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 their common ancestor was close in build to Eothyris. The two together form the clade Caseasauria.
Eothyris is known from a single skull specimen; Oedaleops is known from three partial skulls and some parts of some limbs. Vaughnictis is known from a partial skull, six dorsal vertebrae and some hind-limb bones. The skulls are approximately six centimeters in length, suggesting that the total length of the animals was under one meter.
The species 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 evolved.
Romer, AS & Price L.I (1940), Review of the Pelycosauria. Geol. Soc. Amer. Spec. Papers 28: 1–538.
Brocklehurst, N., Reisz, R. R., Fernandez, V. & Fröbisch, J. (2016) A Re-Description of 'Mycterosaurus' smithae, an Early Permian Eothyridid, and Its Impact on the Phylogeny of Pelycosaurian-Grade Synapsids. PlosOne 11:e0156810.
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