Abiotic drivers of cranial functional disparity in Mesozoic marine reptiles

The wide array of cranial shapes exhibited by Mesozoic marine reptiles reflects both the longevity of certain clades (e.g., Ichthyosauromorpha, Sauropterygia) and their capacity to adapt to diverse ecological roles throughout their evolutionary history. The Mesozoic saw profound environmental changes, including mass extinctions, anoxic oceanic events, and major regressions/transgressions. These profound events may have driven ecological selectivity among marine predators. However, testing these hypotheses has been challenging due to preservation biases that severely affect taxonomic diversity. Since disparity is often decoupled from species richness, quantifying the functional shapes of Mesozoic marine reptile skulls might reveal new connections with changes in environmental factors. In this work, we introduce the largest dataset of functionally informative craniomandibular traits of marine reptiles ever assembled, comprising measurements from over 600 taxa across 21 Mesozoic time bins. Analyses of functional disparity reveals peaks during the Mid Triassic and Early Cretaceous, driven by the substantial expansion of functional morphospaces by coastal durophagous groups (placodonts and sea turtles, respectively). Through generalised differencing, we detrended overall disparity and group-specific disparity curves, as well as fluctuations in abiotic factors such as sea level, continental flooding extension, and continental fragmentation. We then tested for correlations within the same time bin and with a one-bin lag between these datasets. While most clades only showed isolated relationships between disparity and abiotic factors, ichthyosauromorphs consistently exhibited significant negative correlations with both sea level and continental flooding, suggesting sensitivity to these environmental shifts. These findings provide initial insights into the evolutionary drivers influencing Mesozoic marine predators and suggest potential relationships with abiotic conditions.