Sphenacodontoidea

Sphenacodontoidea is a node-based clade that is defined to include the most recent common ancestor of Sphenacodontidae and Therapsida and its descendants (including mammals).^[1] Sphenacodontoids are characterised by a number of synapomorphies concerning proportions of the bones of the skull and the teeth.^[2][3]

Sphenacodontoids Temporal range: Late Carboniferous-Recent, 300–0 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Fossilized skull of two sphenacodontoids: Clelandina (Therapsida, Gorgonopsidae) and a Dimetrodon (Sphenacodontidae).
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Sphenacomorpha
Clade:	Sphenacodontia
Clade:	Pantherapsida
Clade:	Sphenacodontoidea Marsh, 1878
Clades
†Shashajaia †Sphenacodontidae Therapsida

The sphenacodontoids evolved from earlier sphenacodonts such as Haptodus and Ianthodon via a number of transitional stages of small, 1-10 kg, faunivore animals.^[1] The possible common ancestor of sphenacodontids and therapsids was a carnivorous synapsid that reached moderate or large size and more closely resembled the land-dominant Early Permian sphenacodontids than the small Haptodus.^[2] The first predators among Sphenacodontoidea, like Shashajaia, appeared in the tropical western part of Pangea in the Late Carboniferous.^[1] Later, in Permian, sphenacodontoids gave rise to the dominant terrestrial carnivores in both sphenacodontid and therapsid groups.^[2]

Classification

The following taxonomy follows Fröbisch et al. (2011) and Benson (2012) unless otherwise noted.^[4][5]

Class Synapsida

• Sphenacodontoidea
  • Family †Sphenacodontidae
  • Therapsida

Phylogeny

Sphenacodontoidea in a cladogram modified from Huttenlocker et al. (2021):^[1]

Synapsida	Caseasauria Eupelycosauria Varanopidae Ophiacodontidae Sphenacomorpha Edaphosauridae Sphenacodontia Haptodus Ianthodon Palaeohatteria Pantelosaurus Sphenacodontoidea Sphenacodontidae Cutleria Secodontosaurus Cryptovenator Sphenacodon Ctenospondylus Dimetrodon Shashajaia bermani Therapsida Raranimus Dinocephalia Anomodontia Biarmosuchia Gorgonopsia Therocephalia to Mammaliaformes

See also

• Evolution of mammals