Edwards, Ben Thomas; (2001) The Upper Triassic microvertebrate assemblage of Ruthin Quarry, South Wales. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Bone-rich sediments recovered from the Upper Triassic cave systems of Ruthin Quarry, South Wales, preserve a diverse assemblage of small vertebrates. These include procolophonids (2 taxa), archosauriforms (4 taxa), lepidosaurs (2 taxa) and a Trilophosaurus-like animal, Tricuspisaurus thomasi. The most common Ruthin procolophonid had an extensive covering of dermal armour. Skull bones and osteoderms share a pronounced dermal ornament (deeply pitted surface with one or more distinct bosses) characteristic of certain parareptiles (pareiasaurs, lanthanosuchids, Sclerosaurus). The second procolophonid, recognised largely by characters of the tooth-bearing elements, is a rare component of the assemblage and closely resembles Haligonia bolodon from the Wolfville Formation of Nova Scotia. The two forms are considered congeneric. The most common archosauriform is a suchian closely related to the Crocodylomorpha. The suchian shares two derived characters with sphenosuchian crocodylomorphs, osteoderm morphology (paired, elongate, ornamented osteoderms with an anterior articulatory process) and prefrontal-frontal articulation (in which the prefrontal underlaps the frontal); but retains a postfrontal. The second notable archosauriform is a prosauropod dinosaur, known principally from a tiny ilium (4mm long) of a hatchling, or very juvenile animal. The two lepidosaurs found at Ruthin are the sphenodontian Planocephalosaurus and a basal lepidosaur, possibly, but not necessarily, a rhynchocephalian. Contrary to previous reports, Clevosaurus is not present at Ruthin. The previously described, and enigmatic, Tricuspisaurus is a trilophosaurid archosauromorph. Attribution of this form, and others like it (Variodens, Xenodiphyodon, Trilophosaurus jacobsi [=Chinleogomphius]), to the Procolophonidae are not supported. Although wholly terrestrial, the Ruthin assemblage shares many groups (procolophonids, archosaurs, lepidosaurs, trilophosaurids) in common with the more typical aquatic/terrestrial mix from other Upper Triassic localities. The Ruthin fossil assemblage may be correlated with that of the Wolfville Formation of Nova Scotia, dating the fauna as late Carnian.

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