Contrasting macroevolutionary patterns in raptorial tetrapods across the Triassic–Jurassic transition

Ichthyosauria and Eosauropterygia were the two of the most successful lineages of marine raptorial predators, co-existing in the same ecosystems throughout most of the Mesozoic. As such, these animals were affected by similar environmental, ecological, and evolutionary pressures, making them a good example in comparing macroevolutionary trajectories among contemporaneous clades of aquatic tetrapods. The very end of the Triassic represents a key period in their evolution as they seemingly went through a massive bottleneck that strongly reduced their morphological diversity, with pelagic lineages (e.g. Parvipelvia and Plesiosauria) as the only survivors. However, studies analysing their evolution across the Triassic-Jurassic (T/J) transition are rare and usually focussed on coarse morphological and temporal data. In the present study, we comprehensively compare the evolution of ichthyosaurian and sauropterygian disparity and morphospace occupation across the T/J transition by using biomechanically- and ecologically-informative traits. We reveal distinct patterns in the craniodental diversification of these two lineages. The ecomorphospace of eosauropterygians is predominantly shaped by a strong phylogenetic signal, resulting in the clustering of three groups with clearly distinct craniodental architectures: Pachypleurosauroidea, Nothosauroidea, and Pistosauroidea. This pattern of ecomorphospace occupation suggests rapid ‘leaps’ towards novel niches throughout the early evolution of eosauropterygians. By contrast, the diversification of ichthyosaurians lacks a discernible evolutionary trend. We rather find evidence for functional convergences between Early Jurassic parvipelvians and Triassic non-parvipelvians, suggesting high evolvability right after the T/J transition. Our examination of the temporal evolution of morphofunctional disparity, body size, and pelagic adaptations of these reptiles during the Late Triassic does not support the hypothesis of an abrupt macroevolutionary bottleneck at or near the T/J transition. Instead, our findings suggest the presence of a yet temporally elusive turnover event during the lower to middle Norian is suggested, possibly induced by rapid sea-level changes that led the extinction of coastal water species and facilitated the dominance of the parvipelvians and pistosauroids, adapted to a more open ocean lifestyle.