View source for Lystrosaurus

{{Short description|Genus of Late Permian and Early Triassic dicynodont therapsids}}
{{use dmy dates|date=July 2024}}
{{Automatic taxobox
| fossil_range = [[Late Permian]] ([[Lopingian]])–[[Early Triassic]] ([[Olenekian]]), {{Fossil range|255|248}}
| image = Lystrosaurus hedini IMG 4469.jpg
| image_upright = 1.15
| image_caption = ''L.&nbsp;hedini'' skeletal mount from the Natural history museum of [[Zürich]]
| taxon = Lystrosaurus
| authority = [[Edward Drinker Cope|Cope]], [[1870 in paleontology|1870]]
| subdivision_ranks = Species
| subdivision = {{collapsible list|bullets = true|title=<small>List</small>
| ''L.&nbsp;murrayi'' <small>([[Thomas Henry Huxley|Huxley]], 1859) ([[Type species|type]])</small>
| ''L.&nbsp;declivis'' <small>([[Richard Owen|Owen]], 1860)</small>
| ''L.&nbsp;curvatus'' <small>(Owen, 1876)</small>
| ''L.&nbsp;maccaigi'' <small>[[Harry Seeley|Seeley]], 1898</small>
}}
}}

'''''Lystrosaurus''''' ({{IPAc-en|ˌ|l|ɪ|s|t|r|oʊ-|ˈ|s|ɔːr|ə|s}}; 'shovel lizard'; proper [[Ancient Greek]] is {{math|λίστρον}} ''lístron'' ‘tool for leveling or smoothing, shovel, spade, hoe’) is an extinct genus of [[herbivore|herbivorous]] [[dicynodont]] [[therapsid]]s from the [[Lopingian|late Permian]] and [[Early Triassic]] [[epoch (geology)|epochs]] (around 248&nbsp;million years ago). It lived in what is now [[Antarctica]], [[India]], [[China]], [[Mongolia]], [[European Russia]] and [[South Africa]]. Four to six species are currently recognized, although from the 1930s to 1970s the number of species was thought to be much higher. They ranged in size from that of a small dog to 8&nbsp;feet (2.5&nbsp;meters) long.<ref>{{cite book |last1=Cluver |first1=Michael Albert |title=Fossil reptiles of the South African Karoo |date=1978 |publisher=The South African Museum |isbn=9780908407583 |language=en }}</ref>

As a dicynodont, ''Lystrosaurus'' had only two teeth (a pair of [[tusk]]-like [[Canine tooth|canines]]), and is thought to have had a horny beak that was used for biting off pieces of vegetation. ''Lystrosaurus'' was a heavily built, herbivorous animal. The structure of its shoulders and hip joints suggests that ''Lystrosaurus'' moved with a [[Terrestrial locomotion|semi-sprawling gait]]. The forelimbs were even more robust than the hindlimbs, and the animal is thought to have been a powerful digger that nested in burrows.

''Lystrosaurus'' survived the [[Permian-Triassic extinction]], 252 million years ago. In the [[Early Triassic]], they were by far the most common terrestrial vertebrates, accounting for as many as 95% of the total individuals in some fossil beds.<ref name=Barendskraal>{{cite journal |last1=Damiani |first1=R.J. |last2=Neveling |first2=J. |last3=Modesto |first3=S.P. |last4=Yates |first4=A.M. |name-list-style=amp |year=2004 |title=Barendskraal, a diverse amniote locality from the ''Lystrosaurus'' assemblage zone, Early Triassic of South Africa |journal=Palaeontologia Africana |volume=39 |pages=53–62 }}</ref> Researchers have offered various hypotheses for why ''Lystrosaurus'' survived the extinction event and thrived in the early Triassic.

== History of discovery ==
[[File:Wegener fossils-mapped.png|thumb|350px|Map of [[Pangea]] showing locations of ''Lystrosaurus'' remains as yellow disks. Distorted boundaries of modern continents shown as grey lines. (Distributions for lystrosaurs and three other Permian and Triassic fossil groups used as biogeographic evidence for [[continental drift]] and certain [[land bridge]]s.)]]

Dr.&nbsp;Elias Root Beadle, a Philadelphia missionary and avid fossil collector, discovered the first ''Lystrosaurus'' skull. Beadle wrote to the eminent paleontologist [[Othniel Charles Marsh]], but received no reply. Marsh's rival, [[Edward Drinker Cope]], was very interested in seeing the find, and described and named ''Lystrosaurus'' in the ''Proceedings of the American Philosophical Society'' in 1870.<ref name="wallace2000">{{cite book|author=Wallace, David Rains|year=2000|title=The Bonehunters' Revenge: Dinosaurs, Greed, and the Greatest Scientific Feud of the Gilded Age|publisher=Houghton Mifflin Harcourt|isbn=978-0-618-08240-7|pages=44–45}}</ref> Its name is derived from the [[Ancient Greek]] words ''listron'', "shovel", and ''sauros'', "lizard".<ref name=Liddell1980>{{cite book |editor1-last=Liddell |editor1-first=Henry George |editor1-link = Henry George Liddell |editor2-first=Robert |editor2-last=Scott |editor2-link=Robert Scott (philologist) |name-list-style=amp |year=1980 |title=A Greek-English Lexicon |edition=abridged |title-link=A Greek-English Lexicon |publisher=[[Oxford University Press]] |place=Oxford, UK |isbn =978-0-19-910207-5 }}</ref> Marsh belatedly purchased the skull in May&nbsp;1871, although his interest in an already-described specimen was unclear; he may have wanted to carefully scrutinize Cope's description and illustration.<ref name="wallace2000"/>

=== Plate tectonics ===
The discovery of ''Lystrosaurus'' fossils at Coalsack Bluff in the [[Transantarctic Mountains]] by [[Edwin H. Colbert]] and his team in 1969–1970 helped support the hypothesis of [[plate tectonics]] and strengthen the theory, since ''Lystrosaurus'' had already been found in the lower Triassic of southern Africa as well as in India and China.<ref>{{cite news |first=Naomi |last=Lubick |date=February 2005 |title=Investigating the Antarctic |series=Geotimes |publisher=[[American Geosciences Institute]] |url=http://www.agiweb.org/geotimes/feb05/feature_Antarcticfossils.html |via=agiweb.org }}</ref><ref>{{Cite book|title=International Encyclopedia of Geography: People, the Earth, Environment and Technology |pages = 1–9 |last=Trewick |first=Steve |year=2016|publisher=John Wiley & Sons, Ltd|isbn=9781118786352|language=en|doi=10.1002/9781118786352.wbieg0638|chapter = Plate Tectonics in Biogeography}}</ref>

===Distribution and species===
''Lystrosaurus'' fossils have been found in many [[Late Permian]] and [[Early Triassic]] terrestrial [[bone bed]]s, most abundantly in Africa, and to a lesser extent in parts of what are now India, China, Mongolia, European Russia, and Antarctica (which was not over the South Pole at the time).<ref name=Surkov-Kalndze-Benton-2005>{{cite journal |last1=Surkov |first1=M.V. |last2=Kalandadze |first2=N.N. Of |last3=Benton |first3=M.J. |name-list-style=amp |date=June 2005 |title=''Lystrosaurus georgi'', a dicynodont from the Lower Triassic of Russia |journal=Journal of Vertebrate Paleontology |volume=25 |issue=2 |pages=402–413 |url=http://palaeo.gly.bris.ac.uk/benton/reprints/2005lystrosaurus.pdf |doi=10.1671/0272-4634(2005)025[0402:LGADFT]2.0.CO;2| s2cid=59028100 |issn=0272-4634 |url-status=dead |access-date=2008-07-07 |archive-url=https://web.archive.org/web/20081219184100/http://palaeo.gly.bris.ac.uk/benton/reprints/2005lystrosaurus.pdf |archive-date=2008-12-19}}</ref>

====Species found in Africa====

[[File:Lystrosaurus murrayi 7.JPG|thumb|left|''L.&nbsp;murrayi'' skeleton]] 
Most ''Lystrosaurus'' fossils have been found in the [[Balfour Formation|Balfour]] and [[Katberg Formation]]s of the [[Karoo Basin|Karoo basin]] in [[South Africa]]; these specimens offer the best prospects of identifying species because they are the most numerous and have been studied for the longest time. As so often with fossils, there is debate in the [[Scientific community|paleontological community]] as to exactly how many species have been found in the Karoo basin. Studies from the 1930s to 1970s suggested a large number (23 in one case).<ref name=Botha-Smith-2007>{{cite journal |last1=Botha |first1=J. |last2=Smith |first2=R.M.H. |name-list-style=amp |year=2005 |title=''Lystrosaurus'' species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa |journal=Lethaia| volume=40 |issue=2 |pages=125–137 |doi=10.1111/j.1502-3931.2007.00011.x}} <br/>Full version available at {{cite report |last1=Botha |last2=Smith |year=2007 |title=''Lystrosaurus'' species composition across the Permo–Triassic boundary in the Karoo Basin of South Africa |url=http://www.nasmus.co.za/PALAEO/jbotha/pdfs/Botha%20and%20Smith%202007.pdf |access-date=2008-07-02 |url-status=dead |archive-url=https://web.archive.org/web/20080910214110/http://www.nasmus.co.za/PALAEO/jbotha/pdfs/Botha%20and%20Smith%202007.pdf |archive-date=2008-09-10 }}</ref> However, by the 1980s and 1990s, only 6&nbsp;species were recognized in the [[Karoo]]: ''L.&nbsp;curvatus'', ''L.&nbsp;platyceps'', ''L.&nbsp;oviceps'', ''L.&nbsp;maccaigi'', ''L.&nbsp;murrayi'', and ''L.&nbsp;declivis''. A study in 2011 reduced that number to four, treating the fossils previously labeled as ''L.&nbsp;platyceps'' and ''L.&nbsp;oviceps'' as members of ''L.&nbsp;curvatus''.<ref>{{cite book |last1=Grine |first1=F.E. |last2=Forster |first2=C.A. |last3=Cluver |first3=M.A. |last4=Georgi |first4=J.A. |name-list-style=amp |year=2006 |title=Amniote Paleobiology: Perspectives on the evolution of mammals, birds, and reptiles |pages=432–503 |publisher=University of Chicago Press}}</ref>

''L.&nbsp;maccaigi'' is the largest and apparently most specialized species, while ''L.&nbsp;curvatus'' was the least specialized. A ''Lystrosaurus''-like fossil, ''Kwazulusaurus shakai'', has also been found in South Africa. Although not assigned to the same [[genus]], ''K.&nbsp;shakai'' is very similar to ''L.&nbsp;curvatus''. Some paleontologists have therefore proposed that ''K.&nbsp;shakai'' was possibly an ancestor of or closely related to the ancestors of ''L.&nbsp;curvatus'', while ''L.&nbsp;maccaigi'' arose from a different lineage.<ref name=Botha-Smith-2007/>
''L.&nbsp;maccaigi'' is found only in sediments from the [[Permian]] period, and apparently did not survive the [[Permian–Triassic extinction event]]. Its specialized features and sudden appearance in the [[fossil record]] without an obvious ancestor may indicate that it immigrated into the Karoo from an area in which Late Permian sediments have not been found.<ref name=Botha-Smith-2007/>
''L.&nbsp;curvatus'' is found in a relatively narrow band of sediments from shortly before and after the extinction, and can be used as an approximate marker for the boundary between the [[Permian]] and [[Triassic]] periods. A skull identified as ''L.&nbsp;curvatus'' has been found in late Permian sediments from [[Zambia]]. For many years it had been thought that there were no Permian specimens of ''L.&nbsp;curvatus'' in the Karoo, which led to suggestions that ''L.&nbsp;curvatus'' immigrated from Zambia into the Karoo. However, a re-examination of Permian specimens in the Karoo has identified some as ''L.&nbsp;curvatus'', and there is no need to assume immigration.<ref name=Botha-Smith-2007/>

''L.&nbsp;murrayi'' and ''L.&nbsp;declivis'' are found only in Triassic sediments.<ref name=Botha-Smith-2007/>

====Other species====
''Lystrosaurus georgi'' fossils have been found in the Earliest Triassic sediments of the [[Moscow]] Basin in Russia. It was probably closely related to the African ''Lystrosaurus curvatus'',<ref name=Surkov-Kalndze-Benton-2005/> which is regarded as one of the least specialized species and has been found in very Late Permian and very Early Triassic sediments.<ref name=Botha-Smith-2007/>

''L.&nbsp;murrayi'', in addition to two undescribed species presently assigned to ''L.&nbsp;curvatus'' and ''L.&nbsp;declivis'', is known from the Early Triassic [[Panchet Formation]] of the [[Damodar River|Damodar Valley]] and the [[Kamthi Formation]] of the [[Pranhita–Godavari Basin|Pranhita-Godavari Basin]] in [[India]].<ref>{{Cite journal |last1=Bandyopadhyay |first1=Saswati |last2=Ray |first2=Sanghamitra |date=2020-03-01 |title=Gondwana vertebrate faunas of India: Their diversity and intercontinental relationships |journal=Episodes Journal of International Geoscience |lang=en |volume=43 |issue=1 |pages=438–460 |doi=10.18814/epiiugs/2020/020028 |doi-access=free }}</ref> Seven ''Lystrosaurus'' species have been described from the Early Triassic [[Jiucaiyuan Formation|Jiucaiyuan]], [[Guodikeng Formation|Guodikeng]] and [[Wutonggou Formation|Wutonggou]] formations of the [[Bogda Shan|Bogda Mountains]] in [[Xinjiang]], [[China]], although it is possible that only two (''L.&nbsp;youngi'' and ''L.&nbsp;hedini'') are valid; unusually, no Chinese ''Lystrosaurus'' specimens are known below the Permian-Triassic boundary in this region.<ref>{{cite journal |last1=Angielczyk |first1=Kenneth D. |last2=Liu |first2=Jun |last3=Sidor |first3=Christian A. |last4=Yang |first4=Wan |date=2022-11-01 |title=The stratigraphic and geographic occurrences of Permo-Triassic tetrapods in the Bogda Mountains, NW China — Implications of a new cyclostratigraphic framework and Bayesian age model |journal=Journal of African Earth Sciences |volume=195 |pages=104655 |doi=10.1016/j.jafrearsci.2022.104655 |issn=1464-343X |doi-access=free }}</ref><ref>{{Cite journal |last1=Han |first1=Fenglu |last2=Zhao |first2=Qi |last3=Liu |first3=Jun |date=2021-03-18 |title=Preliminary bone histological analysis of Lystrosaurus (Therapsida: Dicynodontia) from the Lower Triassic of North China, and its implication for lifestyle and environments after the end-Permian extinction |journal=[[PLoS One]] |lang=en |volume=16 |issue=3 |pages=e0248681 |doi=10.1371/journal.pone.0248681 |doi-access=free |issn=1932-6203 |pmc=7971864 |pmid=33735263 }}</ref> ''L.&nbsp;curvatus'', ''L.&nbsp;murrayi'', and ''L.&nbsp;maccaigi'' are known from the [[Fremouw Formation]] in the [[Transantarctic Mountains]] of [[Antarctica]].<ref>{{Cite journal |last1=Cosgriff |first1=J.W. |last2=Hammer |first2=W.R. |last3=Ryan |first3=W.J. |year=1982 |title=The Pangaean reptile, ''Lystrosaurus maccaigi'', in the Lower Triassic of Antarctica |url=https://www.jstor.org/stable/1304463 |journal=Journal of Paleontology |volume=56 |issue=2 |pages=371–385 |jstor=1304463 |issn=0022-3360}}</ref>

==Description==
[[File:Lystrosaurus murrayi scale.svg|thumb|right|upright|Size of ''Lystrosaurus murrayi'' relative to a human.]]
''Lystrosaurus'' was a [[dicynodont]] [[therapsid]], between {{cvt|0.6|to|2.5|m|ft|0}} long with an average of about {{cvt|0.9|m|ft|0}} depending upon the species.<ref>{{cite encyclopedia |title=Lystrosaurus |department=Fossils & Facts |encyclopedia=Encyclopaedia Britannica |lang=en |url=https://www.britannica.com/animal/Lystrosaurus |access-date=2019-03-18}}</ref>

Unlike other therapsids, dicynodonts had very short snouts and no teeth except for the [[tusk]]-like upper [[canine teeth|canines]]. Dicynodonts are generally thought to have had [[Horn (anatomy)|horny]] [[beak]]s like those of turtles, for shearing off pieces of vegetation, which were then ground on a horny [[secondary palate]] when the mouth was shut. The jaw joint was weak and moved backwards and forwards with a shearing action, instead of the more common sideways or up and down movements. It is thought that the jaw muscles were attached unusually far forward on the skull and took up a lot of space on the top and back of the skull. As a result, the eyes were set high and well forward on the skull, and the face was short.<ref>{{cite book |last=Cowen |first=R. |year=2000 |title=The History of Life |edition=3rd |publisher=Blackwell Scientific |isbn=978-0-632-04444-3 |pages=167–168 }}</ref>

Features of the skeleton indicate that ''Lystrosaurus'' moved with a [[terrestrial locomotion|semi-sprawling gait]]. The lower rear corner of the [[scapula]] (shoulder blade) was strongly ossified (built of strong bone), which suggests that movement of the scapula contributed to the stride length of the forelimbs and reduced the sideways flexing of the body.<ref name=Surkov-Kalndze-Benton-2005/> The five [[sacral vertebrae]] were massive but not fused to each other and to the [[pelvis]], making the back more rigid and reducing sideways flexing while the animal was walking. [[Therapsida|Therapsids]] with fewer than five sacral vertebrae are thought to have had sprawling limbs, like those of modern lizards.<ref name=Surkov-Kalndze-Benton-2005/> In [[dinosaur]]s and [[mammal]]s, which have erect limbs, the sacral vertebrae are fused to each other and to the [[pelvis]].<ref>{{cite book |last=Benton |first=Michael J. |author-link=Michael J. Benton |editor-last=Weishampel |editor1-first=David B. |editor2-last=Dodson |editor2-first=Peter |editor3-last=Osmólska |editor3-first=Halszka |name-list-style=amp |year=2004 |chapter=Origin and relationships of Dinosauria |title=The Dinosauria |edition=2nd |publisher=University of California Press |place=Berkeley, CA |isbn=978-0-520-24209-8 |pages=[https://archive.org/details/dinosauriandedit00weis/page/n25 7]–19 |url=https://archive.org/details/dinosauriandedit00weis|url-access=limited }}</ref> A buttress above each acetabulum (hip socket) is thought to have prevented [[Joint dislocation|dislocation]] of the [[femur]] (thigh bone) while ''Lystrosaurus'' was walking with a semi-sprawling gait.<ref name=Surkov-Kalndze-Benton-2005/> The forelimbs of ''Lystrosaurus'' were massive,<ref name=Surkov-Kalndze-Benton-2005/> and ''Lystrosaurus'' is thought to have been a powerful burrower.

Mummified specimens recovered from the Karoo Basin and described in 2022 revealed that ''Lystrosaurus'' had dimpled, leathery, and hairless skin.<ref>{{cite journal |author1=Smith, R.M.H. |author2=Botha, J. |author3=Vigilietti, P.A. |year=2022|title=Taphonomy of drought afflicted tetrapods in the Early Triassic Karoo Basin, South Africa |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |volume=604 |at=111207 |doi=10.1016/j.palaeo.2022.111207|bibcode=2022PPP...60411207S |s2cid=251781291 }}</ref><ref>{{cite news |title=These fossil mummies reveal a brutal world long before ''T.&nbsp;rex'' lived | date=20 September 2022 |website=gizmodo.com |url=https://gizmodo.com/these-fossil-mummies-reveal-a-brutal-world-long-before-1849558556 }}</ref>

==Paleoecology==

===Dominance of the Early Triassic===

[[File:Lystr georg1DB.jpg|thumb|alt=A pink/grey four-footed animal. The head is facing slightly toward you, and has two big teeth. It is covered in fur and has sharp claws.|''Lystrosaurus georgi'']]
''Lystrosaurus'' is notable for dominating southern [[Pangaea]] for millions of years during the [[Early Triassic]]. At least one unidentified species of this genus survived the [[Permian–Triassic extinction event|end-Permian mass extinction]] and, in the absence of predators and herbivorous competitors, went on to thrive and [[evolutionary radiation|re-radiate]] into a number of species within the genus,<ref name=Benton-2006>{{cite book |first=M.J. |last=Benton |author-link=Michael J. Benton |year=2006 |title=When Life Nearly Died: The greatest mass extinction of all time |isbn=978-0-500-28573-2 |publisher=Thames & Hudson |location=London, UK }}</ref> becoming the most common group of terrestrial vertebrates during the Early Triassic; for a while, 95% of land vertebrates were ''Lystrosaurus''.<ref name=Benton-2006/><ref name=Cokinos-2007>{{cite magazine |first=Christopher |last=Cokinos |title=The consolations of extinction |orig-date=May-June 2007 |date=12 October 2007 |magazine=[[Orion (magazine)|Orion]] |url=http://www.orionmagazine.org/index.php/articles/article/268/ |archive-url=https://web.archive.org/web/20071012225407/http://www.orionmagazine.org/index.php/articles/article/268/ |archive-date=2007-10-12}}</ref> This is the only time that a single species or genus of land animal dominated the Earth to such a degree.<ref>{{cite episode |title=Clash of titans |series-no=3 |date=November 2005 |series=Life before Dinosaurs (renamed 'Walking with Monsters') |series-link=Walking with Monsters |network=[[BBC]], [[Discovery Channel]] }}</ref> A few other Permian therapsid genera also survived the mass extinction and appear in Triassic rocks—the [[therocephalia]]ns ''[[Tetracynodon]]'', ''[[Moschorhinus]]'', ''[[Ictidosuchoides]]'' and ''[[Promoschorhynchus]]''—but do not appear to have been abundant in the Triassic;<ref name=Botha-Smith-2007/><ref>{{cite journal |last=Huttenlocker |first=A.K. |author2=Sidor, C.A. |author3= Smith, R.M.H.  |year=2011 |title=A new specimen of ''Promoschorhynchus'' (Therapsida: Therocephalia: Akidnognathidae) from the Lower Triassic of South Africa and its implications for theriodont survivorship across the Permo-Triassic boundary |journal=Journal of Vertebrate Paleontology |volume=31 |issue=2 |pages=405–421 |doi=10.1080/02724634.2011.546720 |bibcode=2011JVPal..31..405H |s2cid=129242450 }}</ref> complete ecological recovery took 30 million years, spanning the [[Early Triassic|Early]] and [[Middle Triassic]].<ref>{{cite journal |last1=Sahney |first1=S. |last2=Benton |first2=M.J. |name-list-style=amp |year=2008 |title=Recovery from the most profound mass extinction of all time |journal=Proceedings of the Royal Society B: Biological Sciences |doi=10.1098/rspb.2007.1370 |volume = 275 |issue=1636 |pages = 759–65 |pmid=18198148 |pmc=2596898}}</ref>

Several attempts have been made to explain why ''Lystrosaurus'' survived the [[Permian–Triassic extinction event]], the "mother of all mass extinctions",<ref name="Erwin1993">{{cite book |last=Erwin |first=D.H. |year=1993 |title=The Great Paleozoic Crisis: Life and death in the Permian |publisher=Columbia University Press |isbn=978-0-231-07467-4 }}</ref> and why it dominated Early Triassic fauna to such an unprecedented extent:

[[File:Lystr murr1DB.jpg|thumb|right|''Lystrosaurus murrayi'']]
* Growth marks in fossilized tusks suggest that ''Lystrosaurus'' living in Antarctica [[Early Triassic|~250&nbsp;Mya]] could enter a prolonged state of [[torpor]] analogous to [[hibernation]]. This could be the oldest evidence of a hibernation-like state in a vertebrate animal and indicates that torpor arose in vertebrates before mammals and dinosaurs evolved.<ref name="Lystrosaurus">{{cite news |title=Fossil evidence of 'hibernation-like' state in 250&nbsp;million-year-old Antarctic animal |date=August 2020 |website=phys.org |lang=en |url=https://phys.org/news/2020-08-fossil-evidence-hibernation-like-state-million-year-old.html |access-date=7 September 2020 }}</ref><ref>{{cite news |title=Fossil suggests animals have been hibernating for 250&nbsp;million years |work=[[UPI]] |url=https://www.upi.com/Science_News/2020/08/27/Fossil-suggests-animals-have-been-hibernating-for-250-million-years/8421598530111/ |access-date=7 September 2020 |lang=en}}</ref><ref>{{cite journal |last1=Whitney |first1=Megan R. |last2=Sidor |first2=Christian A. |date=27 August 2020 |title=Evidence of torpor in the tusks of Lystrosaurus from the Early Triassic of Antarctica |journal=Communications Biology |volume=3 |issue=1 |page=471 |doi=10.1038/s42003-020-01207-6 |pmid=32855434 |pmc=7453012 |language=en |issn=2399-3642}} [[File:CC-BY icon.svg|50px]]  Text and images are available under a [https://creativecommons.org/licenses/by/4.0/  Creative Commons Attribution&nbsp;4.0 International License].</ref>
* One of the more recent theories is that the extinction event reduced the atmosphere's oxygen content and increased its [[carbon dioxide]] content, so that many terrestrial species died out because they found breathing too difficult.<ref name=Cokinos-2007/> It has therefore been suggested that ''Lystrosaurus'' survived and became dominant because its burrowing life-style made it able to cope with an atmosphere of "stale air", and that specific features of its anatomy were part of this adaptation: a barrel chest that accommodated large lungs, short internal [[nostril]]s that facilitated rapid breathing, and high [[neural spine]]s (projections on the dorsal side of the [[vertebra]]e) that gave greater leverage to the muscles that expanded and contracted its chest. However, there are weaknesses in all these points: the chest of ''Lystrosaurus'' was not significantly larger in proportion to its size than in other [[dicynodont]]s that became extinct; although Triassic dicynodonts appear to have had longer neural spines than their Permian counterparts, this feature may be related to posture, locomotion or even body size rather than respiratory efficiency; ''L.&nbsp;murrayi'' and ''L.&nbsp;declivis'' are much more abundant than other Early Triassic burrowers such as ''[[Procolophon]]'' or ''[[Thrinaxodon]]''.<ref name=Botha-Smith-2007/>
[[File:Lystrosaurus skeleton.jpg|thumb|right|alt= Full body view of the skeleton of a four footed animal.|''Lystrosaurus'' skeletal diagram]]
* The suggestion that ''Lystrosaurus'' was helped to survive and dominate by being semi-aquatic has a similar weakness: although [[temnospondyl]]s become more abundant in the Karoo's Triassic sediments, they were much less numerous than ''L.&nbsp;murrayi'' and ''L.&nbsp;declivis''.<ref name=Botha-Smith-2007/>
* The most specialized and the largest animals are at higher risk in mass extinctions; this may explain why the unspecialized ''L.&nbsp;curvatus'' survived while the larger and more specialized ''L.&nbsp;maccaigi'' perished along with all the other large Permian herbivores and carnivores.<ref name=Botha-Smith-2007/> Although ''Lystrosaurus'' generally looks adapted to feed on plants similar to ''[[Dicroidium]]'', which dominated the Early Triassic, the larger size of ''L.&nbsp;maccaigi'' may have forced it to rely on the larger members of the ''[[Glossopteris]]'' flora, which did not survive the end-Permian extinction.<ref name=Botha-Smith-2007/>
* Only the {{cvt|1.5|m|ft|0}}–long therocephalian ''[[Moschorhinus]]'' and the large [[archosauriform]] ''[[Proterosuchus]]'' appear to be large enough to have preyed on the Triassic ''Lystrosaurus'' species, and this shortage of predators may have been responsible for a ''Lystrosaurus'' population boom in the Early Triassic.<ref name=Botha-Smith-2007/>
* According to Benton, "Perhaps the survival of ''Lystrosaurus'' was simply a matter of luck".<ref name=Benton-2006/>

==See also==
{{Portal|Geology|Paleontology}}
* {{annotated link|Evolution of mammals}}
* {{annotated link|Lists of synapsids}}

==References==
{{Reflist|25em}}

==External links==
{{Commons category|Lystrosaurus}}
* {{cite web |title=Dicynodontia |website=Palaeos.com |id=400.725 |url=http://www.palaeos.com/Vertebrates/Units/400Therapsida/400.725.html#Dicynodontia  |url-status=dead <!-- presumed --> |archive-url=https://web.archive.org/web/20060222040556/http://www.palaeos.com/Vertebrates/Units/400Therapsida/400.725.html#Dicynodontia |archive-date=2006-02-22 }}
* {{cite web |first=Hugh |last=Rance |title=Mammal-like reptiles of Pangea |series=The present is the key to the past |website=geowords.com |url=http://geowords.com/histbooknetscape/i07.htm |url-status=dead <!-- presumed --> |archive-url=https://web.archive.org/web/20060427060532/http://geowords.com/histbooknetscape/i07.htm |archive-date=2006-04-27 }}

{{Anomodontia|O.}}
{{Geology of South Africa|paleontology}}
{{Taxonbar|from=Q131680}}

[[Category:Dicynodontia]]
[[Category:Triassic synapsids]]
[[Category:Permian–Triassic extinction event]]
[[Category:Prehistoric synapsids of Asia]]
[[Category:Lopingian synapsids of Africa]]
[[Category:Early Triassic synapsids]]
[[Category:Prehistoric vertebrates of Antarctica]]
[[Category:Triassic Antarctica]]
[[Category:Taxa named by Edward Drinker Cope]]
[[Category:Fossil taxa described in 1870]]
[[Category:Lopingian genus first appearances]]
[[Category:Changhsingian genera]]
[[Category:Induan genera]]
[[Category:Early Triassic genus extinctions]]
[[Category:Anomodont genera]]