Friday, December 11, 2015

[Paleontology • 2015] Dimetrodon borealis • Re-evaluation of the Historic Canadian fossil Bathygnathus borealis from the Early Permian of Prince Edward Island

Dimetrodon borealis is shown with an overlay of the "Bathygnathus" fossil from Prince Edward Island), with a Walchia tree in the background (a common fossil found on Prince Edward Island).

 illustration: Danielle Dufault || DOI: 10.1139/cjes-2015-0100


The holotype and only known specimen of Bathygnathus borealis is a partial snout with maxillary dentition of a presumed sphenacodontid from the Lower Permian (Artinskian 283–290 Ma) redbeds of Prince Edward Island, Canada. Due to its incomplete nature, assessment of the taxon’s systematic position within a cladistic analysis had never been performed. However, recent recognition of the phylogenetic utility of tooth characters in sphenacodontids now allows for a modern phylogenetic evaluation of B. borealis. Results show that B. borealis is the sister taxon of Dimetrodon grandis, which is supported by dental characters: crowns with mesial and distal denticles and roots elongate, lacking plicidentine. An autapomorphy of B. borealis is the large facial exposure of the septomaxilla. As Bathygnathus has priority over Dimetrodon in the scientific literature, we suggest a reversal of precedence is required to preserve the familiar name Dimetrodon and to maintain universality, thus recognizing the new species Dimetrodon borealis.

Dimetrodon borealis fossil shows a close up of a tooth with serrations (tiny bumps along the edges of the teeth).
photo: Kirstin Brink 


Results of this study show that dental characters are highly significant for resolving the taxonomic affinities of B. borealis. As noted by Langston (1963), the tooth counts in ANSP 9524 are the same as those of D. grandis, which has the lowest tooth counts for any sphenacodontid (Romer and Price 1940). Also, the combination of denticles on the mesial and distal carinae and elongate tooth roots lacking plicidentine are only known in D. grandis (Brink et al. 2014; Brink and Reisz 2014). Therefore, the sister-taxon relationship between D. grandis and ANSP 9524 is well supported.

Phylogenetic analysis of sphenacodontids and basal therapsids suggests that ANSP 9524 is more closely related to Dimetrodon than to basal therapsids, and is deeply nested within the Dimetrodon clade as the sister taxon of D. grandis. In the context of Sphenacodontidae, we identify the large facial exposure of the septomaxilla in ANSP 9524 as an autapomorphy of B. borealis. Given the lack of other cranial or postcranial material from PEI, and the geographic and temporal separation between B. borealis and D. grandis, we support the conclusion of Langston (1963) that B. borealis represents a distinct sphenacodontid species.

As the genus Bathygnathus (Leidy 1854) has taxonomic priority over Dimetrodon (Cope 1878), Dimetrodon could be synonymized into Bathygnathus, following the rules of the ICZN (ICZN 1999, Article 23). However, given the wide usage and familiarity of the generic name Dimetrodon in both the scientific and popular literature (Angielczyk 2009; Reisz 1986; Romer and Price 1940; Steyer 2012), a case has been made with the ICZN to reverse precedence and retain Dimetrodon (Case 3695; Brink 2015). This would result in the new combination D. borealis for ANSP 9524. With the addition of the first Canadian species, this increases the total number of recognized species of Dimetrodon to 13 (Brink and Reisz 2012).

The recognition of Dimetrodon on PEI is not unexpected, given the paleogeographical location of PEI in the Early Permian (Brink et al. 2012, 2013; Olson and Vaughn 1970). All major Early Permian terrestrial fossil bearing localities, such as the ‘four corner’ states and Texas in the USA (Olson and Vaughn 1970) and the Bromacker quarry in Germany (Martens et al. 2005) are situated around the Early Permian paleoequator, and all share a similar paleofauna, including temnospondyls, diadectids, parareptiles, and other synapsids (Brink et al. 2013). The presence of Seymouria and Dimetrodon suggests a close affinity between the Orby Head Formation of PEI and the Arroyo Formation of Texas (Brink et al. 2013; Olson and Vaughn 1970).

Kirstin S. Brink, Hillary C. Maddin, David C. Evans and Robert R. Reisz. 2015. Re-evaluation of the Historic Canadian fossil Bathygnathus borealis from the Early Permian of Prince Edward Island.  Canadian Journal of Earth Sciences. 2015; 1 DOI: 10.1139/cjes-2015-0100

Canuckosaur! First Canadian 'dinosaur' becomes Dimetrodon borealis

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