The Sphenacodontidae are a family of small to large, advanced, carnivorous, Late Pennsylvanian to middle Permian pelycosaurs. Primitive forms (Haptodus, etc) were generally small in size (60 cm to 1 meter), but during the later part of the early Permian these animals grew progressively larger (up to 3 meters or more), to become the top predators of their environments.
The skull is long, deep and narrow, an adaptation for strong jaw muscles. The front teeth are large and dagger-like, whereas the teeth in the sides and rear of the jaw are much smaller. (hence the name of the well-known genus Dimetrodon - "two-measure teeth", although all members of the family have this attribute).
Several large (c. 3 meters) and advanced members of this group (Secodontosaurus, Ctenospondylus, Dimetrodon) are distinguished by a tall sail along the back, made up of elongated vertebral neural spines, which in life must have been covered with skin and blood vessels, and presumably functioned as a thermoregulatory device. However, possession of a sail does not appear to have been essential for these animals. For example there is the case in which one genus (Sphenacodon - fossils known from New Mexico) lacks a sail, while a very similar and closely related genus (Dimetrodon - fossils known from Texas) has one. During the Permian, these two regions were separated by a narrow sea-way, but it is not clear why one geographically isolated group should evolve a sail, but the other group not.
The family Sphenacodontidae is actually paraphyletic as originally described, defined by shared primitive synapsid characters; these animals constitute an evolutionary gradation from primitive synapsid to early therapsid. The clade Sphenacodontia is used to designate the monophyletic group that includes Sphenacodontids and all their descendants (including mammals), while Sphenacodontidae in the strict sense includes only specialised pelycosaurs, and not earlier more primitive members of the family like Haptodus, Palaeohatteria, Pantelosaurus, and Cutleria (in pre-cladistic classifications all included under the genus Haptodus). The clade Sphenacodontoidea is used by Laurin and Reisz 1997 to designate the most recent common ancestor of Sphenacodontidae and Therapsida and all their descendants, and is defined by certain features of the skull.
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