The periotic of a basal balaenopterid from the Tortonian of the Stirone River, northern Italy (Cetacea, Mysticeti, Balaenopteridae)

Abstract

A new periotic (MuMAB 240508) is described from the Tortonian of the Stirone River, Northern Italy. The new specimen is described and compared to all the known periotics of extant and extinct balaenopterid species. The new specimen shows balaenopterid characters, including the triangular anterior process, the transverse and anteroposterior elongations of the pars cochlearis, as well as in the arrangement of the endocranial foramina of the periotic. The presence of a medial promontorial groove, a lack of separation between the round window and the perilymphatic foramen together with the presence of a protruding set of medial crests suggest that this periotic may belong to an archaic and undescribed balaenopterid species. A phylogenetic analysis was performed to understand the relationships of this specimen within the broader context of balaenopterid phylogeny; this analysis showed that MuMAB 240508 belongs to a basal balaenopterid taxon that is the sister group to all the other Balaenopteridae with the exception of ‘Balaenoptera’ ryani. An ordinary least squares regression analysis showed that a relationship exists linking the length of the anterior process of the periotic and the total body length in Balaenopteridae. By means of such a relationship, the total body length of the individual to which MuMAB 240508 belonged to was reconstructed and resulted in c. 14.4 m. This individual was longer than all the other contemporaneous balaenopterid mysticetes confiming the hypothesis that the origin of large size occurred in this family earlier than previously thought.