Paleoecological reconstruction of Late Pleistocene deer from the Ryukyu Islands, Japan: Combined evidence of mesowear and stable isotope analyses

Abstract

Abstract The Ryukyu Islands (Amami Islands, Okinawa Islands, and Sakishima Islands) form an island arch situated at the southern end of the Japanese archipelago. In this area, numerous fossiliferous localities have been estimated to date from the end of the Pleistocene, which have yielded abundant vertebrate fossil remains. Among the excavated fossils of terrestrial vertebrates, two extinct deer species, Cervus astylodon and Dicrocerus sp., are representative of the Late Pleistocene fauna of the Okinawa Islands. However, their ecological characteristics have been largely unknown to date. In the present study, we reconstructed the paleoecology of the deer species using mesowear and stable isotope analyses of tooth remains excavated from the Hananda-Gama Cave, Okinawa Island. Mesowear analysis estimates the properties of consumed food from the facet development of ungulate cheek teeth, with more abrasive diets (i.e., grass) resulting in more rounded cusps with a lower profile. Carbon isotopic composition (δ13C) reflects relative contribution of C3 (woody and most herbaceous plants) and C4 (grass growing in temperate to tropical regions) plants in diets, with the former showing lower δ13C values. Both extinct deer had sharp cusps with high profiles, which implied a significant amount of browse in diets. Our stable isotope analysis corroborated this: δ13C values were in the range of extant C3 feeders. Species ranges overlapped each other; however, C. astylodon had a wider range into the higher δ13C value range than Dicrocerus sp., which implied dietary niche differentiation between the two species. The dominance of C3 browse in their diet supports a hypothesis that hypsodont molar of C. astylodon was not an adaptive response to abrasive and wear-induced diet but correlated evolution with its elongated longevity, which was considered to be up to 26 years of age.