Diphydontosaurus undetermined

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 : 683-685

DIPHYDONTOSAURUS SP.

FIGS 3A, C, E – H, K, L

Referred material: Specimen BATGM CD3; anterior region of a left dentary displaying emplacements for 13 pleurodont teeth. Specimens BATGM CD5 and BATGM CD6: posterior and mid-region of two left dentaries, respectively.

Remarks: The dentary and maxilla of Diphydontosaurus is unique amongst the sphenodontians in possessing posterior acrodont and anterior rapidly replaced pleurodont teeth. The genus also displays alternating sized acrodont teeth on the maxilla and dentary. In the specimens from Holwell there are no complete jawbones but the fragments can be directly compared with the range of Tytherington specimens held in the Geology collection at the University of Bristol (BRSUG) for identification.

Specimen BATGM CD3 ( Fig. 3A, C) has seven or eight teeth per mm, somewhat more than some D. avonis specimens from Tytherington Quarry, the type locality, that have six teeth per mm in the same region ( Fig. 3B, D). By contrast, in the dentary of Gephyrosaurus bridensis Evans, 1980 there are approximately three anterior teeth per mm (see Evans, 1980: fig. 42). Although the implantation looks to be the same, it is not possible to compare the frequency of replacement in the pleurodont teeth between CD3 and D. avonis , as the Holwell specimen is damaged at the teeth bases. Also, the Holwell specimen does not have the laterally positioned ventrally directed flange, part of the symphysis of the Tytherington dentary ( Fig. 3B), but that region is worn on BATGM CD3. The depth of the dentary at the mandibular symphysis and at the point of curvature behind the mandibular symphysis in CD3 is between 10 and 20% larger than the Tytherington specimen. These features may suggest that BATGM CD3 represents a larger variety of Diphydontosaurus .

There are two other left dentaries that show a mid and posterior region; both have been worn in transport before deposition. BATGM CD5 ( Fig. 3E, F) is the larger of these but has only one partly intact acrodont tooth, wider than any tooth in BATGM CD3, with the simple triangular shape found in Diphydontosaurus in a similar posterior region. The tooth also sits obliquely on the crest of the dentary and the mesial – distal axis of the tooth is slightly labially orientated anteriorly, a feature in common with D. avonis . Noticeably there is a small hump in the dorsal edge of dentary BATGM CD5 behind the tooth. This feature is also found in the D. avonis holotype and in the dentary from Tytherington fissure 2 (BRSUG 29382), one of the largest ( Fig. 3I, J). The mesial – distal length of the CD5 tooth viewed lingually is 0.8 mm, slightly larger than the largest tooth on the Tytherington dentary and also of the holotype BRSUG 23760.

BATGM CD6 ( Fig. 3G, H) is from the mid-region of the dentary and the Meckelian canal is open but narrowing and descending towards the anterior. The most complete tooth is more conical, and again typical of the mid region of Diphydontosaurus where there is a transition from typical pleurodonty to more robust and less frequently replaced teeth. The tooth is between those of BATGM CD3 and BATGM CD 5 in width, and there is a possible replacement pit at the base, but the tooth is too worn to be sure. There is a high lingual wall that leaves the teeth emplaced in a groove, although they are ankylosed to the labial wall. The two anterior teeth bases are in near proximity (and possibly attached) to the lingual wall. Overall, these dentaries are similar in morphology and size to typical specimens from Tytherington, but there are slight differences such as the relatively higher acrodont tooth and greater density of pleurodont teeth. However, the difference in BATGM CD5 may simply relate to the lateral compression of the specimen during diagenesis. The dentary BATGM CD5 is about the same size as one of the largest from Tytherington shown in Figure 3I and J, and is only slightly larger than the holotype. Therefore we conclude that the specimens represent Diphydontosaurus sp. aff. avonis .