New data on the palaeobiogeography of Early Jurassic marine reptiles: the Toarcian ichthyosaur fauna of the Vocontian Basin (SE France)

in Neues Jahrbuch für Geologie und Palaontologie. Abhandlungen (2011), 261(1), 111-127

The Vocontian Basin (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 several Lower Jurassic ... [more ▼]

The Vocontian Basin (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 several Lower Jurassic ichthyosaurs. The specimens discussed here were discovered in lower Toarcian limestone and marl successions in the vicinity of Digne-les-Bains, High-Provence Alps. The best-preserved specimen is identified as Suevoleviathan sp., a rare taxon previously reported only in southern Germany. Along with this specimen, premaxillae and paddle elements of Eurhinosaurus sp. and probable Stenopterygiidae centra were found in neighbouring localities. These specimens were preserved thanks to the deposition of soft anoxic marls or calcarodetritic sediments, coeval with other anoxic shales in Europe (the Toarcian Oceanic Anoxic Event or T-OAE), which allows faunal comparisons between these basins. The localities from the Vocontian Basin are closer to the Tethys than any other sites where identifiable Toarcian ichthyosaurs have been found in Europe. Nevertheless, the Vocontian ichthyosaur assemblage is strikingly similar to those in other basins across Europe. It suggests that Toarcian ichthyosaurs had a wide palaeobiogeographical distribution, reflecting their anatomical adaptations as highly mobile swimmers. [less ▲]

Reconsidering the extinction of ichthyosaurs

Conference (2010)

Despite their extreme adaptation to life in open sea, ichthyosaurs were one of the first major groups of post-Triassic marine reptiles to disappear, at the end of Cenomanian, whereas plesiosaurs ... [more ▼]

Despite their extreme adaptation to life in open sea, ichthyosaurs were one of the first major groups of post-Triassic marine reptiles to disappear, at the end of Cenomanian, whereas plesiosaurs, mosasaurs and numerous families of marine crocodiles and sea turtles disappeared during the Cretaceous/Paleocene Extinction Event. It has been proposed that unique biological factors drove ichthyosaurs to extinction, namely a break in the food chain at the level of belemnites or a progressive ecological replacement by teleost fishes since the Middle Jurassic. However, new discoveries in France and Russia turn both these hypotheses unsatisfactory because ichthyosaur diversity remained high during the Early Cretaceous both from taxonomic and ecological points of view, with the persistence of several Late Jurassic genera into the Early Cretaceous and the colonization of various feeding guilds. The extinction of ichthyosaurs during the Cenomanian was therefore more sudden than previously described. The present study aims at replacing the extinction of ichthyosaurs within the global context of changes in marine ecosystems during the ‘middle’ Cretaceous. The ‘middle’ Cretaceous (Aptian-Turonian) is indeed punctuated by numerous and profound global climatic and oceanic changes, as well as intense underwater volcanism. These factors led to recurrent anoxic events, sometimes of worldwide extension. Interestingly, the peak of changes in the geosphere, taking place during the Cenomanian, coincides with major biological changes within the marine realm: the rise of polycotylid plesiosaurs, marine squamates, teleost fishes and chondrichtyans, the onset of the “Chalk sea”, and an extinction within marine invertebrates communities. Ichthyosaurs disappeared during this profound reorganization of the marine ecosystems. However, the precise mechanisms that lead to the sudden extinction of these successful marine reptiles cannot be understood in the current state of our knowledge given the multitude of possible causes occurring at the same time. [less ▲]

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

Conference (2009)

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 ... [more ▼]

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. [less ▲]