Fossil Marine Mammals from the Koetoi Formation (Middle Late Miocene to Early Pliocene) in Wakkanai City, Northern Hokkaido, Japan

Abstract

A survey on the fossil marine mammals was carried out in Wakkanai City and adjacent area in September, 1991, as oart of the Natural History Research Project of northern Hokkaido led by the National Science Museum during the years 1990 and 1991. No marine mammal fossils have been reported so far from northern Hokkaido except for a list of several findings of cetacean fossils (Kimura, 1985). Several specimens of marine mammal fossils became available for the present study with the help of Mr. Toshiya Miyauchi, a naturalist living in Wakkanai City. All the specimens from the Koetoi Formation which is correlated with the middle Late Miocene to Early Pliocene in age based on the diatom biostratigraphy (Akiba, 1979). One specimen, NSM-PV 19734 from Naipopochi (Fig. 1-1), is an anterior half of the braincase and shows such characters as dorsoventrally flattened and laterally expanded braincase, reduced olfactory bulb that is located on the antedorsal margin of the braincase, and indication of prominent anterolateral corner of braincase. Therefore, the specimen is identified as Otariinae gen. et sp. indet. Another specimen, NSM-PV 19735 from the north shore of Onuma (Fig. 1-2), is an incomplete left mandible which shows such characters as parallel sharp ridges along the tooth row, wide mandibular fossa, short and laterally flattened mandibular ramus, and an outline of ventral margin similar to that of Phocoenoides, and therefore is identified as Phocoenoidinae gen. et sp. indet. The third specimen, NSM-PV 19736 also from the north shore of Onuma, is a partial skull that shows such characters as an eminence (boss) on each premaxilla anterior to the narial opening, an atrophied posterior termination of the premaxilla, and palatine bones that are widely exposed anterior to the narial opening and forms a wide flattened area, and therefore is identified as Phocoenidae subfamily indet. There are a few more specimens known from the Koetoi Formation, but they are not identified at present mainly because the preparation is hardly completed. All three specimens represent the first records of marine mammal fossils from northern Hokkaido. In addition, NSM-PV 19734 is the first record of the Callorhinus-like small fur seal from the north-western Pacific during the time period from the Late Miocene and Early Pliocene, and NSM-PV 19735 and 19736 are the first records of Phocoenidae from the north-western Pacific during the Tertiary. Unfortunately, since none of them has been identified at generic or specific level, it is difficult to contribute to the discussion of phylogeny of either family. Otariidae and Phovoenidae. But, at least two phocoenid specimens have potential to be identified at generic or specific level after the preparation is completed.