Ophiacodon (meaning "snake tooth") is an extinct genus of synapsid belonging to the family Ophiacodontidae that lived from the Late Carboniferous to the Early Permian in North America and Europe. The genus was named along with its type species O. mirus by paleontologist Othniel Charles Marsh in 1878 and currently includes five other species. As an ophiacodontid, Ophiacodon is one of the most basal synapsids and is close to the evolutionary line leading to mammals. Description Ophiacodon has a large skull with a deep snout. It has the longest skull of any early synapsid, reaching up to in one specimen. The jaws are lined with many small teeth. It was larger than most other tetrapods (four-limbed vertebrates) of its time, ranging from in length and in weight. Specimens of Ophiacodon vary greatly in size. These differences in size were once used to distinguish species, but are now recognized as ontogenetic variations related to the ages of individuals. Smaller bones often have more poorly developed joint surfaces than larger bones, implying that they come from juvenile individuals while the larger bones come from adults. Analysis of the histology or microscopic anatomy of bones suggests that differences in size represent different growth stages rather than different species. Range Remains of Ophiacodon have been found in North America and Europe. England: Kenilworth - Cisuralian France: Lower Permian of Autun Canada: (?)Nova Scotia: Joggins Formation - Kasimovian United States: Arizona: Cutler Formation - Cisuralian Colorado: Cutler Formation - Cisuralian Kansas: Fort Riley, Chase Group - Cisuralian New Mexico: Cutler Formation - Cisuralian Ohio: Greene Formation, Dunkard Group Cisuralian Oklahoma: Ada Formation - Pennsylvanian, Clyde Formation, Wellington Formation - Cisuralian Texas: Admiral Formation, Belle Plains Formation, Clyde Formation, Wichita Group - Cisuralian Utah: Cutler Formation - Cisuralian Paleobiology Ophiacodon most likely lived on land, but paleontologists have sometimes thought that it was semi-aquatic. An aquatic habitat for Ophiacodon was first proposed by paleontologist E. C. Case in 1907, although he later dismissed the idea. Anatomical features suggesting that it spent much of its time in the water include broad claws that seemed to be adaptations for paddling, thin jaws and numerous small teeth that seemed to be adapted for eating fish, and weakly developed bones, which are seen in many other secondarily aquatic tetrapods. In 1940, paleontologists Alfred Romer and Llewellyn Ivor Price proposed that hindlimbs with a greater length than forelimbs was another aquatic