Echoes from the Miocene: tracing the anatomy and phylogeny of <i>Prosqualodon australis</i> (Cetacea: Odontoceti)

Odontocetes compose the most ecologically and taxonomically diverse clade of marine mammals. All modern odontocetes are homodont, but the early history of this group (Oligocene–Early Miocene) was dominated by a variety of bizarre forms with archaic features. Among these archaic odontocetes was <i>Prosqualodon australis</i>, a medium-sized, brevirostrine species from the Early Miocene of Patagonia (Argentina). It was originally described based on a partial skull; however, extensive fieldwork in Early Miocene outcrops in Patagonia has yielded several new specimens, including skulls with well-preserved basicranium, ear bones and postcranium. Based on this broad new sample we provide an updated anatomical description and phylogenetic analysis of <i>P. australis</i> and discuss other referred specimens to the species. We also perform a taxonomic revision of the other two species of the genus, <i>P. davidis</i> (Australia) and ‘<i>P</i>.’ <i>hamiltoni</i> (New Zealand). Our studies confirmed the phylogenetic position of <i>P. australis</i> as an early-diverging (i.e. stem) odontocete and add diagnostic characters from the basicranium and periotic to the already known features that define <i>P. australis</i>: presence of a triangular-shaped parafalciform fossa, and a narrow and deep furrow that originates from the periotic fossa and is anteromedially blind, periotic with a promontorial groove associated with an anteroventral tubercle and well-developed dorsal crest. Most anatomical differences between <i>P. australis</i> and the holotype of <i>P. davidis</i> are interpreted as ontogenetic variation, suggesting that <i>P. davidis</i> is a junior synonym of <i>P. australis</i>, making it the first circumpolar odontocete species from the Southern Hemisphere. Conversely, the anatomical and phylogenetic differences between <i>P. australis</i> and ‘<i>P</i>.’ <i>hamiltoni</i> suggest that these taxa are not congeneric. The results of our work provide, for the first time, a thorough anatomical and systematic revision of the genus <i>Prosqualodon</i>, focusing on <i>P. australis</i>, based on one of the most outstanding fossil odontocete samples.