Septidelphis morii n. gen. et sp. from the Pliocene of Italy: new evidence of the explosive radiation of the true dolphins (Odontoceti, Delphinidae)

A new genus and species of true dolphin (Delphinidae), Septidelphis morii, is described on the basis of a partial fossil skeleton (almost complete skull, mandibles, teeth, hyoid bones, 17 vertebrae, several ribs, sternum and sternal ribs), originally belonging to an immature subadult animal, from the late Zanclean-early Piacenzian (Pliocene) marine sands of Piedmont (northern Italy). This new dolphin is characterized by an extreme posterior widening of the dorsal opening of the mesorostral canal and shares with Astadelphis, Etruridelphis, Sousa, and Steno the following combination of characters: (1) long and narrow rostrum; (2) wide premaxillae at the middle of rostrum; and (3) condylobasal length reaching at least 550 mm. A phylogenetic analysis using a molecular scaffold approach placed Septidelphis inside the delphinines in a more apical position than Sousa but basal to the clade formed by all other extant delphinines. Etruridelphis is positioned as the sister taxon of Septidelphis, and Astadelphis is positioned as the sister taxon of a larger grouping that includes this clade and all extant delphinines except Sousa. The phylogenetic position of Septidelphis inside Delphinidae and its stratigraphical age (3.81-3.19 Ma) are consistent with estimated divergence dates reported in a recent molecular study, whereas the first occurrence of Etruridelphis, dated to slightly before 3.98 Ma, could represent a minimal age for the origin of the delphinines and a potential calibration point for molecular clock studies. Septidelphis corroborates the explosive radiation and fast speciation-extinction rate of the delphinids during the Pliocene-Pleistocene.