Vertebrates from the Late Triassic Thecodontosaurus-Bearing Rocks Of

Proceedings of the Geologists’ Association 125 (2014) 317–328 Contents lists available at ScienceDirect Proceedings of the Geologists’ Association jo urnal homepage: www.elsevier.com/locate/pgeola Vertebrates from the Late Triassic Thecodontosaurus-bearing rocks of Durdham Down, Clifton (Bristol, UK) Davide Foffa *, David I. Whiteside, Pedro A. Viegas, Michael J. Benton School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK A R T I C L E I N F O A B S T R A C T Article history: Since the discovery of the basal sauropodomorph dinosaur Thecodontosaurus in the 1830s, the associated Received 12 October 2013 fauna from the Triassic fissures at Durdham Down (Bristol, UK) has not been investigated, largely Received in revised form 3 February 2014 because the quarries are built over. Other fissure sites around the Bristol Channel show that dinosaurs Accepted 7 February 2014 represented a minor part of the fauna of the Late Triassic archipelago. Here we present data on Available online 16 April 2014 microvertebrates from the original Durdham Down fissure rocks, which considerably expand the taxonomic diversity of the island fauna, revealing that it was dominated by the sphenodontian Keywords: Diphydontosaurus, and that archosauromorphs, including sphenosuchian crocodylomorphs, coelophy- Late Triassic soid theropods, and the basal sauropodomorph Thecodontosaurus, were diverse. Importantly, a few fish Fissure fills Archosaurs teeth provide new information about the debated age of the fissure deposit, which is identified as lower Sphenodontians Rhaetian. Thecodontosaurus had been assigned an age range over 20–25 Myr of the Late Triassic, so this Durdham Down narrower age determination (209.5–204 Myr) is important for studies of early dinosaurian evolution. Rhaetian ß 2014 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction tooth that was destroyed in 1940 during the Second World War and it is now considered Archosauria incertae sedis, thus nomen The Late Triassic palaeogeography of the region around Bristol dubium (Benton et al., 2000, 2012; Galton, 2007; Benton, 2012); the and South Wales (UK) consisted of an archipelago of small latter was originally represented by a heterodont phytosaur limestone islands lying in or near the tropics (Fig. 1A; Robinson, posterior tooth to which other undiagnostic material was added, 1957; Whiteside and Marshall, 2008). Well-documented terrestri- and it is also considered invalid (Benton et al., 2000, 2012; Galton, al vertebrate faunas from these fissure fills have been found in 2007; Benton, 2012). Halstead and Nicoll (1971) incorrectly many limestone quarries of the area, some such as Cromhall identified an articulated limb of the sphenodontian Clevosaurus (Fraser, 1994) and Tytherington (Whiteside, 1983; Whiteside and from Durdham Down; the specimen is actually Diphydontosaurus Marshall, 2008; Van den Berg et al., 2012) near Bristol (Fig. 1A), and (Whiteside, 1983; Whiteside and Marshall, 2008). The occurrence Pant-y-ffynon (Galton et al., 2007) in South Wales. The first of Clevosaurus at Durdham Down has, in fact, never been vertebrate fossils to be documented from these fissure fills were confirmed; reports of its presence by Whiteside (1983) and ‘saurians’ from Durdham Down in Bristol, named Thecodontosaurus Whiteside and Marshall (2008) were contradicted by Van den Berg by Riley and Stutchbury (1836), and with the species name T. et al. (2012), as the evidence was poor. antiquus added later (Morris, 1843). Historical materials from Durdham Down are in the collections Some additional taxa were reported from the Durdham Down of Bristol City Museum, the Natural History Museum, London, Yale fissure deposits. Riley and Stutchbury (1836, 1840) identified two Peabody Museum, and the National Academy of Sciences in species of their new genus Palaeosaurus, namely P. cylindrodon Philadelphia (Benton, 2012). We have had the unique opportunity (= Palaeosauriscus Kuhn, 1959) and P. platyodon (= Rileyasuchus to acid prepare a representative sample of these original Kuhn, 1961), both of which are now considered nomina nuda Thecodontosaurus-bearing rocks, collected in the 1830s from the (Benton et al., 2000; Galton, 2007). The first was based on a single quarries at Durdham Down, and can therefore for the first time make a detailed analysis of its poorly known microvertebrate fauna; moreover, we mechanically prepared and re-described controversial material from the historical collection. In this paper, * Corresponding author. Tel.: +44 7741622920. E-mail address: [email protected] (D. Foffa). we present the first extensive study on the microvertebrate fauna http://dx.doi.org/10.1016/j.pgeola.2014.02.002 0016-7878/ß 2014 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. 318 D. Foffa et al. / Proceedings of the Geologists’ Association 125 (2014) 317–328 Fig. 1. Geology of Durdham Down fissure. (A) Palaeogeographical map of the Bristol area in the Late Triassic (205 Ma) showing inferred palaeo-highs with superimposed modern geography. (B) Simplified geological map of Durdham Down, with 19th century quarry locations. Geology mapping and column derived from BGS map 264. The Dolomitic conglomerate, Mercia Mudstone Group and Penarth Group are Late Triassic. The other strata are Early Carboniferous. of the Durdham Down fissures to provide an overview of all Fraser, 1994; Benton and Spencer, 1995; Whiteside and Marshall, identifiable fossils, to reconstruct the contemporary ecosystem in 2008), but exact dating has been possible only for Tytherington. which the early dinosaur Thecodontosaurus lived, and to date the That age assignment is based principally on the association of fauna. Institutional abbreviations: BRSMG, Bristol City Museum & Rhaetian palynomorphs and marine phytoplankton with Theco- Art Gallery collection; BRSUG, Bristol University, School of Earth dontosaurus and/or other reptiles in six fissures including fissure 2 Sciences collection. at Tytherington quarry, described by Marshall and Whiteside (1980) and Whiteside and Marshall (2008), who also reported an 2. Geological and palaeoenvironmental setting association of the terrestrial reptiles with fish fossils found in the Penarth Group, particularly the Westbury Formation (lower The Durdham Down quarries in Clifton were exploited for the Rhaetian). Whiteside and Marshall (2008) also argued that there extraction of Carboniferous Limestone, which was used as a was no evidence that these Late Triassic fissure faunas are older building material. When quarrying ended about 1840, the quarries than the Rhaetian transgression, through a comparison of the were filled and/or became building sites; for this reason, the exact terrestrial faunas, Penarth Group fishes and geological field location of the fissure deposits of the 1830s cannot be certain. The relations. Currently, the consensus is that the Durdham Down Thecodontosaurus bones collected in the 1830s, and now in BRSMG, Thecodontosaurus-bearing deposits can be dated as Rhaetian could have come from anywhere in the string of quarries between (Whiteside, 1983; Galton, 2007; Benton, 2012) based on the Worrall Road to the south, and Upper Belgrave Road to the north conspecific Thecodontosaurus remains from fissure 2 at Tyther- (Fig. 1B). No contemporary documents survive, but the sum of ington. eyewitness accounts, and reported eyewitness accounts, was This has not always been the view, and it is often assumed that summarised in Benton et al. (2000), Galton (2007), and Benton Thecodontosaurus is Carnian in age, some 20–25 Myr older than the (2012). The best surviving quarry wall, with a Triassic-age fissure early Rhaetian. This discrepancy in age is significant because cutting down into the Carboniferous Limestone is at the east end of Thecodontosaurus is phylogenetically a basal sauropodomorph the series of quarries, beside Quarry Steps, and this was designated dinosaur, and the earlier date provides a different pattern of timing as a Site of Special Scientific Interest (Benton and Spencer, 1995). for this part of the dinosaur phylogenetic tree. The Carnian age was Further evidence that this is the most likely location was assumed early: for example, Moore (1881) did report ‘Rhaetic’ established by a team from the Bristol Dinosaur Project, who bone bed fossils from the quarry at Durdham Down, but decided found Thecodontosaurus and Diphydontosaurus fossils in a collec- that the Thecodontosaurus bones were more likely considerably tion of rocks from walls around the old quarry. older, equivalent to Carnian. Robinson (1957) viewed faunas from As in many similar sites in the SW of Great Britain, karstic these types of fissure as Norian, and this view was followed by features such as fissures and cave systems developed in the Benton (1991), who presented the time range of Thecodontosaur- uplifted and steeply dipping Carboniferous limestones. The infills idae as late Carnian to Rhaetian. Benton and Spencer (1995) noted of the cavernous voids frequently contain fossils, following cavern the similarity of Thecodontosaurus from Durdham Down with collapse and meteoritic waters washing lateritic soil and limestone materials from Tytherington, and its early Rhaetian age, but further debris, including blocks of unaltered Carboniferous limestone, into noted also the phylogenetically basal position of the genus among these cavities. The fossil-bearing rock contains layers of conglom- sauropodomorphs, and so hinted at a late Carnian age. Benton et al. erate with grey Carboniferous

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