Eothyrididae (asigned to Pelycosauria by Romer and Price, 1940) were a small group of very primitive, insectivorous synapsids. Only two genera are known, Eothyris and Oedaleops, both from the early Permian of North America. Their main distinguishing feature is the large caniniform tooth in from of the maxilla, eothyridids share with the Caseidae a number of specialised features associated with the morphology of the snout and external naris and it is likely that they were ancestral to them. The two together form the clade Caseasauria. Eothyris parkeyi is known from a single skull specimen; Oedaleops campi is known from three partial skulls and some parts of some limbs. The skulls are approximately 6 centimeters in length, suggesting that the total length of the animals was under 1 meter. Both species, Oedaleops campi and Eothyris parkeyi were found in the lower Permian in what is today North America. In modern cladistics, the Eothyrididae are considered to be a basal group within the Caseasauria. Caseasauria forms a sister group of the Eupelycosauria, out of which the Therapsids (the "mammal-like reptiles") evolved.
Oedaleops campi (Langston, 1965) was a insectivore eothyridid, its kull fragments from three individuals and some limb elements were found in the Abo Cutter formation in New Mexico. It is dated to the Sakmarian age of the early Permian.
Eothyris parkeyi (Romer, 1937) was a insectivore eothyridid, it is found only a single skull in the Belle Plains Formation in Texas. It belongs to the Artinskian age of the Early Permian.