Evidence of dietary niche partitioning in the Squalodelphinidae (Cetacea, Odontoceti): a new large-sized taxon from the early Miocene of Peru

The South Asian river dolphin (Platanista gangetica) is the only extant survivor of the superfamily Platanistoidea (Cetacea, Odontoceti), a clade characterized by a diversified fossil record since the late Oligocene. The early Miocene family Squalodelphinidae differs from other members of the Platanistoidea by having a moderately elongated and tapered rostrum, single-rooted posterior teeth retaining ornamentation, and marked skull asymmetry. Similar to the extant common dolphin, the few squalodelphinids described are all relatively small in size, having an estimated body length (based on the bizygomatic width of the skull) between ca 2 m (Huaridelphis raimondii) and ca 2.5 m (Squalodelphis fabianii). Based on a partial skeleton (MUSM 2545), including a well-preserved skull, collected from the Chilcatay Formation (Peru), we report here a new squalodelphinid genus and species. A volcanic ash layer sampled near MUSM 2545 yielded a 40Ar/39Ar age of 18.80 ± 0.06 Ma (early Burdigalian, early Miocene). In addition to several morphological characters, this new squalodelphinid differs from all the other genera of the family in its markedly larger size: condylobasal length of the skull greater than 770 mm and bizygomatic width equal to 370 mm. Based on this last measurement, the body length of MUSM 2545 can be estimated to ca 3.5 m. Combined with some cranial features, such as the robust rostrum less tapered than in other squalodelphinids, the large temporal fossa, and the prominent nuchal and temporal crests, this large body size suggests that the new taxon consumed larger prey than other members of its family. Together with Huaridelphis raimondii and Notocetus vanbenedeni, both of them also found in the Chilcatay Formation, this new archaic platanistoid increases further the peculiar local diversity of squalodelphinids along the southeastern Pacific coast, a diversity possibly related to their partition in different, size-related dietary niches.