Secodontosaurus

Extinct genus of synapsids

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. Like the well known Dimetrodon, Secodontosaurus is a carnivorous member of the Eupelycosauria family Sphenacodontidae and has a similar tall dorsal sail. However, its skull is long, low, and narrow, with slender jaws that have teeth that are very similar in size and shape—unlike the shorter, deep skull of Dimetrodon ("two measure tooth"), which has large, prominent canine like teeth in front and smaller slicing teeth further back in its jaws. 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. Although no complete skeletons are currently known, Secodontosaurus likely ranged from about in length, weighing up to
Fossils of Secodontosaurus have been found in Texas in North America in the Wichita and the Clear Fork groups of Early Permian formations. 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. These discoveries are mentioned in online blogs but so far have not been formally described.

The name Secodontosaurus comes from Latin seco ('to cut') + Greek ὀδούς, ὀδόντος (, odont, 'tooth') + Greek σαῦρος ( saurus, 'lizard') and is based on the anatomical term "secodont" for teeth with cutting edges designed to tear or slice flesh. Paleontologist Robert Bakker has dubbed Secodontosaurus the "fox faced finback" after its long jaws. Robert R. Reisz and others published the first description of Secodontosaurus material in 1880 as a supposed species of his genus Theropleura ("mammal rib"). Theropleura Cope, 1878 is a junior synonym of Ophiacodon Marsh, 1878 ("snake point tooth"). The specimen, AMNH 4007 collected by Jacob Boll, was actually a composite that included bones of "amphibians" and a sphenacodontid. Cope described the teeth as having "apices are not very acute. The superficial coating is striate with fifteen or sixteen rather obtuse ridges"—features expressed in the species name Theropleura obtusidens (Latin for "obtuse tooth" or "blunt toothed"). E. C. Case described another specimen of Secodontosaurus, AMNH 4091, as Dimetrodon longiramus ["long (lower jaw) ramus"] in 1907. In 1916 S. W. Williston illustrated parts of the upper and lower jaws of a new, then unnamed genus, specimen FMNH (WM) 573, that he took for a member of the ophiacodontids, noting, however, its "broader, flattened, and cutting teeth" compared to Ophiacodon, which has mainly pointed conical teeth. In his 1925 Osteology of the Reptiles, Williston listed the name Secodontosaurus under the Ophiacodontidae, presumably as the new genus from 1916, but did not provide a description or an explanation. Williston likely chose the name Secodontosaurus "cutting tooth lizard" for flat, sharp edged ["secodont"] teeth that contrasted with the conical teeth typically found in ophiacodontids. In 1936 A. S. Romer made Williston's proposed generic name official but identified Secodontosaurus as a sphenacodontid related to Dimetrodon instead of an ophiacodontid. He also chose Case's Dimetrodon longiramus as the type species and erected the new species S. willistoni from the Clear Fork group, noting its later occurrence and larger size. In 1940 Romer and Price gave priority to Cope's species obtusidens over the synonym longiramus. When Reisz and others favor a monophyletic group Sphenacodontinae composed of Dimetrodon, Sphenacodon, and Ctenospondylus, based mainly on shared characters in the skull and the mandible. According to these phylogenetic hypotheses, the long skulled Secodontosaurus represents a separate branch at the base of the Sphenacodontidae. 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. Cladogram after Fröbisch et al., 2011: