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Therapsida
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==Characteristics== [[Image:Pristeroognathus DB.jpg|thumb|left|Illustration of ''[[Alopecognathus]]'', an early [[therocephalia]]n therapsid]] ===Jaw and teeth=== Therapsids' [[temporal fenestra]]e were larger than those of the pelycosaurs. The jaws of some therapsids were more complex and powerful, and the [[teeth]] were differentiated into frontal [[incisor]]s for nipping, great lateral [[canine tooth|canines]] for puncturing and tearing, and [[molar (tooth)|molars]] for shearing and chopping food. ===Posture=== Therapsid legs were positioned more vertically beneath their bodies than were the sprawling legs of [[reptile]]s and pelycosaurs. Also compared to these groups, the feet were more symmetrical, with the first and last toes short and the middle toes long, an indication that the foot's [[Axis (anatomy)|axis]] was placed parallel to that of the animal, not sprawling out sideways. This orientation would have given a more [[mammal]]-like gait than the [[lizard]]-like gait of the pelycosaurs.<ref>{{cite book|last=Carroll|author-link=Robert L. Carroll|first=R. L.|title=Vertebrate Paleontology and Evolution|year=1988|publisher=W. H. Freeman and Company|location=New York|isbn=978-0-7167-1822-2|pages=[https://archive.org/details/vertebratepaleon0000carr/page/698 698]|title-link=Vertebrate Paleontology and Evolution}}</ref> ===Physiology=== The physiology of therapsids is poorly understood. Most Permian therapsids had a pineal foramen, indicating that they had a [[parietal eye]] like many modern reptiles and amphibians. The parietal eye serves an important role in thermoregulation and the [[circadian rhythm]] of ectotherms, but is absent in modern mammals, which are [[endothermic]].<ref name=Benoit2016B/> Near the end of the Permian, dicynodonts, [[therocephalia]]ns and cynodonts show [[parallel evolution|parallel]] trends towards loss of the pineal foramen, and the foramen is completely absent in [[probainognathian cynodont]]s. Evidence from oxygen isotopes, which are correlated with body temperature, suggests that most Permian therapsids were ectotherms and that endothermy evolved convergently in dicynodonts and cynodonts near the end of the Permian.<ref name=Rey2017/> In contrast, evidence from histology suggests that endothermy is shared across Therapsida,<ref name=FraureBrac2020/> whereas estimates of blood flow rate and lifespan in the mammaliaform ''[[Morganucodon]]'' suggest that even early mammaliaforms had reptile-like metabolic rates.<ref name=Newham2020/> Evidence for respiratory turbinates, which have been hypothesized to be indicative of endothermy, was reported in the therocephalian ''[[Glanosuchus]]'', but subsequent study showed that the apparent attachment sites for turbinates may simply be the result of distortion of the skull.<ref name=Hopson2012/> ===Integument=== The evolution of integument in therapsids is poorly known, and there are few fossils that provide direct evidence for the presence or absence of fur. The most basal synapsids with unambiguous direct evidence of fur are [[docodonts]], which are mammaliaforms very closely related to crown-group mammals. Two "mummified" juvenile specimens of the dicynodont ''[[Lystrosaurus murrayi]]'' preserve skin impressions; the skin is hairless, leathery, and dimpled, somewhat comparable to elephant skin.<ref>{{Cite journal |last1=Smith |first1=Roger M.H. |last2=Botha |first2=Jennifer |last3=Viglietti |first3=Pia A. |date=15 October 2022 |title=Taphonomy of drought afflicted tetrapods in the Early Triassic Karoo Basin, South Africa |url=https://linkinghub.elsevier.com/retrieve/pii/S0031018222003777 |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |language=en |volume=604 |pages=111207 |doi=10.1016/j.palaeo.2022.111207|bibcode=2022PPP...60411207S |url-access=subscription }}</ref><ref>{{Cite web |last=Timmons |first=Jeanne |date=2022-09-20 |title=These Fossil Mummies Reveal a Brutal World Long Before T. Rex Lived |url=https://gizmodo.com/these-fossil-mummies-reveal-a-brutal-world-long-before-1849558556 |access-date=2024-07-25 |website=Gizmodo |language=en-US}}</ref> Fossilized facial skin from the dinocephalian ''[[Estemmenosuchus]]'' has been described as showing that the skin was glandular and lacked both scales and hair.<ref name=Chudinov1968/> [[Coprolite]]s containing what appear to be hairs have been found from the [[Late Permian]].<ref name=Smith2011/><ref name=Bajdek2016/> Though the source of these hairs is not known with certainty, they may suggest that hair was present in at least some Permian therapsids. The closure of the pineal foramen in probainognathian cynodonts may indicate a mutation in the regulatory gene Msx2, which is involved in both the closure of the skull roof and the maintenance of hair follicles in mice.<ref name=Benoit2016A/> This suggests that hair may have first evolved in probainognathians, though it does not entirely rule out an earlier origin of fur.<ref name=Benoit2016A/> Whiskers probably evolved in probainognathian cynodonts.<ref name=Benoit2016A/><ref name=Benoit2020/> Some studies had inferred an earlier origin for whiskers based on the presence of foramina on the snout of therocephalians and early cynodonts, but the arrangement of foramina in these taxa actually closely resembles lizards,<ref name=Estes1961/> which would make the presence of mammal-like whiskers unlikely.<ref name=Benoit2020/>
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