Re-assessment of the impact of the Late Triassic extinction events on the diversification of raptorial marine reptiles

The Triassic (259.1–201.3 million years ago) represents a pivotal period in Earth's history, marking the establishment of complex marine trophic networks that still persist nowadays. The recovery of marine life following the largest-ever biodiversity crash at the Permian–Triassic transition occurred during the Early to Middle Triassic and was notably characterized by the rapid radiation of multiple reptile lineages that invaded aquatic ecosystems. These reptiles diversified in nearshore and oceanic habitats, resulting in a wide variety of body sizes, feeding strategies, and locomotion styles. However, the Early Jurassic fossil record is much less diversified and indicates that nearly all ‘typically Middle Triassic’ faunas adapted to coastal environments disappeared, in addition to gigantic forms. Only a fraction of ichthyosaurians (the ‘fish-shaped’ reptiles) and plesiosaurians (initially long-necked eosauropterygians) successfully crossed the Triassic–Jurassic (T/J) transition. This strong reduction in marine reptile disparity has been associated to the end-Triassic mass extinction event (ETME), that forced, under this paradigm, a macroevolutionary bottleneck, in which only pelagic raptorial predatory forms survived. However, studies supporting this hypothesis used, in addition of phylogenetic data to reconstruct the evolution of disparity, coarse temporal intervals, thus potentially confounding multiple temporally-isolated events.
The aim of this Ph.D. project is therefore to quantitatively re-assess the influence of Late Triassic events on the evolutionary history of both ichthyosaurians and eosauropterygians. This project involves a comprehensive examination of their morphological evolution from the Middle Triassic to the end of the Early Jurassic, alongside the quantification of the phylogenetic signal in their extinction patterns and the influence of body size in extinction susceptibility. Furthermore, to perform disparity analyses, I compiled extensive morphological datasets that incorporate a great number of 3D models of specimens digitised for the first time during this project.
The overall findings of this project strongly challenge the hypothesis of a sudden and severe evolutionary bottleneck affecting the diversity and the disparity of raptorial marine reptiles at or close to the T/J transition. Instead, the loss of ‘Triassic’ forms appears to be the result of a more gradual pattern of selective extinctions throughout the Late Triassic, most of which were unrelated from the ETME. A major faunistic turnover, likely driven by marine regression, is thought to have occurred during the early Late Triassic, leading to the extinction of coastal faunas and facilitating the diversification of taxa adapted to life in the open-ocean. While both ichthyosaurians and eosauropterygians were impacted by the reduction of flooded continental shelves, the latter appeared more severely affected. Eosauropterygians indeed experienced a significant drop in disparity, associated with the disappearance of ‘typical Middle Triassic’ lineages that had previously undergone a remarkable ecomorphological diversification, reflecting the occupation of multiple ecological niches. Among ichthyosaurians, the T/J transition has traditionally been associated with a notable decrease in disparity and a substantial shift in morphospace occupation. However, our morphological analyses reveal a high diversity of craniodental phenotypes during the Early Jurassic, as shown by the occupation of previously unexplored regions in morphospace and a substantial overlap with those of their Triassic predecessors. Even if this pattern suggests a minor effect of the end-Triassic events on the morphological evolution of ichthyosaurians, these events coincided with the final demise of the whale-sized shastasaurids, suggesting a certain degree of phylogenetic clustering in extinctions and potentially a greater susceptibility among large-bodied taxa during the ETME.