Secodontosaurus (meaning "cutting-tooth lizard") is an extinct genus of "pelycosaur" synapsids that lived from between about 285 to 272 million years ago during the Early Permian.

[1] Its unusual long, narrow jaws suggest that Secodontosaurus may have been specialized for catching fish or for hunting prey that lived or hid in burrows or crevices.

In recent years, teams from the Houston Museum of Natural Science have recovered remains in the Clear Fork Red Beds of North Texas that appear to be new specimens of Secodontosaurus.

[2] The postcranial skeletal material from different individuals includes parts of the backbone with clear evidence of a tall sail very similar to that of Dimetrodon.

Instead, the long, narrow snout with forward slanting teeth at the mandible tip might have allowed Secodontosaurus to probe after small animals hiding in burrows and other tight spaces.

The American paleontologist Edward Drinker Cope[6] published the first description of Secodontosaurus material in 1880 as a supposed species of his genus Theropleura ("mammal rib").

The deep-skulled, but low-crested, Sphenacodon would be outside the Secodontosaurus-Dimetrodon clade and would retain characteristics of an earlier stage of sphenacodontid evolution before a tall, thin-spined dorsal sail evolved.

Most recent analyses[11][12] favor a monophyletic group Sphenacodontinae composed of Dimetrodon, Sphenacodon, and Ctenospondylus, based mainly on shared characters in the skull and the mandible.

More complete fossils of early sphenacodontids such as Cryptovenator (currently known only from jaw material) and other forms from the Late Carboniferous (Pennsylvanian) could help clarify the evolution of the group, and how many times and at what evolutionary stage sails developed.