Sphenacodon

Sphenacodon (meaning "wedge point tooth") is an extinct genus of synapsid that lived from about 300 to about 280 million years ago (Ma) during the Late Carboniferous and Early Permian periods. Like the closely related Dimetrodon, Sphenacodon was a carnivorous member of the Eupelycosauria family Sphenacodontidae. However, Sphenacodon had a low crest along its back, formed from blade-like bones on its vertebrae (neural spines) instead of the tall dorsal sail found in Dimetrodon. Fossils of Sphenacodon are known from New Mexico and the Utah–Arizona border region in North America.

Researchers currently recognize two species: Sphenacodon ferox (the type species) and Sphenacodon ferocior. Sphenacodon ferocior can be up to 40% larger in overall size (at about long) compared to Sphenacodon ferox (at about ). In addition, the dorsal spines in Sphenacodon ferocior are proportionately 45% taller than in Sphenacodon ferox. The recent discovery of a nearly complete skull of Sphenacodon ferox has helped clarify other distinctions between the two species, including the number of teeth in certain parts of the jaws and the size of the indented notch between the maxillary and premaxillary bones in the upper jaw. The two species occur together in some formations, but Sphenacodon ferox apparently survived later into the Early Permian.