New sperm whale cranium from the late Miocene and a revised family attribution for the small crown physeteroid "Thalassocetus" / Apolline Alfsen

New sperm whale cranium from the late Miocene and a revised family attribution for the small crown physeteroid Thalassocetus / Apolline Alfsen, in colloque "1st Virtual Conference for Women Archaeologists and Paleontologists. Nouveaux apports à l’étude des populations et environnements passés" organisé par le laboratoire Travaux et Recherches Archéologiques sur les Cultures, les Espaces et les Sociétés (TRACES) de l’Université Toulouse Jean Jaurès et le laboratoire Paléontologie Évolution Paléoécosystèmes (PALEVOPRIM) de l'Université de Poitiers, sous la responsabilité scientifique de Julie Bachellerie, Ana Belén Galán López (Traces), Émilie Berlioz et Margot Louail (Palevoprim). Université Toulouse Jean Jaurès, 8-9 mars 2021. Session 1 : Morphological variability, taxonomy and adaptations.

Among cetaceans, toothed whales (Odontoceti) are known to display a unique arrangement in their skull that has been linked to their echolocation abilities. Their nasal and facial regions lack of symmetry and some of their right-sided bones (maxilla, premaxilla) impressively spread onto the left side. Despite the continuously improving fossil record, our understanding of the origin and early radiation of the two modern sperm whale families Kogiidae (incl. the pygmy and dwarf sperm whales Kogia spp.) and Physeteridae (incl. the great sperm whale Physeter) remains limited. The discovery of a new physeteroid cranium from the late Miocene (Tortonian) of Antwerp (Belgium) shed new light on the poorly resolved phylogeny. Among extinct species, based on cranial material from the late early to middle Miocene of Antwerp, the small-sized Thalassocetus antwerpiensis has been recognized as the earliest branching kogiid. The new specimen leads to the description and comparison of a close relative of T. antwerpiensis. Thanks to the relatively young ontogenetic stage of this new cranium, the highly modified plate-like bones making the floor of its facial region could be individually removed, a fact that greatly helped deciphering their identity and geometry. Close morphological similarities with T. antwerpiensis allow the reassessment of several facial structures in the latter; notably the reinterpretation of a crest-like structure, previously identified as a sagittal facial crest, typical for kogiids, and here revised as theleft nasal (lost in known kogiids) and the left maxilla. Implemented in a phylogenetic analysis, the new anatomical interpretations result in the new Belgian specimen and T. antwerpiensis being recovered as sister-groups in the Physeteridae. Consequently, the geologically oldest kogiids are now dated from the Tortonian, further extending the ghost lineage separating these early late Miocene kogiid records from the estimated latest Oligocene to earliest Miocene divergence of kogiids and physeterids.