Latest Pleistocene Japanese Sea lion (Otariidae) fossil from the riverbed of the Hanamurogawa River west of Kasumigaura Lake, Ibaraki Prefecture

Abstract

A limb bone of a marine mammal, recently recovered from the riverbed of the Hanamurogawa River, Tsukuba City, Ibaraki Prefecture, central Japan, is identified as the left humerus of an adult male Japanese Sea Lion, Zalophus japonicus (pinniped). A metapodial specimen reported by Nakashima et al. (2002) was re-analyzed and identified as an astragalus of an adult female of Z. japonicus. The humerus of the present study is inferred to have been derived from the Sakuragawa Terrace Deposits or their equivalents, based on radiocarbon dating (27,900±120 ¹⁴C BP and 31,950-31,300 cal BP) of bone collagen and the mode of fossil occurrence. This evidence indicates that the deposits bearing mammalian fossils such as Palaeoloxodon naumanni and associated cervids were deposited in the latest Pleistocene. The taphonomical processes by which the humerus and astragalus of Z. japonicus ended up in a fossil assemblage dominated by land mammals should be further investigated in view of human activity during the Late Paleolithic period.