An Early Miocene kentriodontoid (Cetacea: Odontoceti) from the western North Pacific, and its implications for their phylogeny and paleobiogeography

Abstract

So–called ‘kentriodontids’ are extinct dolphin–like odontocetes known from the Early to Late Miocene worldwide. Although recent studies have proposed that they were monophyletic, their taxonomic relationships still remain controversial. Such a controversy exists partly because of the predominance of primitive morphologies in this taxon, but the fact is that quite a few ‘kentriodontids’ are known only from fragmentary skulls and/or isolated periotics. A new ‘kentriodontid’ Platysvercus ugonis gen. et sp. nov. is described based on a nearly complete skull from the upper Lower Miocene Sugota Formation, Akita Prefecture, northern Japan. Based on the phylogenetic analysis of P. ugonis described here, the monophyly of the ‘kentriodontids’ is confirmed, and it is recognized as the superfamily Kentriodontoidea. This new superfamily is subdivided into two families as new ranks: Kentriodontidae and Lophocetidae. Based on the paleobiogeographic analysis of the Kentriodontoidea, their common ancestor emerged in the North Pacific Ocean and spread over the Northern Hemisphere. Initial diversification of the Kentriodontidae in the North Pacific Ocean and the Lophocetidae in the North Atlantic Ocean was recognized as a vicariance event. The diversification and extinction of the Kentriodontoidea could have been synchronously influenced by climate events during the Middle Miocene.

Dawn of the Delphinidans: New Remains of Kentriodon from the Lower Miocene of Italy Shed Light on the Early Radiation of the Most Diverse Extant Cetacean Clade

“…For example, Kimura and Hasegawa [58] built upon their phylogenetic analysis (which recovers K. diusinus as the basalmost Kentriodon species) to suggest that Kentriodon originated in the Pacific Ocean and then repeatedly migrated into the Atlantic Ocean. On the other hand, Guo and Kohno [7] regarded K. schneideri from the North Atlantic as the earliest branching Kentriodon species. These discrepancies are also due to the fragmentary nature of the fossil record, as many Kentriodon species are only known by a single specimen and/or from a single fossiliferous locality.…”

“…represents an artificial, polyphyletic set of archaic delphinidans. Therefore, Kentriodontidae has subsequently been redefined ("Kentriodontidae s.s.") and limited in scope to include the type genus Kentriodon and a few other closely related genera [6,7,54,[56][57][58][59][60]. That said, which and how many genera belong to this family remains controversial.…”

“…Delphinida represents the most diverse extant cetacean clade, with delphinids alone accounting for more than 40% of the total number of living cetacean species [2]. In addition to the many extinct representatives of the aforementioned superfamilies, the fossil record of delphinidans includes a polyphyletic set of more archaic taxa, most of which have often been subsumed into the largely Miocene family Kentriodontidae [3][4][5][6][7]. Overall, the extinct and extant members of Delphinida share a small to intermediate body size, a roughly to fully homodont dentition, a reduced posterior extension of the premaxilla, a posteriorly elongated lateral lamina of the palatine and some peculiar ear bone characters [1,5,8].…”