Revision of the Late Jurassic crocodyliform Alligatorellus, and evidence for allopatric speciation driving high diversity in western European atoposaurids

Abstract

Atoposaurid crocodyliforms represent an important faunal component of Late Jurassic to Early Cretaceous Laurasian semi-aquatic to terrestrial ecosystems, with numerous spatiotemporally contemporaneous atoposaurids known from western Europe. In particular, the Late Jurassic of France and Germany records evidence for high diversity and possible sympatric atoposaurid species belonging to Alligatorellus, Alligatorium and Atoposaurus. However, atoposaurid taxonomy has received little attention, and many species are in need of revision. As such, this potentially high European diversity within a narrow spatiotemporal range might be a taxonomic artefact. Here we provide a taxonomic and anatomical revision of the Late Jurassic atoposaurid Alligatorellus. Initially described as A. beaumonti from the Kimmeridgian of Cerin, eastern France, additional material from the Tithonian of Solnhofen, south-eastern Germany, was subsequently referred to this species, with the two occurrences differentiated as A. beaumonti beaumonti and A. beaumonti bavaricus, respectively. We provide a revised diagnosis for the genus Alligatorellus, and note a number of anatomical differences between the French and German specimens, including osteoderm morphology and the configuration and pattern of sculpting of cranial elements. Consequently, we restrict the name Alligatorellus beaumonti to include only the French remains, and raise the rank of the German material to a distinct species: Alligatorellus bavaricus. A new diagnosis is provided for both species, and we suggest that a recently referred specimen from a coeval German locality cannot be conclusively referred to Alligatorellus. Although it has previously been suggested that Alligatorellus, Alligatorium and Atoposaurus might represent a single growth series of one species, we find no conclusive evidence to support this proposal, and provide a number of morphological differences to distinguish these three taxa that appear to be independent of ontogeny. Consequently, we interpret high atoposaurid diversity in the Late Jurassic island archipelago of western Europe as a genuine biological signal, with closely related species of Alligatorellus, Alligatorium and Atoposaurus in both French and German basins providing evidence for allopatric speciation, potentially driven by fluctuating highstand sea levels.

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

“…One exception to this is Salisbury & Naish (2011), who provided a detailed differential diagnosis for Theriosuchus, drawing heavily on Schwarz & Salisbury (2005). However, several of the listed characters are present more broadly within Atoposauridae; for example the 'slit-like, horizontally oriented and rostrally positioned external nares, separated from each other by the rostral-most extent of the nasals' (p. 338), is a feature also found in both Alligatorellus (Gervais, 1871) and Alligatorium meyeri (Vidal, 1915) (see also Wellnhofer, 1971 andMannion, 2014), as well as the 'Glen Rose Form'/ Wannchampsus (Langston, 1974;Rogers, 2003;Adams, 2014). Additional characters, such as a shallow sulcus on the maxilla posterior to the triple junction of the maxilla, premaxilla, and nasal, and features of the posterior mandibular morphology (Schwarz & Salisbury, 2005: 797), cannot be assessed on the Skye specimen.…”

“…Alligatorellus, Alligatorium, Atoposaurus, and Montsecosuchus, as well as the putative atoposaurids Brillanceausuchus and Karatausuchus, all preserve skulls, but are dorsally or dorsolaterally flattened and do not preserve the dentary and dental arcade in a manner that facilitates comparison with NMS G. 2014.52.1 (Wellnhofer, 1971;Efimov, 1976;Buscalioni & Sanz, 1990a;Michard et al, 1990;Storrs & Efimov, 2000;Tennant & Mannion, 2014). Amongst the multispecific genus Theriosuchus, T. grandinaris preserves the anterior-most dentary in ventral view, and it is also three-dimensionally preserved in T. guimarotae (Schwarz & Salisbury, 2005), T. ibericus (Brinkmann, 1989(Brinkmann, , 1992, T. pusillus (Owen, 1878;Salisbury, 2002;Young et al, 2014b), and material ascribed to T. sympiestodon (Martin et al, 2014b).…”

Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover

“…Metasuchia, the dominant clade within Mesoeucrocodylia, comprises two major clades of crocodylomorphs: the extinct clade Notosuchia, and Neosuchia, which includes Eusuchia and extant Crocodylia. Basal neosuchians, including the exclusively Laurasian semi-aquatic goniopholids, appear to have passed comparatively unscathed through the J/K boundary (Martin, Rabi & Csiki, 2010;Andrade et al, 2011), although terrestrial atoposaurids seem to have been affected, with Cretaceous occurrences dominated by the shallow marine Theriosuchus lineage (Tennant & Mannion, 2014;Young et al, 2016). This pattern of decline is reflected in subsampled diversity estimates of non-marine crocodyliforms, which decreased through the J/K boundary (Mannion et al, 2015).…”

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

“…from the Middle Jurassic of Skye (Tennant et al 2016c;Young et al 2016a) has deeper rugosities in the symphyseal plate, more prominent ornamentation on the lateral surface of the dentary and a different alveolar morphology. Compared with NMS G.2016.21.1, Alligatorium has a curved dorsal margin of the dentary, and Atoposaurus and Alligatorellus have the splenial contributing to the mandibular symphysis; although the latter two most likely represent skeletally immature specimens (Tennant & Mannion 2014;Tennant et al 2016c). Montsecosuchus, a small-bodied, phylogenetically uncertain neosuchian from Spain (Buscalioni & Sanz 1990;Tennant et al 2016c), has more strongly developed dentary ornamentation than NMS G.2016.21.1.…”

“…Several fossil crocodyliforms represented by mature specimens have a smooth or weakly pitted external surface of the dentary. These include several ''protosuchian'' species (Yang 1973;Wu et al 1997;Pol & Norell 2004a, b), various metriorhynchid genera (e.g., Young et al 2013), the atoposaurid Alligatorium (Tennant & Mannion 2014), and Pachycheilosuchus (Rogers 2003). Similarly, shallow rugosities in the symphyseal plate of NMS G.2016.21.1 may be phylogenetically informative, but these rugosities become deeper through ontogeny in extant crocodylians (Fig.…”