Cranial anatomy of the Middle Eocene Egyptian crocodylian Herugavialis cairensis provides new insights into gavialoid diversity in the Mediterranean region

Abstract Gavialoidea is presently composed of only two extant crocodylian species, the Indian gharial Gavialis gangeticus, and the false gharial Tomistoma schlegelii, both from southeast Asia. However, Gavialoidea displays a rich and near-cosmopolitan diversity in the fossil record, with over 40 species currently recognized. Several of these gavialoids are recovered from Cenozoic deposits in the Mediterranean region, with many assigned historically to the genus Tomistoma. However, recent phylogenetic analyses do not typically recover these species as closely related to the false gharial, with several fossil species in need of taxonomic revision. Here, we present a detailed re-evaluation of one of these species, Tomistoma cairense from the Middle Eocene Mokattam Formation of Egypt, based on the holotype skull and several referred specimens. Supplemented by endocranial anatomy revealed via computed tomography (CT), we identify numerous anatomical differences with T. schlegelii and other gavialoids, including the approximately contemporaneous Egyptian species, Eogavialis africanum and Paratomistoma courti. These differences are reinforced through phylogenetic analysis under maximum parsimony, in which T. cairense is recovered as a gavialoid that does not cluster with T. schlegelii or Mediterranean taxa. We erect a new genus name for T. cairense, with the resultant new combination of Herugavialis cairensis diagnosed by several characteristics, including telescoped orbits. An apparent decline in Mediterranean gavialoid diversity during the Late Eocene–Oligocene is potentially driven by a reduction in habitat, as a result of global cooling and declining eustatic sea level. Warming in the Miocene might have facilitated a subsequent rebound in gavialoid diversity, prior to the extirpation of crocodylians in the Mediterranean region in the Late Miocene, which might have been caused by the Messinian Salinity Crisis. The taxonomic revision of additional Mediterranean gavialoid species should facilitate greater insight into their evolutionary history and demise.