Habitat availability, Jurassic and Cretaceous origins of the deep-bodied shark morphotype, and the rise of pelagic sharks

Macroevolutionary trends in morphology fundamentally shape our understanding of marine ecosystems through deep time. Sharks (Elasmobranchii: Selachii) have been suggested to broadly exhibit two discrete body forms: one ‘shallow-bodied’ form associated with slow-moving benthic species, and a ‘deep-bodied’ form typified by highly active pelagic taxa. Until now, the validity or evolution of these body forms have not been tested in a phylogenetic framework. In this study, we reconstruct the evolutionary history of shark body form and provide statistical evidence to suggest three broadly discrete body forms among extant species. We find support for a benthic origin of sharks, with four discrete transitions to a pelagic-type morphology occurring during the Jurassic and Cretaceous. Increased habitat availability during this time, driven by a combination of elevated sea temperature, eustatic sea level rise, continental fragmentation, and diversification trends of actinopterygians and marine reptiles could have facilitated the colonisation of the pelagic realm by Mesozoic sharks, and the repeated independent evolution of body form consistent with extant pelagic species. We also propose that habitat availability and its taphonomic consequences may explain discordance between origination times suggested by molecular phylogenies and the fossil record.