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

TLDR

Metriorhynchoid crocodylians were the apex of marine specialization in Archosauria, dominating Middle Jurassic–Early Cretaceous seas and demonstrating that extinct crocodilians were not all similar to modern relatives. The study aims to investigate the macroevolutionary history of Metriorhynchoidea. To do so, the authors applied a suite of methods—including phylogenetic reconstruction, geometric morphometrics, diversity counts, discrete character disparity analysis, and finite‑element biomechanical modeling. Analyses reveal that the clade diversified in form, function, and biodiversity up to the Jurassic–Cretaceous boundary, after which no further radiation occurred; they also document a trend toward hypercarnivory in Dakosaurus, specialized piscivory in Rhacheosaurus and Cricosaurus, increasing marine specialization with convergent traits such as loss of the deltopectoral crest and retracted nares, and clear distinctions between two newly erected subfamilies, underscoring how quantitative diversity, morphology, and function illuminate the clade’s macroevolutionary pattern.

Abstract

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 characterdisparity 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.