Caseasauria is one of the two main clades 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, herbivorous Caseidae. These two groups share a number of specialised features associated with the morphology of the snout and external naris.
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 therapsids. The last caseasaurs became extinct at the end of the Guadelupian (Middle Permian).
Among the most conspicuous characteristics uniting caseasaurs are an enlarged nostril and a snout tip that overhangs the tooth row.[1]
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 4 to 6 meters (13 to 20 ft) and a mass of 330 to 500 kilograms (730 to 1,100 lb).[2]
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.[3] The earliest definitive caseasaur is Eocasea.[4]
Caseids thrived during the Kungurian age, and numerous large herbivorous caseids are known from the Kungurian of the United States.