Kirstin S. Brink,ab Hillary C. Maddin,c David C. Evans,d Robert R. Reisza

aDepartment of Biology, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada.

bDepartment of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON M5S 3B2, Canada.

cDepartment of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada.

dDepartment of Natural History, Royal Ontario Museum, Toronto, ON M5S 2C6, Canada.



Paper handled by associate editor Hans-Dieter Sues

Received June 3, 2015. Accepted August 26, 2015.

Canadian Journal of Earth Sciences, 2015, 52(12): 1109-1120, https://doi.org/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 .

Given that B. borealis has priority over all other sphenacodontid taxa following the rules of the International Code of Zoological Nomenclature ( ICZN 1999 , Article 23), synonymization of B. borealis with any other sphenacodontid could have serious ramifications on the taxonomy of Sphenacodontoidea. As such, we carefully redescribe ANSP 9524 in light of the recent identification of phylogenetically significant tooth characters in sphenacodontids ( Brink et al. 2014 ; Brink and Reisz 2014 ; Evans et al. 2009 ). We use CT scans to examine the internal anatomy of the specimen, and perform the first phylogenetic analyses that include B. borealis and assess its relationships within Sphenacodontoidea (Sphenacodontidae and Therapsida).

Two phylogenetic analyses were performed to assess the relationships of ANSP 9524 within Sphenacodontidae and Therapsida. The matrix of Fröbisch et al. (2011) , as modified in Brink and Reisz (2014) , was used to analyse ANSP 9524 within the context of sphenacodontids. One new character was added based on the results of Brink et al. (2014) on tooth root shape (Appendix 1). Tetraceratops , Hipposaurus , and Raranimus were included in the analysis to assess the relationships between sphenacodontids and early therapsids, for a total of 14 operational taxon units (OTUs). To analyse ANSP 9524 within the context of Therapsida, the matrix of Liu et al. (2009) , as modified by Amson and Laurin (2011) , was used. The therapsid analysis had 24 OTUs with the inclusion of ANSP 9524. A parsimony-based branch-and-bound analysis with ACCTRAN optimization, 1000 bootstrap replicates, and a decay analysis was performed in PAUP v4.0 ( Swofford 2002 ) for both matrices.

CT scans were undertaken at the Princess Margaret Hospital, Toronto, Canada, on a Toshiba Aquilion One machine at 135 kVp and 500 mA with an exposure time of 3000 ms. Scans were output as dicoms and the volume renderings were created using Amira v.5 ( VisageImaging 2005 ). Individual dicom slices were chosen at specific levels to reveal details of the tooth crown and root cross sectional shape.

In this paper Top of page Introduction Methods Systematic paleontology « Discussion References

Systematic paleontology

Amniota

Synapsida Osborn, 1903

Eupelycosauria Kemp, 1982

Family Sphenacodontidae Williston, 1912

Genus Bathygnathus Leidy, 1853

Bathygnathus borealis Leidy, 1853

Figs. 2–5

holotype: ANSP 9524, a partial snout including a left premaxilla, nasal, maxilla, and septomaxilla. The incomplete maxilla bears eight preserved teeth.

revised diagnosis: Large sphenacodontid characterized by the presence of a large facial exposure of the septomaxilla. Similar to Dimetrodon grandis in having tooth roots that lack plicidentine, tooth roots equal in length to tooth crowns, postcaniniform teeth bearing longitudinal distal sulci, and mesial and distal carinae bearing denticles. Differs from other sphenacodontids (Dimetrodon milleri, D. limbatus, Sphenacodon spp., Ctenospondylus, spp., and Secodontosaurus obtusidens) in lacking tooth roots with plicidentine and tooth crowns with enamel ornamentation. Differs from the therapsid Tetraceratops insignis in having teeth with mesial and distal carinae bearing denticles and a facial exposure of the septomaxilla. Differs from the therapsid Raranimus dashankouensis in having a tall septomaxillary process and tall maxilla relative to the height of the snout. Differs from other therapsids in having two caniniform teeth, precaniniform maxillary teeth, and no postcanine diastema on maxilla.

description: As ANSP 9524 has been described in detail (i.e., Langston 1963), the following description focuses only on previously unreported anatomical characteristics, including morphology obtained from CT data. The specimen consists of a left premaxilla, maxilla, septomaxilla, and nasal (Fig. 2). No teeth are preserved in the premaxilla, and the maxilla bears eight preserved teeth. The specimen is broken in two pieces, allowing for examination of the caniniform tooth root. A hole dorsal to the caniniform tooth was suggested to be a possible bite mark or other taphonomic damage (Langston 1963). Several foramina line the ventral edge of the maxilla, slightly above the tooth row, as is typical for sphenacodontids (Reisz 1986; Romer and Price 1940). The maxilla is large, contacting the premaxilla and septomaxilla rostrally, and the nasal dorsally (Fig. 2). Dimetrodon has an extensive nasal-maxilla contact and a small nasal-lacrimal contact, whereas Sphenacodon and Ctenospondylus retain a nasal-maxilla and nasal-lacrimal contact that are equal in length (Eberth 1985). Although the proportions of the lacrimal in ANSP 9524 cannot be determined, given the preserved length of the nasal-maxilla contact, the anterior extent of the lacrimal is likely short, as in Dimetrodon. No sutures are preserved where the maxilla contacts the jugal posteriorly.