A new ichthyosaur from the Late Jurassic of north-west Patagonia (Argentina) and its significance for the evolution of the narial complex of the ophthalmosaurids

Abstract

Ophthalmosaurids were highly derived and the youngest clade of ichthyosaurs. Their evolutionary history comprises ~76 Myr (from the early Middle Jurassic to the final extinction of the group at the Cenomanian-Turonian boundary). Fossil records indicate that soon after they emerged they achieved a widespread geographical distribution. Analyses of disparity based on craniodental morphologies, and the co-occurrence of different bone histology and microstructure among the members of the group, indicate that ophthalmosaurids are not only taxonomically but also ecologically diverse. A region of the skull that is particularly complex among ophthalmosaurid taxa is the external nares and surrounding areas, but there have been few attempts to explore this topic. We describe a new ophthalmosaurid from the Late Jurassic of Patagonia (Argentina) in the genus Arthropterygius and analyse its phylogenetic relationships. A detailed exploration of the skull, using computed tomography, reveals the division of the external nares by means of a stout bony pillar. This condition was previously known only in some Cretaceous forms and, within a phylogenetic framework, its distribution among ophthalmosaurids indicates that this morphology was the result of convergent evolution. Based on available data on extant (and some extinct) forms, we explore putative soft structures involved in the narial region. We propose that the division of the external nares (complete or partly osseous) was related functionally to the separation of the air passage from the outlet of nasal salt glands in such a way that the salt glands could be evacuated underwater, while the air passage could be closed by a valvular system.