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Dimetrodon borealis Temporal range: Middle Permian, 270 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Holotype jaw of D. borealis displayed at the Royal Ontario Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Family:	†Sphenacodontidae
Genus:	†Dimetrodon
Species:	†D. borealis
Binomial name
†Dimetrodon borealis Leidy, 1854
Synonyms
Bathygnathus borealis Leidy, 1854

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 Sciences in 1854. Leidy identified the bone as a lower jaw, a mistake that was not corrected until English paleontologist Richard Owen reinterpreted it as an upper jaw in 1876. Leidy erected the new genus and species Bathygnathus borealis, which means "northern deep jaw" in Greek as a reference to the height of the jaw and its discovery in Canada. He identified it as belonging to a dinosaur, although he never called Bathygnathus a dinosaur in the paper (Dawson later described it as "a carnivorous reptile... one of that giant reptile aristocracy which constituted the highest animal type in the middle or secondary period of geologic time"). Leidy compared Bathygnathus with Thecodontosaurus from the Triassic red beds of the United Kingdom, one of the first dinosaurs to have been described scientifically.

Dawson inferred that Bathygnathus was a fast-moving carnivore, reasoning that its deep skull was similar to the short skulls of fast-moving snakes and unlike the long skulls of slow-moving crocodilians. Local naturalist Francis Bain popularized the image of Bathygnathus as a dinosaur in the late 1800s, describing it as a "deep jawed monster" that could attack prey "with a bound of sixteen or eighteen feet... bearing it to the ground with its great weight, while the powerful claws prevented its escape, and the sabre-armed jaws completed the sanguinary work of destruction."

American paleontologist E. C. Case reclassified Bathygnathus as a pelycosaur (a type of "mammal-like reptile") in 1905, noting its similarities with the genus Dimetrodon, a sail-backed synapsid that was discovered in Texas in the 1870s. In their 1940 overview of pelycosaurs, Alfred Romer and Llewellyn Ivor Price suggested that Bathygnathus might be synonymous with Dimetrodon.

References

1. "News Room | University of Toronto Mississauga". www.utm.utoronto.ca.
2. ^ Spalding, D.A.E. (1995). "Bathygnathus, Canada's first "dinosaur"". In Sarjeant, W.A.S. (ed.). Vertebrate Fossils and the Evolution of Scientific Concepts. Taylor & Francis US. pp. 245–254. ISBN 2881249965.
3. Benson, R. B. J. (2012). "Interrelationships of basal synapsids: Cranial and postcranial morphological partitions suggest different topologies". Journal of Systematic Palaeontology. 10: 601–624. doi:10.1080/14772019.2011.631042.
4. "Canuckosaur! First Canadian 'dinosaur' becomes Dimetrodon borealis". phys.org.
5. Langston, W. (1963). "Fossil vertebrates and the late Palaeozoic red beds of Prince Edward Island". Bulletin of the National Museum of Canada. 5 (56): 1–36.
6. ^ Dawson, J.W. (1855). "Chapter VII. The New Red Sandstone". Acadian Geology: an account of the geological structure and mineral resources of Nova Scotia and portions of the neighbouring provinces of British America. Olivier & Boyd. pp. 104–115.
7. Romer, A.S.; Price, L.I. (1940). "Review of the Pelycosauria". Geological Society of America Special Paper. 28: 1–538. doi:10.1130/spe28-p1.

v t e Synapsida
Kingdom: Animalia Phylum: Chordata Superclass: Tetrapoda Clade: Amniota Tetrapodomorpha see Tetrapodomorpha Reptiliomorpha see Reptiliomorpha Synapsida see below↓
Synapsida Synapsida †Diadectomorpha? †Caseasauria? Eothyrididae Asaphestera? Eothyris Oedaleops Vaughnictis Caseidae Alierasaurus Angelosaurus Arisierpeton Callibrachion Casea Caseoides Caseopsis Cotylorhynchus Datheosaurus Ennatosaurus Eocasea Euromycter Lalieudorhynchus Martensius Oromycter Phreatophasma Ruthenosaurus Trichasaurus Eupelycosauria †Varanopidae? Apsisaurus Archaeovenator Ascendonanus Eoscansor Dendromaia Nitosaurus? Pyozia Thrausmosaurus Mesenosaurinae Anningia Cabarzia Elliotsmithia Heleosaurus Mesenosaurus Microvaranops Varanodontinae Aerosaurus Mycterosaurus? Ruthiromia Tambacarnifex Varanops Varanodon Watongia Metopophora see below↓ Incertae sedis †Dimacrodon †Driveria †Gorgodon †Knoxosaurus †Mastersonia †Protoclepsydrops?
Metopophora †Ophiacodontidae Archaeothyris Baldwinonus Clepsydrops Echinerpeton Ophiacodon Stereophallodon Stereorhachis Varanosaurus Haptodontiformes †Ianthodon †Milosaurus †Edaphosauridae Bohemiclavulus Edaphosaurus Glaucosaurus Gordodon Ianthasaurus Lupeosaurus Melanedaphodon Ramodendron Remigiomontanus Xyrospondylus Sphenacodontia †Cutleria †Haptodus †Hypselohaptodus †Kenomagnathus †Tetraceratops †Palaeohatteriidae Palaeohatteria Pantelosaurus Sphenacodontoidea †Shashajaia †Sphenacodontidae Cryptovenator Ctenorhachis Ctenospondylus Cutleria? Dimetrodon Eosyodon Macromerion Neosaurus Secodontosaurus Sphenacodon Steppesaurus? Tappenosaurus? Tetraceratops? Therapsida see below↓
Therapsida Therapsida †Raranimus †Biarmosuchia see Biarmosuchia †Dinocephalia see Dinocephalia †Anomodontia see Anomodontia includes dicynodonts †Gorgonopsia see Gorgonopsia Eutheriodontia †Therocephalia see Therocephalia Cynodontia see Cynodontia includes mammals Incertae sedis †Dinosaurus †Kamagorgon †Niaftasuchus
See also: Pelycosauria Phthinosuchia Theriodontia Category

• Fossils of Canada
• Paleontology in Prince Edward Island
• Sphenacodontidae
• Cisuralian synapsids of North America
• Fossil taxa described in 1854
• Taxa named by Joseph Leidy
• Paleozoic life of Prince Edward Island
• Cisuralian genus first appearances
• Cisuralian genus extinctions