Revision of the Late Jurassic crocodyliformAlligatorellus, 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.

Upper Jurassic sauropod record in the Lusitanian Basin (Portugal): Geographical and lithostratigraphical distribution

“…Other vertebrate groups have been reported, such as fishes (Sauvage, 1897-98;Kriwet, 2000;Balbino, 2003), amphibians (Wiechmann, 2000), turtles (Sauvage, 1897-98;Bräm, 1973;Antunes et al, 1988;Gassner, 2000;Pérez-García and Ortega, 2011;Pérez-García, 2015); sphenodonts Malafaia et al, 2010a), squamates (Seiffert, 1973;Broschinski, 2000;Caldwell et al, 2015), pterosaurs (Dantas, 1987;Wiechmann and Gloy, 2000), crocodyliforms (Sauvage, 1897-98;Krebs and Schwarz, 2000;Schwarz and Fechner, 2004;Schwarz and Salisbury, 2005;Tennant and Mannion, 2014;Russo et al, 2017), mammals (e.g., Hahn, 1971;Krusat, 1980;Hahn and Hahn, 2000;Krebs, 2000;Martin, 2000;Martin and Nowotny, 2000;Martin, 2005), and possible plesiosaurs (Castanhinha and Mateus, 2007).…”

Peer Review #1 of "Late Jurassic theropod dinosaur bones from the Langenberg Quarry (Lower Saxony, Germany) provide evidence for several theropod lineages in the central European archipelago (v0.1)"

“…Pseudosuchian phalanges, for example from the Late Jurassic crocodyliform Alligatorellus sp., are usually more elongate and gracile than observed for DfMMh/FV2/19 (e.g. Tennant & Mannion, 2014). Dinosauria Owen, 1842 Theropoda Marsh, 1881 Material: DfMMh/FV3/19, proximal part of a small right fibula ( Figure 4F-J).…”

Late Jurassic theropod dinosaur bones from the Langenberg Quarry (Lower Saxony, Germany) provide evidence for several theropod lineages in the central European archipelago

“…Phalanges of Jurassic turtles such as thalassochelydians have flatter shafts, often more broadly expanded proximal and distal ends, and the articular surfaces are less pronounced than in DfMMh/FV2/19 (e.g., Eurysternum wagleri, BSPG 1960 VIII 43). Pseudosuchian phalanges, for example from the Late Jurassic crocodyliform Alligatorellus sp., are usually more elongate and gracile than observed for DfMMh/FV2/19 (e.g., Tennant & Mannion, 2014). Dinosauria Owen, 1842Theropoda Marsh, 1881 Material: DfMMh/FV3/19, proximal part of a small right fibula (Figs.…”