Computed tomography reveals the endocranial anatomy of Crocodylia: Implications for phylogenetic relationships and ecomorphological convergence across Crocodylomorpha

Living crocodylians (alligators, caimans, gavials and 'true' crocodiles) are the remnants of a broader clade, Crocodylomorpha, that displays high ecomorphological diversity, particularly in skull shape. However, there are several instances of ecomorphological convergence between distant lineages, especially pertaining to the independent acquisition of a long, slender, longirostrine snout. In some instances, it is difficult to determine whether extinct species belong to one longirostrine lineage or another, with this problem especially prevalent with regards to the gavialoid crocodylian clade. The recent application of computed tomography (CT) scanning to the skulls of vertebrate species has revealed a plethora of previously hidden information on their endocranial anatomy. This in turn has the potential to shed new light on a group's evolutionary interrelationships, with endocranial data potentially helping to distinguish between shared phylogenetic ancestry of an anatomical feature and ecomorphological convergence. Although CT scans of skulls of several crocodylian species have been evaluated, previous studies have focused on describing the endocranial anatomy of an individual species or assessing intraclade variation of a specific morphological feature. Here, we evaluate the endocranial anatomy of Crocodylia based on a combination of published and newly presented CT-scan data for 43 extant and extinct species, including representatives from the three major subclades, Alligatoroidea (13 species), Crocodyloidea (14 species), and Gavialoidea (16 species), alongside four non-crocodylian eusuchians. Alligatoroids possess a sigmoidal encephalic endocast, an endosseous labyrinth wherein the area of the anterior semicircular canal is more than three times that of its posterior counterpart, as well as snout sinuses lateral and ventral to the nasal cavity. Crocodyloids have dorsoventrally tall pharyngotympanic tubes, antorbital and postvestibular sinuses, and a cerebrum that has its greatest transverse width at its midpoint. Gavialoids have a relatively 'simple' endocranial morphology, with a straight encephalic endocast, and reduced or absent snout sinuses. Several extinct gavialoids are also characterised by depressions on the internal surface of the prefrontal bones, which have been previously hypothesised to be osteological correlates for salt glands; however, similar depressions in some extant species of Crocodylus appear to be associated with airspace instead. We identify several differences between the endocranial anatomy of phylogenetically distant longirostrine crocodylomorphs: by contrast with thalattosuchians, crocodylians lack a confluent dorsal dural venous sinus and orbital arteries, whereas they possess an intertympanic sinus and nasolacrimal ducts. Our study demonstrates the viability of endocranial data for identifying phylogenetically informative morphological features in crocodylomorphs more broadly.