What is "sphenacodontoidea"

Sphenacodontoidea is a node-based clade that defined to include the most recent common ancestor of the Sphenacodontidae and the Therapsida and their descendants (including mammals). They are defined according to a number of specialised characteristics concerning proportions of the bones of the skull and the teeth.

The Sphenacodontoidea evolved from earlier Sphenacodontia such as Haptodus via a number of transitional stages of small, unspecialised pelycosaurs.