Habitat Availability, Jurassic and Cretaceous Origins of the Deep-Bodied Shark Morphotype and the Rise of Pelagic Sharks.

Macroevolutionary trends in vertebrate morphology fundamentally shape our understanding of marine ecosystems through deep time. Body form influences interactions between organisms and their environment, dictating their locomotor capabilities and ability to hunt/escape from other species. 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, no study has addressed the validity or evolution of these body forms in a phylogenetic framework. Hence, we lack understanding of when, why and how the body forms observed in extant species originally evolved. 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.