Abstract
Teleosauroids were a successful group of semi-aquatic crocodylomorphs that were an integral part of coastal marine/lagoonal faunas during the Jurassic. Their fossil record suggests that the group declined in diversity and abundance in deep water deposits during the Late Jurassic. One of the few known teleosauroid species from the deeper water horizons of the well-known Kimmeridge Clay Formation is ‘Teleosaurus’ megarhinus Hulke, 1871, a poorly studied, gracile longirostrine form. The holotype is an incomplete snout from the Aulacostephanus autissiodorensis Sub-Boreal ammonite Zone of Kimmeridge, England. The only other referred specimen is an almost complete skull from the slightly older A. eudoxus Sub-Boreal ammonite Zone of Quercy, France. Recently, the validity of this species has been called into question. Here we re-describe the holotype as well as the referred French specimen and another incomplete teleosauroid, DORCM G.05067i-v (an anterior rostrum with three osteoderms and an isolated tooth crown), from the same horizon and locality as the holotype. We demonstrate that all specimens are referable to ‘Teleosaurus’ megarhinus and that the species is indeed a valid taxon, which we assign to a new monotypic genus, Bathysuchus. In our phylogenetic analysis, the latest iteration of the ongoing Crocodylomorph SuperMatrix Project, Bathysuchus megarhinus is found as sister taxon to Aeolodon priscus within a subclade containing Mycterosuchus nasutus and Teleosaurus cadomensis. Notably Bathysuchus has an extreme reduction in dermatocranial ornamentation and osteoderm size, thickness and ornamentation. These features are mirrored in Aeolodon priscus, a species with a well-preserved post-cranial skeleton and a similar shallow and inconspicuous dermal ornamentation. Based on these morphological features, and sedimentological evidence, we hypothesise that the Bathysuchus + Aeolodon clade is the first known teleosauroid lineage that evolved a more pelagic lifestyle.
Highlights
Teleosauroids were a successful group of semiaquatic Jurassic crocodylomorphs
We conducted a phylogenetic analysis to test the evolutionary relationships of Bathysuchus megarhinus gen. nov. within Thalattosuchia, using a modified version of the dataset published by Ösi et al (2018), which is continuously being updated, as it forms the foundation of the ongoing Crocodylomorph SuperMatrix Project
Well-developed anterior premaxillary projections seen in B. megarhinus (NHMUK PV OR 43086 and Dorset County Museum (DORCM) G.050671i—but not in LPP specimen, as its narial cavities are infilled with matrix) are present in M. nasutus (CAMSM J.1420, DF pers. obs.) (Figs. 2–3)
Summary
Teleosauroids (one of the subgroups of Thalattosuchia) were a successful group of semiaquatic Jurassic crocodylomorphs. They were abundant in marine/lagoonal faunas for most of the Jurassic of Europe, Asia and Africa (Andrews, 1909; Andrews, 1913; Buffetaut, Termier & Termier, 1981; Lepage et al, 2008; Young et al, 2016; Jouve et al, 2016; Johnson et al, 2018), and continued into the Early Cretaceous of Africa (Fanti et al, 2016). ‘Teleosaurus’ megarhinus may provide pivotal insights into the palaeobiology and evolution of teleosauroids
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