Mesozoic marine crises and ichthyosaur history: a non-direct relationship

Marine ecosystems underwent several profound crises throughout the Mesozoic; many of them are correlated with supra-regional to worldwide anoxic events. The impact of these crises is relatively well understood among animals occupying lower trophic levels but their influence on top predators of that time, fishes and marine reptiles, remains unclear. The fossil record of marine reptiles is of fluctuating quality but thoroughly revised taxonomic and phylogenetic frameworks now allow assessment of the influence of past climatic and oceanic changes on these top predators, by quantifying their cladogenesis and extinction rates across selected intervals. Here, I focus on ichthyosaurs, a successful clade of marine reptiles that colonized marine ecosystems during most of the Mesozoic. The ichthyosaur record indicates relatively few severe turnovers and a non direct relationship with environmental drivers, notably anoxic events. New fossils from France reveal the middle-late Norian extinctions did not eradicate the clade of whale-sized shastasaurid ichthyosaurs; similarly, the severe early Toarcian anoxic event, the end-Jurassic climate changes and several Cretaceous anoxic events did not impacted ichthyosaurs significantly, at least at the suprageneric level. On the other hand, severe turnovers or extinctions occurred during the latest Triassic and the Cenomanian and are coincident with a number of profound environmental and biotic changes. This indicates that unique drivers fail to explain the turnover patterns in ichthyosaur evolutionary history. Notably, oceanic anoxic events only impacted ichthyosaurs during the end-Cenomanian and did so during a period of intense climatic and biological upheavals. These major crises in ichthyosaur history are seemingly protracted over several million years and are probably best explained by a conjunction of causes. This is part of a wider project that will incorporate data from other contemporaneous groups to shed a new light on the general turnover patterns among marine top predators of the Mesozoic and the influence of ancient environmental changes in shaping their biodiversity.