Neurocranial bones are key to untangling the sea cow evolutionary tree: osteology of the skull of Sobrarbesiren cardieli (Mammalia: Pan-Sirenia)

Abstract

The pan-sirenian Bauplan is conservative, probably owing to the constraints of adaptation to an aquatic lifestyle. Gathering morphological data from extinct forms is complex, resulting in poorly resolved phylogenies for stem pan-sirenians. Extant sirenians ossify the falx cerebri and the tentorium cerebelli, membranes of the dura mater of the brain attached to the parietal bone. Nevertheless, these ossifications are not present in some pan-sirenians. The basioccipital bone has received little attention in the literature except for establishing the relative age of individuals. Here, we present new cranial elements and a detailed description of the skull of Sobrarbesiren cardieli, a stem pan-sirenian from the Lutetian of Spain represented by eight individuals; we study its intraspecific variation and palaeoecological implications and explore the evolution of the endocranial structures and the basioccipital bone in pan-sirenians. Six new phylogenetic characters are added to the latest pan-sirenian dataset, resulting in a well-resolved topology where Sobrarbesiren is recovered close to the root, in a clade with Prototherium and Eotheroides aegyptiacum. The basioccipital bone and the ossified endocranial membranes have a phylogenetic signal, and the absence of such endocranial structures represents the plesiomorphic condition for pan-sirenians and is not diagnostic for the family Protosirenidae as previously believed.