first1=Y. |last2=Aoki |first2=S. |last3=Aoki |first3=K. |last4=Kato |first4=D. |last5=Saneyoshi |first5=M. |last6=Tsogtbaatar |first6=K. |last7=Windley |first7=B. F. |last8=Ishigaki |first8=S. |title=Calcite U–Pb age of the Cretaceous vertebrate-bearing Bayn Shire Formation in the Eastern Gobi Desert of Mongolia: usefulness of caliche for age determination |journal=Terra Nova |date=2020 |volume=32 |issue=4 |pages=246–252 |doi=10.1111/ter.12456|bibcode=2020TeNov..32..246K |doi-access=free }}</ref> The remains were found in poorly cemented, gray sands containing intraformational [[Conglomerate (geology)|conglomerates]], gravel, and gray claystone.<ref name=Dinosauria1/> The Bayan Shireh Formation overlies the [[Baruunbayan Formation]] and underlies the Javkhlant Formation. The sediments of these formations were deposited by [[meandering rivers]] and lakes on an [[alluvial plain]] (flat land consisting of sediments deposited by highland rivers) with a [[semi-arid]] climate.<ref name="Arid">{{cite journal |last1=Khand |first1=Y. |last2=Badamgarav |first2=D. |last3=Ariunchimeg |first3=Y. |last4=Barsbold |first4=R. |title=Cretaceous system in Mongolia and its depositional environments |journal=Developments in Palaeontology and Stratigraphy |date=2000 |volume=17 |pages=49–79 |doi=10.1016/S0920-5446(00)80024-2 |publisher=Elsevier|isbn=9780444502766 }}</ref><ref name="Trackway"/>

Therizinosaurs were the most abundant theropods in the Bayan Shireh Formation in terms of [[biodiversity]]; in addition to ''Segnosaurus'', members of the group included ''Erlikosaurus'', ''Enigmosaurus'', and ''[[Duonychus]]''.<ref name="Trackway">{{cite journal |last1=Lee |first1=Yu.-N. |last2=Lee |first2=H.-J. |last3=Kobayashi |first3=Y. |last4=Paulina-Carabajal |first4=A. |last5=Barsbold |first5=R. |last6=Fiorillo |first6=A. R. |last7=Tsogtbaatar |first7=K. |title=Unusual locomotion behaviour preserved within a crocodyliform trackway from the Upper Cretaceous Bayanshiree Formation of Mongolia and its palaeobiological implications |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |date=2019 |volume=533 |pages=109239 |doi=10.1016/j.palaeo.2019.109239|bibcode=2019PPP...53309239L |s2cid=197584839 }}</ref><ref>{{Cite journal |last1=Kobayashi |first1=Yoshitsugu |last2=Zelenitsky |first2=Darla K. |last3=Fiorillo |first3=Anthony R. |last4=Chinzorig |first4=Tsogtbaatar |date=2025-03-25 |title=Didactyl therizinosaur with a preserved keratinous claw from the Late Cretaceous of Mongolia |journal=[[iScience]]  |issue=4 |language=English |volume=28 |doi=10.1016/j.isci.2025.112141 |issn=2589-0042|doi-access=free |bibcode=2025iSci...28k2141K |pmc=12131255 }}</ref> Other theropods included the tyrannosaur ''[[Khankhuuluu]]'', the ornithomimosaur ''[[Garudimimus]]'', and the dromaeosaur ''[[Achillobator]]''.<ref>{{Cite journal |last=Voris |first=Jared T. |last2=Zelenitsky |first2=Darla K. |last3=Kobayashi |first3=Yoshitsugu |last4=Modesto |first4=Sean P. |last5=Therrien |first5=François |last6=Tsutsumi |first6=Hiroki |last7=Chinzorig |first7=Tsogtbaatar |last8=Tsogtbaatar |first8=Khishigjav |date=2025-06-11 |title=A new Mongolian tyrannosauroid and the evolution of Eutyrannosauria |journal=[[Nature (journal)|Nature]] |doi=10.1038/s41586-025-08964-6}}</ref><ref>{{cite journal|last1=Barsbold|first1=R.|date=1981|title=Беззубые хищные динозавры Монголии|trans-title=Toothless carnivorous dinosaurs of Mongolia|journal=Transactions, Joint Soviet–Mongolian Palaeontological Expedition|volume=15|pages=28−39|s2cid=195060070|language=ru}} [https://paleoglot.org/files/Barsbold_81.pdf Translated paper]</ref><ref name=Perle1999>{{cite journal|last1=Perle|first1=A.|last2=Norell|first2=M. A.|last3=Clark|first3=J.|title=A new maniraptoran theropod – ''Achillobator giganticus'' (Dromaeosauridae) – from the Upper Cretaceous of Burkhant, Mongolia|journal=Contributions from the Geology and Mineralogy Chair, National Museum of Mongolia|date=1999|issue=101|pages=1–105|oclc=69865262}}</ref> Other dinosaurs included the ankylosaur ''[[Talarurus]]'',<ref name="Magnetic"/> the hadrosaur ''[[Gobihadros]]'',<ref name="Gobihadros">{{cite journal |last1=Tsogtbaatar |first1=K. |last2=Weishampel |first2=D. B. |last3=Evans |first3=D. C. |last4=Watabe |first4=M. |last5=Mpodozis |first5=J. M. |title=A new hadrosauroid (Dinosauria: Ornithopoda) from the Late Cretaceous Baynshire Formation of the Gobi Desert (Mongolia) |journal=PLOS ONE |date=2019 |volume=14 |issue=4 |pages=e0208480 |doi=10.1371/journal.pone.0208480|pmid=30995236 |pmc=6469754 |bibcode=2019PLoSO..1408480T |doi-access=free }}</ref> the sauropod ''[[Erketu]]'',<ref>{{cite journal |last1=Ksepka |first1=D. T. |last2=Norell |first2=M. |title=''Erketu ellisoni'', a long-necked sauropod from Bor Guvé (Dornogov Aimag, Mongolia)|journal=American Museum Novitates|issue=3508 |pages=1–16|date=2006 |hdl=2246/5783 |doi=10.1206/0003-0082(2006)3508[1:EEALSF]2.0.CO;2 |s2cid=86032547 |url=https://www.biodiversitylibrary.org/bibliography/169093 }}</ref> and the ceratopsian ''[[Graciliceratops]]''.<ref>{{cite book |last1=Sereno |first1=P. C. |chapter=The fossil record, systematics and evolution of pachycephalosaurs and ceratopsians from Asia |year=2000 |title=The Age of Dinosaurs in Russia and Mongolia|pages=480–516 |isbn=978-0-521-54582-2 |location=Cambridge|publisher=Cambridge University Press| editor1 =Benton, M. J.| editor2 =Shishkin, M. A.| editor3 =Unwin, D. M.| editor4 =Kurochkin, E. N.}}</ref> Dinosaur eggs, some of which were identified as Dendroolithidae, as well as footprints of dinosaurs and [[crocodyliforms]], have also been found. The formation is distinctive for its variety and abundance of turtles, and invertebrates include [[ostracods]] and freshwater molluscs.<ref name="Magnetic">{{cite journal |last1=Hicks |first1=J. F. |last2=Brinkman |first2=D. L. |last3=Nichols |first3=D. J. |last4=Watabe |first4=M. |title=Paleomagnetic and palynologic analyses of Albian to Santonian strata at Bayn Shireh, Burkhant, and Khuren Dukh, eastern Gobi Desert, Mongolia |journal=Cretaceous Research |date=1999 |volume=20 |issue=6 |pages=829–850 |doi=10.1006/cres.1999.0188|bibcode=1999CrRes..20..829H }}</ref><ref name=Dinosauria1/><ref name="Trackway"/> The Bayan Shireh Formation is possibly coeval with the [[Iren Dabasu Formation]] of the [[Inner Mongolia]] region of China, from where therizinosaur fossils similar to those of ''Segnosaurus'' and ''Erlikosaurus'' have also been found.<ref name="IrenDabasu">{{cite journal |last1=Currie |first1=P. J. |last2=Eberth |first2=D. A. |title=Palaeontology, sedimentology and palaeoecology of the Iren Dabasu Formation (Upper Cretaceous), Inner Mongolia, People's Republic of China |journal=Cretaceous Research |date=1993 |volume=14 |issue=2 |pages=127–144 |doi=10.1006/cres.1993.1011|bibcode=1993CrRes..14..127C }}</ref>

==See also==
{{Portal|Dinosaurs}}
* [[Timeline of therizinosaur research]]

==References==
{{Reflist}}

==External links==
* {{Commons category-inline|Segnosaurus|''Segnosaurus''}}
* {{Wikispecies-inline|Segnosaurus|''Segnosaurus''}}

{{Maniraptora|M.}}
{{Taxonbar|from=Q131102}}

{{featured article}}

[[Category:Therizinosauria]]
[[Category:Dinosaur genera]]
[[Category:Turonian dinosaurs]]
[[Category:Taxa named by Altangerel Perle]]
[[Category:Fossil taxa described in 1979]]
[[Category:Dinosaurs of Mongolia]]