Quaternary Rodent Faunas in the Japanese Islands (Part 1)

Abstract

The results of the study on the rodent remains from the Japanese Quaternary are described and discussed. The remains comprise more than 10, 000 specimens collected from 30 localities in Honshu, Kyushu and the Ryukyu Islands. Their systematic analysis results in the recognition of 18 forms including two new species, Clethrionomys japonicus and Microtus epiratticepoides. On the other hand, the fossiliferous sediments of those localities are examined precisely from the viewpoints of stratigraphy and geochronology. Based on these results, the following faunal succession is reconstructed. From the Early Pleistocene to early Middle Pleistocene, the ancestors of living species migrated into the Honshu, Shikoku and Kyushu areas from various parts of the continent such as Europe, South Siberia, North and South China, and evolved into endemic species to those areas due to the influences of geographical isolation and adaptation to the characteristic temperate forest environments. Among those rodents, sciurids, murids and glirids have not changed in species level since the middle Middle Pleistocene, while arvicolids have taken on drastic changes with new immigrations. Namely, Myopus schisticolor appeared transiently in the middle Middle Pleistocene, and Clethrionomys japonicus underwent rapid divergence into Phaulomys smithii and P. andersoni from the late Middle to Late Pleistocene. Moreover, in the late Middle Pleistocene, Microtus montebelli appeared in the fauna as the last immigrant from the continent. This new arrival came to replace gradually the former inhabitants, M. epiratticepoides and M. cf. brandtioides, from the late Middle to Late Pleistocene. Finally, the last two forms were extinguished in those areas at the end of the Late Pleistocene. Apart from those families, cricetids appeared temporally in the middle Middle Pleistocene. As a whole, the exchange in rodent faunas between those areas and others (continental regions and Hokkaido) ceased in the late Middle Pleistocene, and never took place afterwards. In the Ryukyu Islands, the Late Pleistocene and Holocene rodent faunas are represented by two endemic murids such as Tokudaia osimensis and Diplothrix legata. In spite of the opinions of the previous authors, the former possibly arose from the Parapodemus-Apodemus lineage in Pliocene, while the latter was probably originated from a primitive murid in Pliocene which was rather near to Rattus than to Lenothrix and Lenomys. They might flourish and have wide ranges in the islands until the Late Pleistocene, whereas their distribution was strongly reduced to restricted areas in Holocene.