History of commensal rodents on Ishigaki Island (southern Ryukyus) reconstructed from Holocene fossils, including the first reliable fossil record of the house mouse Mus musculus in Japan

The history of commensal rodents in mainland Japan and the Ryukyu Islands (Ryukyus) has been poorly understood owing to the paucity of reliable fossil records. Recent excavation of the Shiraho-Saonetabaru cave site on Ishigaki Island of the southern Ryukyus yielded a large number of rodent fossils from well-dated sediments ranging from the Late Pleistocene to the Holocene. Some of the fossils were preliminarily assigned to Mus musculus and Rattus sp. They are described here to confirm their taxonomic positions. These forms are considered to be commensal rodents.A history of commensal rodents on Ishigaki Island is reconstructed based on the stratigraphic position and relative abundance of the commensal forms as well as the non-commensal native rodent, Niviventer sp., which is very abundant in most of the horizons of the sediments. Before the end of the Pleistocene, no commensal forms inhabited the island, and the rodent fauna consisted only of the non-commensal form. In the early Holocene, one of the commensal forms, M. musculus, invaded the island, but the continued abundance of the non-commensal form afterward indicates that the invasion did not affect the fauna. In the late Holocene, a second invasion, by Rattus sp., probably caused the rapid decline and extinction of the non-commensal form. Since the extinction, the rodent fauna of the island has comprised only the commensal forms.In Japan, there are no known reliable fossil occurrences of M. musculus, in contrast to several occurrences of Rattus at Middle Pleistocene to Holocene fossil localities. Thus, the fossil of M. musculus described here records the first reliable occurrence of the species in Japan, and is very important for documenting the first appearance of M. musculus in the southern Ryukyus in the early Holocene. The species is considered to have appeared much later in mainland Japan and the central Ryukyus.