Caseasauria is one of the two main

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

s of early

synapsids Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes rept ...

, the other being the Eupelycosauria. Caseasaurs are currently known only from the

Late Carboniferous Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effe ...

and the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...

, and include two superficially different families, the small insectivorous or carnivorous

Eothyrididae Eothyrididae is an extinct family of very primitive, insectivorous synapsids. Only three genera are known, '' Eothyris'', '' Vaughnictis'' and '' Oedaleops'', all from the early Permian of North America. Their main distinguishing feature is the la ...

, and the large, herbivorous

Caseidae Caseidae are an extinct family of basal synapsids that lived from the Late Carboniferous to Middle Permian between about 300 and 265 million years ago. Fossils of these animals come from the south-central part of the United States (Texas, Okla ...

. These two groups share a number of specialised features associated with the morphology of the snout and external

naris A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called tu ...

. The ancestor of caseasaurs can be traced back to an insect eating or an omnivorous

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...

-like synapsid from the Pennsylvanian time of the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

, possibly resembling '' Archaeothyris'', the earliest known synapsid. The caseasaurs were abundant during the later part of the Early Permian, but by the

Middle Permian The Guadalupian is the second and middle series/ epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± ...

caseasaur diversity declined because the group was outcompeted by the more successful therapsids. The last caseasaurs became extinct at the end of the Guadelupian (Middle Permian).

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 .

Evolution

Caseasaurs first appear in the fossil record in the late

, alongside many other early amniote groups. The earliest known synapsid, '' Asaphestera'' from the

Bashkirian The Bashkirian is in the ICS geologic timescale the lowest stage or oldest age of the Pennsylvanian. The Bashkirian age lasted from to Ma, is preceded by the Serpukhovian and is followed by the Moscovian. The Bashkirian overlaps with the ...

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 The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

. Caseasaurs are one of the two synapsid clades known to have survived into the

Guadalupian The Guadalupian is the second and middle series/ epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 � ...

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''.

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,

and

. 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.

List of species

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.

References

Bibliography

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External links

Palaeos Vertebrates 390.100 Synapsida
{{Taxonbar, from=Q131807 Pennsylvanian first appearances Guadalupian extinctions Taxa named by Samuel Wendell Williston