New data on the palaeobiogeography of Toarcian (Lower Jurassic) ichthyosaurs

The Vocontian Basin of SE France was formed along the northwestern border of Tethys during Mesozoic times. Mainly known for its rich ammonite fauna, this basin has also yielded Jurassic and Cretaceous ichthyosaur fossils. The specimens discussed here were discovered in lower Toarcian limestone and marl successions in the vicinity of Digne-les-Bains, High-Provence Alps. One of best-preserved specimens is identified as Suevoleviathan sp., a relatively rare genus previously reported only in the German basins. The specimen is fairly complete but crushed and embedded in limestone from the Falciferum ammonite zone. Because the skull is too damaged to see bone sutures, assignation to this genus is mostly based on fin and coracoid morphology. Along with this specimen, a premaxilla and paddle elements of Eurhinosaurus longirostris and possible Stenopterygiidae centra were found in nearby black marls of the Exaratum ammonite subzone (Falciferum zone, Lower Toarcian; see Floquet et al., 2003 for a detailed stratigraphy). These soft marls were deposited in anoxic waters and are coeval with the Posidonia shales of southwest Germany, the Jet rock formation of northeast England and Grandcourt shales of Luxembourg, which allows faunal comparisons between these basins. The localities from the Vocontian Basin are closer to the Tethys than other sites where Toarcian identifiable ichthyosaurs have been found in Europe. Nevertheless all identifiable specimens from the Vocontian Basin are also identified in other basins across Europe. It suggests that Toarcian ichthyosaurs had wide palaeogeographical distributions and were highly mobile swimmers, perfectly adapted to open marine environments.