The anatomy and taxonomy of the North African Early Miocene crocodylian ‘Tomistoma’ dowsoni and the phylogenetic relationships of gavialoids

Abstract

For decades, the interrelationships of the extant crocodylians Gavialis gangeticus and Tomistoma schlegelii have been debated, with only recent morphological phylogenetic analyses recovering the sister-taxon relationship between these two gavialoid species that has long been apparent in topologies based on molecular data. Several extinct species from the Mediterranean region are currently assigned to Tomistoma; however, their phylogenetic placement is labile, and often they do not form a clade with Tomistoma schlegelii. Here, we present a revision of Tomistoma dowsoni from the Early Miocene of Egypt and Libya, based on the type specimen (a partial snout and mandible) and referred material, including a nearly complete skull. These specimens show no notable anatomical differences and diagnostic features include: (1) a heart-shaped naris; (2) a prefrontal and lacrimal that are equidimensional in anteroposterior length; and (3) a prominent posterior process of the supraoccipital with a convex posterior margin. Maximum parsimony analysis, under both equal and extended implied weighting, recovers Tomistoma dowsoni as a phylogenetically nested gavialine, distantly related to Tomistoma schlegelii and other Mediterranean species currently referred to Tomistoma. Instead, Tomistoma dowsoni forms a sister-taxon relationship with Eogavialis andrewsi, from the Late Miocene of Kenya. Given that this clade does not consistently cluster with the type species of Eogavialis, and that Tomistoma dowsoni is diagnostic and clearly not referrable to Tomistoma, we herein erect the genus Sutekhsuchus, with the new combination Sutekhsuchus dowsoni. Our phylogenetic analyses recover the European Miocene gavialoids in a monophyletic group with the North American Thecachampsa, forming a clade of early-diverging gavialines that underwent transoceanic dispersal. We also recover a monophyletic group of thoracosaurs, comprising Eothoracosaurus, Portugalosuchus, and Thoracosaurus, which is expanded to include other Late Cretaceous–Early Paleogene European, North American, and North African species under extended implied weighting. https://zoobank.org/urn:lsid:zoobank.org:pub:2DE71417-8088-4749-8672-72E4A40C0B39