**2. Geographic setting and paleofauna of the Ryukyu Islands**

The chain of Ryukyu Islands stretches roughly 1200 km between Kyusyu Island of Japan and Taiwan in the west Pacific. More than 150 relatively small islands (<1207 km2 ) are dispersed from the northeast to the southwest at roughly 27° north in latitude (**Figure 1**). A dry and sandy climate generally exists at this latitude around the world, but the Ryukyus are covered with subtropical forests nourished by the hot and humid atmosphere created by the Kuroshio ocean current.

Previously, the Ryukyu Islands were at the eastern end of the Eurasian continent. The tectonic plate movements of the Eurasian and Philippine plates formed the deep ocean basin named the "Okinawa trough" to the west of the Ryukyus and divided the islands from the continent. This geological event probably occurred no later than the early Pleistocene [27]. Today the islands are divided into three parts by the sea, which is over 1000 meters deep: the northern, central, and southern Ryukyus. Some of the northern Ryukyu Islands were connected to the Kyushu Island of Japan during the Last Glacial Maximum (LGM) around 20,000 years ago. The fauna and prehistoric culture of these islands were closely related to those of Kyusyu. By contrast, the central and the southern Ryukyus were isolated throughout the Pleistocene.

The islands now are located at the margin of two biogeographic areas: the Oriental and Sino-Japanese regions [28]. The fauna and flora of the Ryukyus

#### **Figure 2.**

*Overviewed stratigraphy of the Minatogawa site. The uppermost part of the fissure deposit contained Holocene Jomon. The lower level (layers III and IV) contained late Pleistocene animal and human fossils. Although only medium-sized animals are drawn in the figure, many small animal fossils were found in the lower deposit.*

gradually shifted from Sino-Japanese species to Oriental species from the north to the south. The basis of the terrestrial fauna was formed before the isolation from the Eurasian Continent in the early Pleistocene [29–31].

Nearly two million years of isolation have fostered many of the islands' endemic animals. Today, the fauna of the Ryukyus consists of many endemic small animals but lacks large and middle-sized animals except for the wild boar. The late Pleistocene fauna throughout the islands has not been studied thoroughly; it has only been cursorily reviewed by several authors [31–33]. The consensus is that the Pleistocene fauna was similar to the existing fauna of each island. There were several middle-sized animals that are now extinct: one or two species of middle-sized deer and a species of middle-sized tortoise. They became extinct almost simultaneously at the end of the Pleistocene, but the precise timing of extinction is not known yet.

The flora of the Pleistocene Ryukyus was shrouded in mystery. Kuroda and Ozawa (1998) studied pollen samples obtained from a boring core specimen extracted from Izena Island, which is northwest of Okinawa Island [34]. They suggested that the evergreen broad-leaved forest reduced in size and the pine tree became dominant during the LGM. However, the composition of the terminal-Pleistocene fossil-amphibian species of the southern area was similar to the modern fauna of the northern broad-leaved evergreen forest area of Okinawa Island [35]. A dominance of forest species is also reported among avifauna [36, 37], reptiles, and mice [38–40] at the Minatogawa site, a terminal-Pleistocene site of Okinawa Island (**Figures 1** and **2**, **Table 1**). Based on the composition of these fossil species, we can assume that a large area of Okinawa Island was covered with a broad-leaved evergreen forest during the late Pleistocene and even the LGM period.
