The evolution of Metriorhynchoidea (Mesoeucrocodylia, Thalattosuchia): an integrated approach using geometric morphometrics, analysis of disparity and biomechanics

Metriorhynchoid crocodylians represent the pinnacle of marine specialization within Archosauria. Not only were they a major component of the Middle Jurassic–Early Cretaceous marine ecosystems, but they provide further examples that extinct crocodilians did not all resemble their modern extant relatives. Here, we use a varied toolkit of techniques, including phylogenetic reconstruction, geometric morphometrics, diversity counts, discrete character disparity analysis, and biomechanical finite-element analysis (FEA), to examine the macroevolutionary history of this clade. All analyses demonstrate that this clade became more divergent, in terms of biodiversity, form, and function, up until the Jurassic–Cretaceous boundary, after which there is no evidence for recovery or further radiations. A clear evolutionary trend towards hypercarnivory in Dakosaurus is supported by phylogenetic character optimization, morphometrics, and FEA, which also support specialized piscivory within Rhacheosaurus and Cricosaurus. Within Metriorhynchoidea, there is a consistent trend towards increasing marine specialization, with the hypermarine Cricosaurus exhibiting numerous convergences with other Mesozoic marine reptiles (e.g. loss of the deltopectoral crest and retracted external nares). In addition, biomechanics, morphometrics, and character disparity analyses consistently distinguish the two newly erected metriorhynchid subfamilies. This study illustrates that together with phylogeny, quantitative assessment of diversity, form, and function help elucidate the macroevolutionary pattern of fossil clades.