The study of an isolated skull discovered in early late Pliocene (Piacenzian) sands at Antwerp, northern Belgium, southern margin of the North Sea Basin, leads to the description of a new genus and species of porpoise (Odontoceti, Phocoenidae), Septemtriocetus bosselaersi. This is the first clear record of a fossil phocoenid from the North Atlantic. With a cranium slightly larger than the largest Recent phocoenid (Dall's porpoise Phocoenoides dalli), the new species is characterized by a constriction of the premaxilla anterior to the rostrum base, a longitudinally elongated frontal boss, and a large temporal fossa. A cladistic analysis of fossil and Recent phocoenids confirms the monophyly of the Recent phocoenid subgroup and suggests a sister-group relationship between Septemtriocetus and the Japanese Pliocene phocoenid Haborophocoena. The geographic pattern of fossil and Recent phocoenids mainly in the Pacific Ocean, combined with phocoenid phylogenetic relationships, suggests an initial early to early late Pliocene north dispersal via the Arctic Ocean to the North Atlantic for the lineage of Septemtriocetus. A subsequent late Pliocene-Pleistocene dispersal event accounts for the Recent distribution of the harbour porpoise Phocoena phocoena in the North Atlantic.
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