Variodens inopinatus, ROBINSON, 1957

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 : 693-695

VARIODENS INOPINATUS ROBINSON, 1957 B

FIG. 7A – C

Referred material: Specimen BATGM CD4, an anterior mid-region fragment of a left dentary from Holwell ‘ Microlestes ’ quarry, Somerset, UK ( Fig.

7A – C).

Remarks: BATGM CD4 is a left dentary with a different tooth implantation than is found in the other jaws. There is a clear Meckelian canal ( Fig. 7A ), and on the labial side a foramen has a deep groove that is directed posteriorly ( Fig. 7B ). The dentition is ankylothecodont, with the base of the teeth ankylosed on the dental shelf rather than to a labial wall, and with the base of the tooth lying below the surface of the bone. From the broken tooth it is clear that they sit and are ankylosed in shallow alveoli ( Fig. 7C ). One tooth, identified as emergent because it is significantly below the estimated full heights of the teeth on either side, has a tricuspid apex, but the others are too damaged to describe any details of cusps. The emerging tooth has a wear facet on the central cusp and there is ankylosing bone formed around the base on the labial side. The two teeth behind the tricuspid are larger, and these three teeth have a slightly more ovoid base, wider transversely than anteroposteriorly, compared with the more anterior first tooth that is subconical. Also, the teeth bases form a pattern with the two larger posterior teeth lying almost the entire width across the jaw, with their medial side near to or at the lingual margin; the two anterior teeth are emplaced more mid-dentary, and the medial bases do not come as close to the lingual edge.

This specimen is clearly referable to V. inopinatus , and from the description of the paratype NMHUK PV R36849 by Robinson (1957b), we can judge that it is from the anterior mid-region behind the symphysis roughly in tooth positions 5 – 8. The anteroposterior lengths of the tooth bases fit the pattern of size in the paratype, with the smallest tooth in the sixth position between a larger anterior tooth and posteriorly the largest teeth. The fully erupted teeth are in the same size range (~0.4 – 0.6 mm) as found in the paratype. The emerging tooth conforms to Robinson’s view that tooth replacement was frequent in this species, and the wear on the middle cusp of the tricuspid ( Fig. 7C ) also accords with the paratype ( Robinson, 1957b: table, p. 284).

SUBCLASS PARAREPTILIA OLSON, 1947 SUPERFAMILY PROCOLOPHONOIDEA ROMER, 1956 FAMILY PROCOLOPHONIDAE SEELEY, 1888 PROCOLOPHONID INDET.

FIG. 7D – F

Remarks: BATGM CD8 is a jaw fragment with four robust teeth ( Fig. 7D – F ). The dental implantation is acrodont but the teeth are more columnar than the triangular shape of the sphenodontians, such as Clevosaurus and Diphydontosaurus . There are no ridges characteristic of Planocephalosaurus , and there is a lack of wear on the jaws from occlusion found in the sphenodontians. Rather, it is the type of acrodonty described for the procolophonid Soturnia by Cabreira & Cisneros (2009). One of the teeth has a wear facet on the labial side, and from this and the Meckelian groove we have identified the specimen as the anterior of a right dentary. There are no distinguishing apomorphies that can assign this specimen to a specific genus.

Figures

Figure 7. Small reptile bones from Holwell. Scale bars: 1 mm. A–C, BATGM CD4, anterior region of left dentary of Variodens inopinatus from Holwell in medial (A), lateral (B), and occlusal (C) view, showing wear on the tricuspid tooth. D–F, BATGM CD8, dentary fragment of procolophonid in medial (D), occlusal (E), and lateral (F) views. G, H, BATGM CD62, fragment of?dentary with?acrodont teeth, of indeterminate reptilian affinities in lateral (G) and medial (H) views. I, J, BATGM CD63, isolated archosauromorph tooth in lateral (I) and medial–distal (J) views. K, L, BATGM C24, ungual phalanx of?procolophonid in side views.