Dimetrodon is Bathygnathus? Or Bathygnathus is Dimetrodon?

While the west of Canada is known for Late Cretaceous dinosaur fossils, the east has a number of Paleozoic outcrops with some early terrestrial tetrapods. In 1845, before Canada was even a country, a fossil of an upper jaw and some teeth was found on Prince Edward Island, and was first described in 1854. As the second ever vertebrate fossil to be found in Canada, this specimen has had an interesting history.

It was first identified as an extinct 'saurian', then as a dinosaur, followed by a theriodont, and finally correctly identified as a sphenacodontid, a group of early synapsids (a group of tetrapods with no temporal fenestra, or holes, in their skulls, consisting of mammals today) which includes the famous extinct sail-backed reptile Dimetrodon grandis. It hails from the Lower Permian, 283-290 million years ago, and was called Bathygnathus borealis. The fragmentary nature of the fossil made it difficult to determine the exact affinities of this specimen. Over the years it has been studied by a number of people, and similarities have been identified with Dimetrodon, Sphenacodon, and Ctenospondylus, but the similarities have never been major enough to warrant an official change. That is, until now.

ANSP 9524 - type specimen of 'Bathygnathus' borealis (Brink et al. 2015)

Dr. Kristin Brink worked on Dimetrodon and similar animals during her PhD research at the University of Toronto (Mississauga) and studied the specimen which is now housed at the Academy of Natural Sciences in Philadelphia. Primarily working on tooth of these similar animals, she was able to study the specimen using CT scans and compare it to other sphenacodontids[1]. She re-described the specimen and underwent phylogenetic analysis and found some interesting results, including that dental characters appear to be extremely important in sphenacodontid taxonomy. Based on phylogenetic analysis of morphological characters, Dr. Brink found that 'Bathygnathus' borealis appeared as a sister taxa to Dimetrodon grandis, and nestled within 3 species of Dimetrodon.

Cladogram showing position of 'Bathygnathus' borealis (Brink et al. 2015)

'Bathygnathus' borealis has the same tooth count, denticles, and tooth roots as Dimetrodon grandis, characters only found  in D. grandis. Brink et al. (2015) concluded that 'Bathygnathus' borealis was actually Dimetrodon borealis, making this specimen the first and only Dimetrodon in Canada.

The interesting and very important thing about this paper is related to the the rules of taxonomic nomenclature and priority. Bathygnathus borealis was named 20 years before Dimetrodon, meaning that by the law of priority, Bathygnathus should have priority and replace Dimetrodon. However, Dimetrodon is a well known and very famous fossil and no one wants to lose that name. Exceptions are occasionally made when there is a strong reason to retain initial names, and they have started a case with the International Commission of Zoological Nomenclature (ICZN), the group responsible for taxonomic names and problems like this. If they succeed, we won't lose Dimetrodon, but gain a new species of 'Dimetrodon' borealis!

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References
Brink KS, Maddin HC, Evans DC, and Reisz RR. 2015. Re-evaluation of the historic Canadian fossil Bathygnathus borealis from the Early Permian of Prince Edward Island. Canadian Journal of Earth Sciences 52: 1109-1120.