Abstract :
[en] Ichthyosauria is a group of reptiles that colonized numerous niches in Mesozoic marine ecosystems. Their Cretaceous representatives were however regarded as undiversified, especially compared to their Triassic and Jurassic ancestors, because only a few species are regarded as valid in the Cretaceous. Besides, most of them belong to a single genus, Platypterygius, whose taxonomy and phylogeny are problematic. Therefore, Cretaceous ichthyosaurs have been widely considered as the last members of a group on the decline, although the timing, the cause and the magnitude of their extinction phases greatly vary among authors. Yet, a great part of the Cretaceous ichthyosaur record, the Eurasian material, has been neglected from more than 100 years; theories explaining the evolution of the last ichthyosaurs therefore miss the biggest part of the picture. Notably, how ichthyosaurs diversified and went extinct during the Cretaceous is basically unknown. To tackle these topics, I assess the taxonomy of Cretaceous ichthyosaurs from Eurasia, test their phylogenetic position within a global analysis of post-Triassic ichthyosaurs, evaluate their ecological diversity and analyse their phases of radiation and extinction.
Assessment of the Eurasian material led to the discovery of seven new taxa (“Iraq animal”, Acamptonectes densus, “Lenin Form”, Sveltonectes insolitus, Sveltonectes “Ebbo Form”, “Sisteron Form”, “Colossal Form”) and the re-installation of several other taxa as valid (Simbirskiasaurus birjukovi, P. bannovkensis, Cetarthrosaurus walkeri, P. hercynicus). Morphological and phylogenetic evidence suggest Platypterygius hides a very large generic diversity and should be limited to its type species P. platydactylus; however, the interrelationships of the large platypterygiine ichthyosaurs from the Aptian–Cenomanian interval are poorly constrained. Phylogenetic analyses provide robust evidence for disparate origins of Cretaceous ichthyosaurs; these animals actually belonged to three distinct clades that diverged several tens of million years before the Cretaceous. These three clades had contrasted evolutionary histories and colonized disctinct ecological niches during the Cretaceous. Survival and extinction rates demonstrate ichthyosaurs suffered no extinction event at the Jurassic–Cretaceous boundary. They were therefore highly diverse during the Cretaceous, although the sampling of some stages remains inadequate. Notably, up to eight genera were present during the late Albian, a few million years before their final extinction. This extinction appears staggered over four phases that span the entire Cenomanian stage; it also forms one of the facets of a much wider event that affected most levels of marine trophic chains during the Cenomanian. Compilation of published data and collaborations with other specialists show that the radiation of snakes, mosasaurs, and numerous lineages of modern fishes, the severe extinctions among cephalopods and rudist bivalves and the complete extinction of ichthyosaurs likely belong to the same global turnover event. The magnitude, diversity and simultaneity of the biotic responses strongly suggest that global physico-chemical factors triggered this profound reorganization of the marine ecosystems, notably the conjunction of a rather brutal onset of extremely high temperatures and sea level, intense marine volcanism and phases of worldwide ocean anoxia.
