REPTILIA, LAURENTI, 1768

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 : 695

REPTILIA INCERTAE SEDIS

Remarks: The small jaw fragment BATGM CD62 ( Fig. 7G, H ), with two near-overlapping teeth, derives from a different taxon. It is unclear where in the jaw the specimen is located, but a dentary fragment is most likely as there is some wear on both lingual and labial sides. The teeth are distinctively truncated and have a basin, developed lingually, particularly in one tooth. It is unclear where the affinities of this specimen lie; it may be sphenodontian but the most similar tooth form occurs in the anterior dentary dentition of the enigmatic Xenodiphyodon petraios Sues & Olsen, 1993 . Sues & Olsen (1993) were unsure of the affinities of Xenodiphyodon , suggesting that it may be related to trilophosaurs such as Variodens or procolophonids. The tooth implantation of BATGM CD 62 is quite different from our Variodens specimen (BATGM CD4), and may be of an unusual acrodont implantation; however, there is a shallow sulcus at the base of the shorter?posterior tooth, indicating that the tooth may lie in a socket. It is therefore possibly ankylothecodont, suggesting procolophonid affinities.

REPTILIA INDET

Remarks: BATGM CD63 ( Fig. 7I, J ) is an isolated, probable archosauromorph, tooth nearly 3 mm long with an apparently non-serrated carina on one side. The specimen is worn but has a blunt apex. The tooth does not readily fit the 16 morphotypes of Heckert (2004), or those from phytosaurs, and its affinities are unclear.

BATGM C24, an ungual phalanx ( Fig. 7K, L ), is difficult to assign, but the size and general shape suggests that it might be from a burrowing animal, as the morphology is reminiscent of the third digit ungual of the manus of extant fossorial golden moles. It may belong to a procolophonid. Procolophonids probably used their claws for digging, with the two innermost digits of the manus being the most robust ( Colbert & Kitching, 1975).

CLASS OSTEICHTHYES HUXLEY, 1880 FAMILY PHOLIDOPHORIDAE WOODWARD, 1890 GENUS PHOLIDOPHORUS AGASSIZ, 1832 PHOLIDOPHORUS SP.

Remarks: Premaxillae: This is the first record of bony fish that can be found in non-marine environments to be reported from Holwell. BATGM 42f ( Fig. 8A, B ) is a worn premaxilla that has the shape of an elongate triangle, measuring 2.18 mm long and 1 mm high at the deepest point (at the short ascending process). The posterior margin is inclined and the whole element is slightly bowed longitudinally. The ventral or oral margin is narrow and marked by eight tooth bases; no complete teeth are present. The external surface of the element is not ornamented, with the original ganoin covering having been removed by post-mortem wear during bone transport, and shows evidence of a shelf of bone along the upper external margin. A second premaxilla in the collection (BATGM 42h; Fig. 8C, D ), perhaps also belonging to a pholidophorid, is rather deeper and has ten tooth bases along the oral margin.

Pholidophorus is a basal teleost genus that was originally erected, but not illustrated, by Agassiz (1832, 1833 – 1844) for two species, each described rather cursorily, from the Upper Triassic of Seefeld (Austrian Tyrol). A number of closely related genera have been described since Agassiz’s pioneering work, mostly from the Carnian to Norian of Europe. The group has been little understood but is the subject of a recent revision by Arratia (2013).

The Holwell specimens described here were compared with specimens from a range of pholidophoriforms in the collections of the NHMUK. Unfortunately, the premaxillae, being the anteriormost components of a protrusible complex of upper jaw bones, are often lost from the front of the snout, even in well-preserved and well-articulated specimens, or are obscured by enclosing sediment. The size of the Holwell specimens, their roughly triangular shape, and the presence of a short ascending process only, together with a single row of tooth bases along the oral margin, compares well with the few accessible examples of pholidophorid premaxillae in the NHMUK collections, and those described by Arratia (2013). Less worn comparative specimens containing complete teeth and retaining the ornamented surface layer of ganoin are needed to confirm the identities suggested here. It is worth noting that virtually complete and well-articulated pholidophorids (e.g. NHMUK PV P578) are known from the Cotham Marble of Aust ( Fig. 8E ), although none of the specimens examined in the present study contain clearly preserved premaxillae.

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.

Figure 8. Pholidophorid fish jaws from Holwell. Pholidophorus from Aust. Scale bars: A–D, 1 mm; E, 10 mm. A, B, BATGM 42f, right premaxilla of Pholidophorus sp. in medial (A) and lateral (B) views. C, D, BATGM 42h, right premaxilla of Pholidophorus sp. in medial (C) and lateral (D) views. E, anterior two-thirds of a specimen of Pholidophorus higginsi Egerton, 1854, from the Cotham Marble of Aust (NHMUK PV P578 in left lateral view), South Gloucestershire, UK.