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Anatomy of a Late Triassic Bristol Fissure: Tytherington Fissure 2

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Anatomy of a Late Triassic Bristol Fissure: Tytherington Fissure 2 Proceedings of the Geologists’ Association 131 (2020) 73–93 Contents lists available at ScienceDirect Proceedings of the Geologists’ Association journa l homepage: www.elsevier.com/locate/pgeola Anatomy of a Late Triassic Bristol fissure: Tytherington fissure 2 a b,c b b, Giovanni Mussini , David I. Whiteside , Claudia Hildebrandt , Michael J. Benton * a Department of Zoology, University of Oxford, 11a Mansfield Road, Oxford, OX1 3SZ, UK b School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK c Palaeontology Section, Earth Science Department, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK A R T I C L E I N F O A B S T R A C T Article history: The Late Triassic and Early Jurassic fissures around Bristol and South Wales are famous as sources of Received 14 October 2019 excellent fossils of early mammals, dinosaurs, sphenodontians, and other tetrapods. However, the ways Received in revised form 7 December 2019 in which these fissures filled with sediment have not often been documented. Moreover, systematic Accepted 9 December 2019 faunal sampling up a working face exposure has rarely been attempted for the UK Triassic fissures. Here Available online 26 December 2019 we show in detail how a complex fissure in Tytherington Quarry, north-east of Bristol, filled in multiple steps, and document both sedimentary facies and faunas. Tytherington Quarry is a key Late Triassic Keywords: Bristol sauropsid fissure site, having previously yielded abundant Thecodontosaurus remains and a Late Triassic new genus of sphenodontian, Diphydontosaurus. Our findings comprise specimens of terrestrial reptiles, Fissure fills including sphenodontians and archosaurs. However, abundant remains of fishes and marine Rhaetian Transgression Systematics invertebrates were also recovered, indicating a significant marine influence during parts of the Sphenodontians fissure-filling process. Comparisons of our results with those of previous studies on Tytherington and Gastropods other Late Triassic SW UK fissure sites indicate some consistent patterns of faunal association and provide Ostracods further evidence of a Rhaetian age for the Late Triassic palaeo-island fissures north of Bristol. Our analysis Archosaurs supports a model of fissure formation and filling entailing punctuated episodes of sedimentation through Tytherington a relatively short time. © 2019 The Geologists' Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction sub-tropical archipelago (Fig.1A) swept by episodic rains (Whiteside and Robinson,1983) that washed lateritic soils into the fissures. The The close of the Triassic was a time of widespread palaeogeo- sediments often contain the remains of insular animals, such as the graphical and environmental change. At the beginning of the dinosaur Thecodontosaurus, other archosaurs, and abundant sphe- Rhaetian stage (duration 4.1 Myr; 205.5–201.4 Ma; Kent et al., 2017), nodontians. This assemblage, dominated by terrestrial reptiles, was the breakup of Pangaea initiated the Rhaetian Transgression, a designated the ‘sauropsid’ fauna by Robinson (1957) to distinguish Europe-wide event entailing the flooding of continental red-bed it from fissure deposits containing plentiful mammals, of Early environments by epicontinental seas (Fischer et al., 2012; Suan et al., Jurassic age (Robinson, 1957; Whiteside and Marshall, 2008; 2012). In the area around Bristol and the Severn Estuary, Rhaetian Whiteside et al., 2016). sediments, all components of the Penarth Group, rest horizontally The chronostratigraphic and palaeoenvironmental contexts of either conformably on underlying units of the Mercia Mudstone the Bristol sauropsid fissures (Cromhall, Durdham Down, Tyther- Group such as the Blue Anchor Formation, or unconformably on an ington and Woodleaze) have been the subject of longstanding uplifted and eroded Carboniferous Limestone landscape (Robinson, debate (Walkden and Fraser, 1993; Simms et al., 1994; Whiteside 1957; Savage, 1993; Benton and Spencer, 1995; Whiteside and and Marshall, 2008; Whiteside et al., 2016). Fossils in all the Marshall, 2008; Whiteside et al., 2016). Here and there, the sauropsid fissure deposits, including those at Tytherington, have Carboniferous limestones formed palaeo-highs characterized by been documented from sediments processed in the laboratory karstic features (Robinson, 1957) including fissures, of which the (Whiteside, 1983; Whiteside and Marshall, 2008; van den Berg Tytherington fissures are an example (Whiteside and Marshall, et al., 2012; Whiteside et al., 2016). Although Whiteside and 2008; Whiteside et al., 2016). These outcrops were part of a small Marshall (2008, fig. 17) made a systematic count of sphenodontian taxa in all the Tytherington fissures, and van den Berg et al. (2012) presented a comprehensive faunal analysis of an exposure of * Corresponding author. Fissure 2, only at Cromhall Quarry have detailed attempts been E-mail address: [email protected] (M.J. Benton). made to match sedimentology and fossil faunas with Late Triassic https://doi.org/10.1016/j.pgeola.2019.12.001 0016-7878/© 2019 The Geologists' Association. Published by Elsevier Ltd. All rights reserved. 74 G. Mussini et al. / Proceedings of the Geologists’ Association 131 (2020) 73–93 Fig.1. Palaeogeography of the Bristol Channel area near Bristol, during the Rhaetian and the Tytherington site. (A) In the palaeogeographic map, about eight limestone islands (stippled) were scattered across a narrow seaway (blank). Fissure localities are marked by blue spots. Green squares show the locations of Penarth Group palynological profiles. Dotted line shows current Severn Estuary. (B) Geological map in the vicinity of Tytherington Quarry indicating fissure 2 and 4 positions. Ex17 is marked with an arrow. Note dash lines also outline quarries, fault lines run approximately N-S. Abbreviations: BRD, Black Rock Dolostone; BRL, Black Rock Limestone; Fm, Formation; Lst, Limestone; Sst, Sandstone. From Whiteside and Marshall (2008) and Klein et al. (2015). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) climatic and eustatic events (Walkden and Fraser, 1993). However, (Thorne et al., 2011; Fischer et al., 2014) and multiple invertebrate excepting sites 4, 5 and 5a of Fraser (1985), even at Cromhall exact phyla (Hallam, 2002). Conversely, only minor perturbations in sample locations are not recorded as specific points in the fissures. taxonomic diversity and functional disparity affected bony fishes In general, although most Bristol fissure reptile fossils have been (Friedman and Sallan, 2012; Romano et al., 2014; Smithwick and collected in situ (e.g. at Tytherington, Whiteside and Marshall, Stubbs, 2018). 2008) the treacherous rock faces, particularly in working quarries, In the terrestrial realm tetrapod faunas underwent major have made up-sequence systematic sampling almost impossible. restructuring. A significant decline in the diversity and disparity of On the basis of sedimentary associations, Fraser (1985), crurotarsan archosaurs in the late Rhaetian coincided with an Walkden and Fraser (1993) and Simms et al. (1994) outlined a increase in dinosaur abundance and diversity, continuing a trend model of the formation and infilling of the Bristol sauropsid that had begun with the demise of the large dicynodonts in the fissures that involved repeated episodes of deposition and erosion Norian (Benton et al., 2014). Basal clades among lissamphibians, over an extended timespan, possibly from Carnian to early turtles, lepidosaurs, crocodylomorphs, and mammals also diversi- Rhaetian. This hypothesis was challenged by Whiteside (1983) fied at this time (Sues and Fraser, 2010; Benton et al., 2014), and Whiteside and Marshall (2008) who argued that evidence establishing the broad structure of modern terrestrial ecosystems. from geomorphology, palynomorphs and animal fossils points Here we focus on the Tytherington site, where fissure 2 instead to correlation with the start of the marine transgression of provides an extensive microvertebrate and invertebrate fossil the early Rhaetian (and not earlier), and that this applies to the record through the timeframe of fissure infilling (Whiteside, 1983; Tytherington fissures, as well as other Bristol fissures such as Whiteside and Marshall, 2008; van den Berg et al., 2012; Whiteside Cromhall, Durdham Down and Woodleaze (Whiteside et al., 2016). et al., 2016). By comparing this fossil biota with those of other parts Understanding the faunal successions of the Bristol fissures will of fissure 2 and the sauropsid fissures in general, we infer the help in understanding Late Triassic global faunas, especially in impact of palaeoenvironmental factors on the fauna of a Late regard to the end-Triassic mass extinction (ETME), as emphasised Triassic island ecosystem. We provide evidence on the palae- by Whiteside and Marshall (2008). The duration, turnover oenvironment and dating of the Tytherington Quarry fissures, as patterns, and timing of the ETME are debated, as is the possibility well as other similar Late Triassic sites in south-western England. of a spate of extinctions at the Norian-Rhaetian boundary (Tanner Institutional acronyms. BRSMG, Bristol Museum and Art et al., 2004; Deenen et al., 2010; Schoene et al., 2010; Kent et al., Gallery, Bristol; BRSUG, University of Bristol, Geology Collection. 2017; Percival et al., 2017; Dunhill et al., 2018). There is evidence for a high rate of species turnover in both marine and terrestrial
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