Gephyrosaurus evansae, Whiteside & Duffin, 2017

Whiteside, David I. & Duffin, Christopher J., 2017, Late Triassic terrestrial microvertebrates from Charles Moore’s ‘ Microlestes’ quarry, Holwell, Somerset, UK, Zoological Journal of the Linnean Society 179 (3), pp. 677-705 : 686

GEPHYROSAURUS EVANSAE SP. NOV.

FIG. 3M – P, R

Type locality: Rhaetian fissure fills in Carboniferous Limestone, Holwell quarry, Somerset, UK.

Holotype: Specimen BATGM CD1 : anterior and mid region of the lower part of a right maxilla displaying emplacements for 24 pleurodont teeth.

Diagnosis: A species of Gephyrosaurus with a similar pleurodont implantation to G. bridensis , but with significantly smaller pointed teeth. The tooth density per mm is therefore much higher than in any similar sized G. bridensis specimen.

Derivation of species name: A tribute to Susan Evans, whose work has transformed our understanding of Mesozoic lepidosaurs; without her detailed account of Gephyrosaurus we would have not be able to identify this specimen.

Remarks: BATGM CD1 is a right maxilla with implantation positions for 24 teeth. Viewed medially ( Fig. 3M, O), the pleurodont implantation is clear and there are resorption pits, or gaps for shed teeth, on at least ten teeth. This high number indicates a rapid replacement of teeth, although the proportion of resorption pits is not as high as the pleurodont region of the Diphydontosaurus maxilla, BRSUG paratype 23764, or on other specimens illustrated in Whiteside (1986: pl. 1). They are within the variability of specimens of G. bridensis , however: some G. bridensis have 2/8 teeth with replacement pits in the anterior maxillary region, others have 5/8. Evans (1985) also noted variability in G. bridensis , with some Pontalun anterior maxillae specimens having low (or ‘suppressed’) replacement, and with others (18%) having none at all. The resorption pits are distinctive and some are relatively large ( Fig. 3O), similar to Diphydontosaurus . Only one tooth is nearly intact; the others all have missing apices. The apex of the most complete tooth ( Fig. 3P, R) is fractured, but a sharpened edge is apparent and recalls the pointed teeth of G. bridensis ( Fig. 3Q, S), rather than the slightly recurved teeth of Diphydontosaurus ( Fig. 3T). The ventral edge of the premaxillary process has a slightly downward orientation ( Fig. 3N), compared with the rest of the bone, and there is a small notch on the anterior of the process ( Fig. 3P) similar to, but larger than, a slight indentation on some specimens of G. bridensis ( Fig. 3Q). The facet for the premaxilla forms a large proportion of the pronounced premaxillary process ( Fig. 3O), as in G. bridensis . The downward orientation of the anterior lower edge of the bone is apparent when viewed medially, and is significantly more pronounced than in Diphydontosaurus and in G. bridensis (see Evans, 1980: fig. 31B). As in G. bridensis , the anterior dental shelf is wider than that in the mid region ( Fig. 3P, Q). The other distinctive feature is the large anterior alveolar foramen ( Fig. 3N), also pronounced in Diphydontosaurus and G. bridensis , which is at the end of a series of five foramina where branches of the superior alveolar nerve exited. The number of teeth per unit length of the anterior and mid region of the maxilla, (3.5 per mm) is similar to the four of Diphydontosaurus , but is significantly higher than the 2.5 of G. bridensis . Furthermore, the minimum of 24 pleurodont teeth is a much higher number than in Diphydontosaurus (which has a total of 16 in the paratype), and sequentially they have a more consistent anteroposterior width like G. bridensis , but unlike the posteriorly increasing size found in Diphydontosaurus . The maxilla is about twice as large as the equivalent region of Diphydontosaurus , but is similar in size to G. bridensis .

Other specimen referable to Gephyrosaurus : Specimen BATGM CD2 ( Fig. 3U, V), an anterior region of the left dentary displaying emplacements for 15 pleurodont teeth ankylosed to the inside of the lateral wall, is referable to Gephyrosaurus sp. (with a close affinity to or actually G. bridensis ), with nearly three pleurodont teeth per mm of the same jaw region (see Evans, 1980: fig. 42A). That is fewer than the equivalent anterior region of the type dentary of D. avonis (approximately 4.5 per mm). The mandibular symphysis, although worn, is more similar to Gephyrosaurus than the laterally flanged feature of D. avonis ( Fig. 3B).