[4] They first appear during the Late Pennsylvanian (Upper Carboniferous) epoch.

Basal Sphenacodontia constitute a transitional evolutionary series from early pelycosaurs to ancestral therapsids (which in turn were the ancestors of more advanced forms and finally the mammals).

The defining characteristics include a thickening of the maxilla visible on its internal surface, above the large front (caniniform) teeth; and the premaxillary teeth being set in deep sockets.

All other (sister group and more primitive) synapsid clades have teeth that are set in shallow sockets.

[3][4][5] Class Synapsida Sphenacodontia in a cladogram after Fröbisch et al., 2011:[5] Edaphosauridae Haptodus garnettensis Palaeohatteria longicaudata Pantelosaurus saxonicus Ianthodon schultzei Cutleria wilmarthi Therapsida Secodontosaurus obtusidens Cryptovenator hirschbergeri Sphenacodon Ctenospondylus Dimetrodon