Caseasauria

 ( Pennsylvanian First Appearances ) 

Caseasauria is one of the two main of early synapsids, the other being the Eupelycosauria. Caseasaurs are currently known only from the Late Carboniferous and the Permian, and include two superficially different families, the small insectivorous or carnivorous Eothyrididae, and the large, herbivory Caseidae. These two groups share a number of specialised features associated with the morphology of the snout and external nares.

The ancestor of caseasaurs can be traced back to an insect eating or an omnivorous reptile-like synapsid from the Pennsylvanian time of the Carboniferous, possibly resembling Archaeothyris, the earliest known synapsid. The caseasaurs were abundant during the later part of the Early Permian, but by the Middle Permian caseasaur diversity declined because the group was outcompeted by the more successful . The last caseasaurs became extinct at the end of the Guadalupian (Middle Permian).

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Description Among the most conspicuous characteristics uniting caseasaurs are an enlarged nostril and a snout tip that overhangs the tooth row.

Early caseasaurs, including all eothyridids, were relatively small animals. However, most caseids reached larger sizes, and some caseids, such as Cotylorhynchus and Alierasaurus, were among the largest terrestrial animals of the early Permian. These large herbivorous taxa reached a length of and a mass of .

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Evolution Caseasaurs first appear in the fossil record in the late Carboniferous, alongside many other early amniote groups. The earliest known synapsid, Asaphestera from the Bashkirian age, may be an eothyridid caseasaur. The earliest definitive caseasaur is Eocasea.

Caseids thrived during the Kungurian age, and numerous large herbivorous caseids are known from the Kungurian of the United States.

Caseasaurs are one of the two pelycosaur clades known to have survived into the Guadalupian epoch, along with varanopids. Two caseasaur taxa are known from the Guadalupian of Russia, representing the geologically youngest known caseasaurs: the small, possibly omnivorous or insectivorous Phreatophasma, and the large, herbivorous Ennatosaurus.

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Classification Caseasauria is generally regarded as the most basal clade of synapsids, with all other synapsids being grouped in the clade Eupelycosauria. However, not all studies have supported this position. In 2012, Roger Benson argued that most of the characters supporting a basal position for caseasaurs pertained to the skull, and presented a phylogenetic analysis incorporating more postcranial data that resolved a clade comprising ophiacodontids and varanopids as the basalmost synapsid clade. However, new postcranial data from eothyridids and basal caseids established that caseasaurs were more basal than ophiacodontids and varanopids after all, with the characters supporting a more derived position for caseasaurs being the result of convergent evolution between caseids and more derived synapsids. The diadectomorphs, conventionally regarded as anamniote tetrapods, may prove to be synapsids even more basal than Caseasauria.

Most caseasaurs are divided into two families, Eothyrididae and Caseidae. The affinities of the earliest definitive caseasaur, Eocasea, are uncertain, with some phylogenetic analyses finding it to be a caseid and others finding it to be a basal caseasaur belonging to neither family.

Three genera are typically regarded as belonging to the family Eothyrididae: Eothyris, Oedaleops, and Vaughnictis. However, some phylogenetic analyses have failed to resolve the eothyridids as a clade, instead finding them to be paraphyletic with respect to Caseidae. Asaphestera has been provisionally regarded as an eothyridid as well, without being included in a phylogenetic analysis.

The remaining caseasaurs belong to the family Caseidae.

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List of species

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Eocasea	E. martini	2014	incertae sedis	Gzhelian		May be a caseid
Asaphestera	A. platyris	1934	Eothyrididae	Bashkirian		Synapsida incertae sedis; may be an eothyridid
Eothyris	E. parkeyi	1937
Oedaleops	O. campi	1965
Vaughnictis	V. smithae	1965		Asselian–Sakmarian		Originally assigned to the genus Mycterosaurus
Callibrachion	C. gaudryi	1893	Caseidae
Datheosaurus	D. macrourus	1904
Oromycter	O. dolesorum	2005
Phreatophasma	P. aenigmatum	1954		Roadian
Martensius	M. bromackerensis	2020
Ruthenosaurus	R. russellorum	2011
Casea	C. broilii	1910
	C. halselli	1954
	C. nicholsi	1954
Euromycter	E. rutenus	1974				Originally described as Casea rutena
Ennatosaurus	E. tecton	1956				The geologically youngest known caseasaur
Caseopsis	C. agilis	1962
Caseoides	C. sanangelensis	1953
Arisierpeton	A. simplex	2019
Alierasaurus	A. ronchii	2014
Cotylorhynchus	C. romeri	1937
	C. hancocki	1953
	C. bransoni	1962
Angelosaurus	A. dolani	1953
	A. greeni	1962
	A. romeri	1962
Trichasaurus	T. texensis	1910

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Paleoecology The paleoecology of caseids is debated. They are typically interpreted as terrestrial animals of dry, upland habitats. However, Caseids exhibit a similar bone microstructure to cetaceans and pinnipeds, which has led to the hypothesis that they led an aquatic lifestyle. This hypothesis has been challenged on the grounds that their bone microstructure specifically resembles fully pelagic animals, and is unlike the bone microstructure of semiaquatic animals, but that the body plan of caseids is inconsistent with a pelagic lifestyle. Moreover, caseid fossils are predominantly associated with arid upland deposits.

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See also

• Evolution of mammals
• Vertebrate paleontology
• Permian tetrapods

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Bibliography

• (2018). 9783110341553, De Gruyter. . ISBN 9783110341553
• Robert R. Reisz (1986). 9783437304866, Gustav Fischer Verlag. ISBN 9783437304866
• (2025). 9789400768406, Springer Netherlands. ISBN 9789400768406

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External links

• Palaeos Vertebrates 390.100 Synapsida

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