Eothyris is a genus of extinct synapsid in the family Eothyrididae from the early Permian. It was a carnivorous insectivorous animal, closely related to Oedaleops. Only the skull of Eothyris, first described in 1937, is known. It had a 6-centimetre-long (2.4-inch) skull, and its total estimated length is 30 centimetres (12 inches).[2][3] Eothyris is one of the most primitive synapsids known and is probably very similar to the common ancestor of all synapsids in many respects. The only known specimen of Eothyris was collected from the Artinskian-lower.[2][4][5]
Eothyris parkeyi was one of many new species of "pelycosaurs" discovered by Alfred Sherwood Romer as part of a series of paleontological expeditions for the Museum of Comparative Zoology (MCZ). The genoholotype (type specimen of a genus) and only known specimen of Eothyris is a complete skull and associated jaws, with the collection number MCZ 1161. This skull was collected about one mile west of the former Woodrum ranch-house, in Early Permian sediments of the Artinskian-lower Kungurian Belle Plains Formation, Wichita Group, south of Dundee, Archer County, Texas, USA.[2] in rocks which are about 275 million years old.[5] The generic name is Greek for "dawn opening", likely in reference for its status as one of the most primitive known synapsids. The specific name refers to J.R. Parkey, of Mankins, Texas, a local landowner who assisted the MCZ field crew in collecting fossils on his property. Eothyris was briefly described by Romer in 1937, and given a more comprehensive description by Romer and Llewellyn Ivor Price in 1940.[3]
Eothyris is known only from its complete skull, however, the postcranial skeleton is unknown. Its skull is short and broad with a total skull length of 5.7 cm (2.25 in), Benson et al. estimated the body to be 30 cm (12 in) long.[6] The major distinguishing features of the skull are that it possessed a pair of long, large, fang-like teeth on each side of the upper jaw. The main differences between Eothyris and Oedaleops are related to the degree of specialization in the dentition of the geologically younger Eothyris.[7]
The superficial dermal elements are preserved, and the occiput is visible. The skull is clearly low, very broad, and short. With normal length of postorbital and orbital regions. The face is much shorter than any other pelycosaur except edaphosaurs.[8] Lacrimal expanded laterally upper the canines. Maxilla contributes to the edge of the orbit as there is no lacrimal jugal contact in the orbit. The squamosal and postorbital are extensively in contact because the temporal opening is small and of oval shape. Also, because the cheek slants outward to very higher degree than is usual in pelycosaurs, the temporal opening is exposed in dorsal view. The premaxillae and nasals are short and the frontals are of moderate length. The supratemporal in Eothyris parkeyi is much more extremely developed than in any other pelycosaur. In Eothyris parkeyi, the postorbital bone is probably the largest for any synapsid, with large lateral and dorsal portions to the skull roof. The slope from skull table to quadrate is gentle, due to shortness of skull. The quadratojugal is very long and jaw articulation is in line with the tooth row. Typical pterygoid flanges and slender are present back in the skull, below the anterior part of the temporal opening. The quadrate is on the right side and is exposed dorsally. The interparietal and tabulars occupy the dorsal rim of the occipital surface.[3][7][9][10][11]