Sphenacodontia
Sphenacodontia is a stem-based clade of derived synapsids. It was defined by Amson and Laurin as "the largest clade that includes Haptodus baylei, Haptodus garnettensis and Sphenacodon ferox, but not Edaphosaurus pogonias". They first appear during the Late Pennsylvanian epoch. From the end of the Carboniferous to the end of the Permian, most of them remained large, with only some secondarily becoming small in size.
Basal Sphenacodontia constitute a transitional evolutionary series from early pelycosaurs to ancestral therapsids. One might say that the sphenacodontians are proto-therapsids.
Characteristics
The defining characteristics include a thickening of the maxilla visible on its internal surface, above the large front teeth; and the premaxillary teeth being set in deep sockets. All other synapsid clades have teeth that are set in shallow sockets.Classification
The following taxonomy follows Fröbisch et al., Benson and Spinder unless otherwise noted.Class Synapsida
- Eupelycosauria
- * Sphenacodontia
- ** †Haptodus
- ** †Hypselohaptodus
- ** †Ianthodon
- ** Pantherapsida
- *** †Kenomagnathus
- *** †Tetraceratops
- *** †Palaeohatteriidae
- *** Sphenacodontoidea
- **** †Shashajaia
- **** †Sphenacodontidae
- **** Therapsida
Phylogeny