Rapid evolution and cranial morphospace expansion during the terrestrial to marine transition in elapid snakes

Ecological transitions can trigger rapid phenotypic evolution and novelty, yet the tempo and mode of such changes remain poorly understood in clades that diversify across broad geographic scales, such as continents and oceans. We analysed skull shape variation across 91 terrestrial, amphibious, and fully marine species of elapid snakes (Elapidae). We observed a significant increase in rates of skull shape evolution during the land-to-sea transition of viviparous sea snakes. This coincides with a shift into a new region of morphospace, defined by a higher frontoparietal region, more depressed snout, and a wider suspensorium. The acceleration of skull shape evolution in sea snakes was closely followed by a major dichotomy in the evolutionary trajectories of the Hydrophis and Aipysurus clades, which exhibit narrow and wide skulls, respectively. We suggest that narrow skulls in the Hydrophis group provided ecological opportunities that subsequently facilitated the rapid evolution of the axial skeleton (previously documented by Sherratt et al., 2022), with both morphological shifts preceding the increase in speciation rates in core Hydrophis. This study highlights the asynchronous nature of phenotypic and lineage diversification rates during the radiation of geographically widespread clades shaped by major ecological transitions.