GEOLOGY AND AGE

The Dota fauna is from the outcrops intermittently exposed on the left bank of Kiso River near Dota, Kani City, Gifu Prefecture, central Japan (Tomida and Setoguchi 1994, figure 1-1). Stratigraphically, those exposures represent about 10 m of sediment near the top of the upper member of the Nakamura Formation (Shikano 1995, figure 10). That part of the formation consists of alternating thin beds of fine sandstone, siltstone, and mudstone, relatively thick sandstone with or without cross-beddings, and thin lignite beds (Shikano 1995). Other than small mammals, many isolated bones of fresh water fishes, including amiids (Yabumoto et al. 1999) and numerous isolated or partly articulated pharyngeal teeth of cyprinids (Yasuno 1982), and freshwater bivalves (mostly articulated) are known from near the top of the formation. Carbonized plant material is also occasionally found from the sandstones (Shikano 1995).

The geologic age of the Nakamura Formation has been generally thought to be slightly younger than the underlying Hachiya Formation, which is supposed to be 22 – 19 Ma, based on fission track dating. However, recent fission track ages obtained from the upper member of Nakamura Formation (Kobayashi 1989; Shikano 1995) and more detailed stratigraphic study of both formations suggest that the Nakamura and Hachiya Formations may have been deposited at about the same time, and the age of the upper member of the Nakamura Formation likely falls between 21 and 18 Ma at most (Shikano 1995). However, if this wide range of ages is correct, the maximum makes the age of the lower member too old. Therefore, the age of the upper member is more likely around 19 Ma.

The Dota fauna currently consists of the following eight taxa: one insectivore (Plesiosorex sp.), one lagomorph (cf. Amphilagus sp.), three castorid rodents (Youngofiber sinensis, Anchitheriomys sp., Eucastor ? sp.), and three eomyid rodents (Megapeomys sp. (described below), gen. et sp. nov. (= ancestral form of Keramidomys), gen. et sp. indet.) (Tomida 2000). They may be, in general, correlated with MN 3 or 4 of the European land mammal age zonation, but considering the absolute age mentioned above, the fauna is more likely correlative with MN 3. The absolute age of MN 3 is considered to be between 20.5 and 18 Ma in Europe (Steininger 1999).