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An Unusual Small-Bodied Crocodyliform from the Middle

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An Unusual Small-Bodied Crocodyliform from the Middle Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 1–12, 2017 An unusual small-bodied crocodyliform from the Middle Jurassic of Scotland, UK, and potential evidence for an early diversification of advanced neosuchians Hongyu Yi1,2, Jonathan P. Tennant3*, Mark T. Young2, Thomas J. Challands2,#, Davide Foffa2#, John D. Hudson4#, Dugald A. Ross5# and Stephen L. Brusatte2,6 1 Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, 100044, China 2 School of GeoSciences, Grant Institute, The King’s Buildings, University of Edinburgh, James Hutton Road, Edinburgh EH9 3FE, UK 3 Department of Earth Science and Engineering, Imperial College London, London, SW6 2AZ, UK Email: [email protected] 4 Department of Geology, University of Leicester, University Road, Leicester LEI 7RH, UK 5 Staffin Museum, 6 Ellishadder, Staffin, Isle of Skye IV51 9JE, UK 6 National Museums Scotland, Chambers Street, Edinburgh EH1 1JF, UK *Corresponding author # These authors listed alphabetically ABSTRACT: The Middle Jurassic is a poorly sampled time interval for non-pelagic neosuchian crocodyliforms, which obscures our understanding of the origin and early evolution of major clades. Here we report a lower jaw from the Middle Jurassic (Bathonian) Duntulm Formation of the Isle of Skye, Scotland, UK, which consists of an isolated and incomplete left dentary and part of the splenial. Morphologically, the Skye specimen closely resembles the Cretaceous neosuchians Pachycheilosuchus and Pietraroiasuchus, in having a proportionally short mandibular symphysis, shallow dentary alveoli, and inferred weakly heterodont dentition. It differs from other crocodyliforms in that the Meckelian canal is dorsoventrally expanded posterior to the mandibular symphysis and drastically constricted at the 7th alveolus. The new specimen, together with the presence of Theriosu- chus sp. from the Valtos Formation and indeterminate neosuchians from the Kilmaluag Formation, indicates the presence of a previously unrecognized, diverse crocodyliform fauna in the Middle Jurassic of Skye, and Europe more generally. Small-bodied neosuchians were present, and ecologically and taxonomically diverse, in nearshore environments in the Middle Jurassic of the UK. KEY WORDS: Isle of Skye, Duntulm Formation, Neosuchia Modern crocodiles, caiman, gharials and alligators are the pholidids and atoposaurids (Andrade et al. 2011; Young et al. only living members of Crocodyliformes, a group of pseudosu- 2016a; Tennant et al. 2016a, c), whereas in other parts of the chian archosaurs whose evolutionary history spans about 200 world, small-bodied sphenosuchian and protosuchian-grade million years (Whetstone & Whybrow 1983; Benton & Clark taxa were widely distributed (Gao 2001; Clark et al. 2004; 1988; Bronzati et al. 2015; Mannion et al. 2015). The Middle Go¨hlich et al. 2005; Pol et al. 2013). By the Early Cretaceous, Jurassic was a key period in their evolution, with thalattosu- European faunas were composed of a more diverse assemblage chians rapidly diversifying in the marine realm (e.g., Gasparini of neosuchians, including goniopholidids, pholidosaurids, ber- and Chong 1977; Young et al. 2010, 2011; Wilberg 2015). How- nissartiids, putative atoposaurids and hylaeochampsids (e.g., ever, much less is known about the non-pelagic crocodyliforms Salisbury & Naish 2011; Sweetman et al. 2015). Ghost lineage from this time. Apparently, terrestrial and freshwater species analyses imply that, depending on the systematic position of were stuck in a low point of their diversity, although this could the Cretaceous clades, some major neosuchian lineages may be an artefact of the notoriously poor Middle Jurassic tetrapod have originated during the dark period of the Middle Jurassic fossil record (Tennant et al. 2016a). Sparse sampling during (e.g., Bernissartiidae, Hylaeochampsidae, Paralligatoridae and this interval complicates our understanding of the diversifica- Pholidosauridae: Bronzati et al. 2015; Turner 2015; Tennant tion patterns of non-pelagic crocodyliforms (Bronzati et al. et al. 2016b). 2015; Mannion et al. 2015; Tennant et al. 2016a), meaning The problem, however, is that there are currently few known that new Middle Jurassic fossils are critical. places where Middle Jurassic, non-pelagic crocodyliforms can Indeed, crocodyliforms from terrestrial and nearshore assemb- be found. Jurassic fossil-bearing beds of terrestrial, freshwater lages during the Middle Jurassic remain poorly understood com- and lagoonal origins are sparsely distributed in China (Gao pared with those from other geological intervals. During the Late 2001; Maisch et al. 2003; Fu et al. 2005), Kyrgyzstan (Averianov Jurassic, most non-pelagic crocodyliforms in Europe were gonio- 2000), parts of Europe (Evans & Milner 1994; Kriwet et al. 6 2017 The Royal Society of Edinburgh. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/3.0/), which permits unrestricted re-use, distribution and reproduction in any medium, provided the original work is properly cited. doi:10.1017/S1755691017000032 2 HONGYU YI ET AL. (A) B (B) (C) Tulm Bay C )RUPDWLRQ 0\D 6NXGLEXUJK Duntulm Castle .LOPDOXDJ 1HRVXFKLDLQGHW "*RQLRSKROLGLGDH F 1 D NP W 5°30’ 'XQWXOP 106* D N 57° %DWKRQLDQ 7KHULRVXFKLVVS E 9DOWRV 1 0LGGOH-XUDVVLF /HDOW6KDOH 0 0.5 km (OJRO6DQGVWRQH &XOODLGK6KDOH A855 0\D Duntulm Kilmaluag Igneous %DMRF &DOOR Formation Formation Intrusions (D) PP Figure 1 Locality and geological context of NMS G.2016.21.1: (A) stratigraphic occurrences of fossil crocodyli- forms discovered from the Great Estuarine Group; (B) map of the Isle of Skye, showing three localities of known fossil crocodyliforms: (a) ¼ Duntulm; (b) ¼ Valtos; (c) ¼ Cladach a’Ghlinne; (C) geological context at the Duntulm locality (a); (D) the new specimen NMS G.2016.21.1. 1997; Knoll et al. 2013; Knoll & Lo´pez-Anton˜anzas 2014; cations for the diversity and ecology of non-pelagic crocodyli- Young et al. 2016a) and Russia (Kuzmin et al. 2013), which forms of the poorly-sampled Middle Jurassic. has yielded some crocodyliform fossils. Furthermore, there are Note, that herein we use the term Hylaeochampsidae in a undescribed crocodyliform teeth of this age from Thailand broad sense, referring to taxa previously referred to Hylaeo- (Tong et al. 2002) and possible remains from Madagascar (Dal champsidae (compare this clade’s taxonomic composition in Sasso & Pasini 2003; Flynn et al. 2006), but these are fragmen- Buscalioni et al. 2011; Pue´rtolas-Pascual et al. 2014; Narva´ez tary. The vast majority of Middle Jurassic crocodyliform fossils et al. 2015; Turner 2015; Schwarz et al. 2017). We use the term are isolated specimens from microvertebrate localities – limited ‘putative hylaeochampsid’ for species with no consensus on evidence that makes it difficult to assign these fossils to higher- their phylogenetic position; i.e., Pachycheilosuchus trinquei level clades, which in turn hinders our understanding of the Rogers, 2003 and Pietraroiasuchus ormezzanoi Buscalioni et al., origin and early evolution of these groups that would later go 2011, as some phylogenetic analyses fail to recover them within on to great diversity and success in the Cretaceous and beyond. Hylaeochampsidae or within a derived neosuchians position Here we report a new crocodyliform lower jaw from the (e.g., Narva´ez et al. 2015; Tennant et al. 2016c). In addition, Middle Jurassic of the Isle of Skye, Scotland, UK (Fig. 1). a recent study using the Turner (2015) matrix recovered the This study is part of an ongoing investigation of the world- susisuchids Susisuchus and Isisfordia to be paraphyletic, and class Middle Jurassic vertebrate fossil record of Skye, led both to be within Hylaeochampsidae sensu lato (Schwarz et al. by the PalAlba Group (Brusatte et al. 2015, 2016; Brusatte & 2017), highlighting the uncertainty in the taxonomic composi- Clark 2015; Young et al. 2016a). Previously identified croco- tion of this clade at present. dyliforms from Skye include indeterminate remains (Evans Institutional abbreviations. AMNH, American Museum of et al. 2006; Wills et al. 2014) and a partial skeleton (Evans & Natural History, New York, USA; IPFUB, Institut fu¨rPala¨on- Waldman 1996) from Cladach a’Ghlinne (Fig. 1B) and, most tologie der Freie Universita¨t Berlin, Berlin, Germany; recently, a partial lower jaw assigned to Theriosuchus sp. NHMUK, Natural History Museum, London, UK; NMS, (Young et al. 2016a) from Valtos (Fig. 1B). The new specimen National Museums Scotland, Edinburgh, UK; PC-1, provisional here represents a hitherto unknown small-bodied taxon from specimen number for Pietraroiasuchus ormezzanoi Buscalioni the Middle Jurassic Skye tetrapod assemblage, and has impli- et al., 2011 at Museo Regionale di Scienze Naturali di Torino, MIDDLE JURASSIC CROCODYLIFORM FROM SCOTLAND, UK 3 Italy; SMU, Shuler Museum of Paleontology, Southern Meth- (Andrews & Walton 1990). The trace fossil Thalassinoides indi- odist University, Dallas, Texas, USA. cates the presence of burrowing crustaceans (Myrow 1995). With that said, the Duntulm facies are not simply marine. Cyanobacterial (‘algal’) limestones interbedded with the oyster- 1. Systematic Palaeontology bearing limestones show evidence of both subaerial exposure and freshwater flushing (Andrews 1986). Carbon, oxygen and Crocodyliformes Benton & Clark, 1988 strontium isotopic studies of the oysters show that the lagoons Mesoeucrocodylia
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