The evolution of extreme hypercarnivory in metriorhynchidae (Mesoeucrocodylia Thalattosuchia) based on evidence from microscopic denticle morphology

Metriorhynchids were a peculiar group of fully marine Mesozoic crocodylomorphs. The derived genera Dakosaurus and Geosaurus exhibit a macroevolutionary trend towards extreme hypercarnivory, underpinned by a diverse array of craniodental adaptations, including denticulate serrated (ziphodont) dentition. A comparative analysis of serrations in Metriorhynchidae shows that known Dakosaurus species had conspicuous denticles, in contrast to the microscopic denticles of Geosaurus. A new tooth from the Nusplingen Plattenkalk of Germany provides evidence for a previously unknown large species of Geosaurus. Metriorhynchid specimens from the upper Kimmeridgian-lower Tithonian of Southern Germany show that ziphodont species of Dakosaurus and Geosaurus co-occurred in the Nusplingen and Solnhofen Seas. Although these genera are similarly denticulate, they diverge in overall crown morphology. Therefore, resource/niche partitioning via craniodental differentiation is posited as maintaining two contemporaneous genera of highly predatory metriorhynchids. Additionally, the new generic name Torvoneustes is proposed for "Geosaurus" carpenteri, the only known metriorhynchid with false-ziphodont dentition. A cladistic analysis shows that ziphodont dentition may have evolved independently in Dakosaurus and Geosaurus, or been acquired earlier by their common ancestor and secondarily lost in Torvoneustes and related taxa.