| Named By: | Alfred S. Romer in 1936 in more detail in main text |
| Time Period: | Cisuralian, 285-272.3 Ma |
| Location: | USA, Texas - Arroyo Formation, Belle Plains Formation |
| Size: | Between 2 and 2.7 meters long |
| Diet: | Carnivore |
| Fossil(s): | Several individuals represented by skulls and partial post cranial remains |
| Classification: | | Chordata | Synapsida | Pelycosauria | Sphenacodontidae | |
| Also known as: | | Dimetrodon longiramus | |
Secodontosaurus (meaning "cutting-tooth lizard"), an extinct genus of "pelycosaur" synapsid 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 2 to 2.7 metres (7 to 9 ft) in length, weighing up to 110 kilograms (250 lb).
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 odous (odont-) "tooth" + Greek sauros "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.