Dimetrodon borealis, formerly known as Bathygnathus borealis, is an extinct species of pelycosaur-grade synapsid that lived about 270 million years ago (Ma) in the Early Middle Permian. A partial maxilla or upper jaw bone from Prince Edward Island in Canada is the only known fossil of Bathygnathus. The maxilla was discovered around 1845 during the course of a well excavation in Spring Brook in the New London area and its significance was recognized by geologists John William Dawson and Joseph Leidy. It was originally described by Leidy in 1854 as the lower jaw of a dinosaur, making it the first purported dinosaur to have been found in Canada, and the second to have been found in all of North America (the first was Clepsysaurus from Pennsylvania, now known to be a phytosaur rather than a dinosaur). The bone was later identified as that of a pelycosaur. Although its current classification as a sphenacodontid synapsid was not recognized until after the discovery of its more famous relative Dimetrodon in the 1870s, Bathygnathus is notable for being the first discovered sphenacodontid. A 2015 study by the researchers from the University of Toronto Mississauga, Carleton University and the Royal Ontario Museum reclassified the species into the genus Dimetrodon. Description The teeth of Dimetrodon borealis are long, recurved, and distinctively teardrop-shaped, being widest at the middle rather than the base. The teardrop shape of the teeth is an indication that Dimetrodon borealis belongs to the family Sphenacodontidae. The shape of the maxilla indicates that Dimetrodon borealis had a deep skull like those of other advanced sphenacodontids like Dimetrodon. Like most other species of Dimetrodon, Dimetrodon borealis has an enlarged caniniform tooth near the front of the jaw. History of study The maxilla of Bathygnathus was found around 1845 in a community in the north of Prince Edward Island called French River. The bone was uncovered by a landowner named Donald McLeod in a layer of shale at the bottom of his well. This layer was part of a red sandstone formation that bears similarities to younger Triassic sandstones in the United Kingdom, leading geologists to think that the deposit dated back to the Triassic rather than the Permian. Canadian geologist John William Dawson purchased the fossil and was the first to recognize its significance. Dawson brought it to the attention of American paleontologist Joseph Leidy, who described it to the Philadelphia Academy of Natural