Paranasal sinus system of Geosaurus araucanensis and the homology of the antorbital fenestra of metriorhynchids (Thalattosuchia: Crocodylomorpha)

Abstract

ABSTRACT Metriorhynchids have been interpreted as the only archosaurs entirely adapted to pelagic marine life, given the deep morphological modifications of their skeletons. The most conspicuous feature in the skull involves the “fenestra and fossa antorbitalis complex.” Exceptionally preserved natural cast of snout cavities of Geosaurus araucanensis, found in the Late Jurassic of the Neuquén Basin (northwestern Patagonia, Argentina), allow exploration of the rostral anatomy of this metriorhynchid. The presence of a paranasal sinus system, hypothetically reconstructed by other authors using EPB method approach, can now be confirmed based on direct morphological evidence. We propose that the openings classically identified in the literature as internal and external antorbital fenestrae of metriorhynchid have been misidentified; the preorbital opening of metriorhynchids is a neomorph associated with a novel salt gland that evolved independent of the antorbital fenestra, and that the true antorbital cavity of G. araucanensis, as well as this cavity in all other metriorhynchids, is internalized. Although this hypothesis could be considered as unorthodox, it is the one that requires the least ad-hoc assumptions to explain observations. Many phylogenetic studies depict the Thalattosuchia nested within Neosuchia. In these trees, the successive outgroups of Thalattosuchia are Dyrosaurids/Pholidosaurids, followed by derived Neosuchians (Goniopholids, Bernissartia, Eusuchians). All these taxa have a closed antorbital fenestra. Within this phylogenetic scenario, the internalization of the antorbital cavity did not occur in the ancestor of thalattosuchians, but in the ancestor of a much more inclusive clade of neosuchians.