The first definitive Middle Jurassic atoposaurid (Crocodylomorpha, Neosuchia), and a discussion on the genus Theriosuchus

Abstract

Atoposaurids were a clade of semiaquatic crocodyliforms known from the Late Jurassic to the latest Cretaceous. Tentative remains from Europe, Morocco, and Madagascar may extend their range into the Middle Jurassic. Here we report the first unambiguous Middle Jurassic (late Bajocian-Bathonian) atoposaurid: an anterior dentary from the Isle of Skye, Scotland, UK. A comprehensive review of atoposaurid specimens demonstrates that this dentary can be referred to Theriosuchus based on several derived characters, and differs from the five previously recognized species within this genus. Despite several diagnostic features, we conservatively refer it to Theriosuchus sp., pending the discovery of more complete material. As the oldest known definitively diagnostic atoposaurid, this discovery indicates that the oldest members of this group were small-bodied, had heterodont dentition, and were most likely widespread components of European faunas. Our review of mandibular and dental features in atoposaurids not only allows us to present a revised diagnosis of Theriosuchus, but also reveals a great amount of variability within this genus, and indicates that there are currently five valid species that can be differentiated by unique combinations of dental characteristics. This variability can be included in future broad-scale cladistics analyses of atoposaurids and closely related crocodyliforms, which promise to help untangle the complicated taxonomy and evolutionary history of Atoposauridae.

An unusual small-bodied crocodyliform from the Middle Jurassic of Scotland, UK, and potential evidence for an early diversification of advanced neosuchians

“…anterior The Middle Jurassic of the Isle of Skye yields a diverse array of archosaurs, lepidosaurs, choristoderes, stem mammals and marine reptiles (e.g., Waldman & Savage 1972;Clark et al 1995;Evans & Waldman 1996;Evans et al 2006;Wills et al 2014;Brusatte et al , 2016Close et al 2016;Young et al 2016a). NMS G.2016.21.1 differs from lepidosaurs and choristoderes in having thecodont dentition with rounded alveoli and a single row of neurovascular foramina on the lateral surface (Romer 1956;Gao & Fox 1998).…”

“…1B) and, most recently, a partial lower jaw assigned to Theriosuchus sp. (Young et al 2016a) from Valtos (Fig. 1B).…”

“…Jurassic fossil-bearing beds of terrestrial, freshwater and lagoonal origins are sparsely distributed in China (Gao 2001;Maisch et al 2003;Fu et al 2005), Kyrgyzstan (Averianov 2000), parts of Europe (Evans & Milner 1994;Kriwet et al 1997;Knoll et al 2013;Knoll & Ló pez-Antoñ anzas 2014;Young et al 2016a) and Russia (Kuzmin et al 2013), which has yielded some crocodyliform fossils. Furthermore, there are undescribed crocodyliform teeth of this age from Thailand (Tong et al 2002) and possible remains from Madagascar (Dal Sasso & Pasini 2003;Flynn et al 2006), but these are fragmentary.…”

“…During the Late Jurassic, most non-pelagic crocodyliforms in Europe were goniopholidids and atoposaurids Young et al 2016a;Tennant et al 2016b, c), whereas in other parts of the world, small-bodied sphenosuchian and protosuchian-grade taxa were widely distributed (Gao 2001;Clark et al 2004;Gö hlich et al 2005;Pol et al 2013). By the Early Cretaceous, European faunas were composed of a more diverse assemblage of neosuchians, including goniopholidids, pholidosaurids, bernissartiids, putative atoposaurids and hylaeochampsids (e.g., Salisbury & Naish 2011;Sweetman et al 2015).…”

“…1). This study is part of an ongoing investigation of the worldclass Middle Jurassic vertebrate fossil record of Skye, led by the PalAlba Group (Brusatte et al , 2016Young et al 2016a). Previously identified crocodyliforms from Skye include indeterminate remains (Evans et al 2006;Wills et al 2014) and a partial skeleton (Evans & Waldman 1996) from Cladach a'Ghlinne (Fig.…”