The oldest record of Alligator sinensis from the Late Pliocene of Western Japan, and its biogeographic implication

Highlights

• The oldest record of Alligator sinensis is reported from the Pliocene of Japan.

• Japanese A. sinensis is large in size (201 cm).

• They might be wiped out due to tectonic and climatic changes in the Plio-Pleistocene.

Abstract

The late Cenozoic fossil record of alligators in East Asia is crucial in understanding the origin and past distribution of Asian alligators that are now represented by a single species, Alligator sinensis. This study reports a partial skeleton of A. sinensis from the Late Pliocene (approximately 3.0 Ma) of western Japan. This Japanese A. sinensis is large in size (>200 cm total length), comparable to the maximum size of extant individuals. It demonstrates the oldest record of A. sinensis and wider distribution of this species in the past. Tectonic and geographic history of East Asia suggests that alligators presumably dispersed into Japan before 25 Ma or after 10 Ma, yet finally were wiped out from Japan due to the semi-isolated condition of the Japanese island arc and the deteriorated climate during the Plio-Pleistocene.

Introduction

Alligator sinensis is the only extant alligatorid species found outside the Americas. It is one of the critically endangered species of the world’s crocodylians and is now restricted to a small area in Anhui Province, China (Jiang, 2010, Thorbjarnarson and Wang, 1999). Fossil and archaeological evidence (Fig. 1; Table 1) indicates that its past distribution extended north to Wangyin, Shandong Province, China (Fig. 1: #10), and south to Penghu Channel, west of Taiwan (Fig. 1: #4). If the potential A. sinensis materials are included, the distribution is extended further south to Ban Si Liam, Nakhon Ratchasima Province, Thailand (Fig. 1: #1). Alligator bones and skins also have been collected in other Neolithic and Bronze Age sites of China (Dawenkou cultural site in Tai’an, Shandong Province; Longshan cultural sites in Linqu and Sishui, Shandong Province, and Linfen, Shanxi Province; Shang Dynasty royal tombs in Anyang, Henan Province), although those remains were related to burials of high-ranked people and could have been brought from other regions (Liu, 1994, Thorbjarnarson and Wang, 2010). However, compared with the solid archaeological evidence, fossil records of A. sinensis hinting the origin and past distribution of the species are still limited; the sole definitive A. sinensis is a skull from the Late Pleistocene of Taiwan (Shan et al., 2013).

In late 1990s, a series of excavations of the Pliocene Tsubusagawa Formation at Ajimu-machi, Usa City, Oita Prefecture, Japan, yielded numerous vertebrate fossils including fishes (Cyprinidae and Bagridae), a salamander (Cryptobranchidae), crocodylians (Alligatoridae and Crocodylidae), turtles (Platysternidae, Trionychidae, and Bataguridae), birds (Pelecaniformes, Falconiformes, Anseriformes, and Gruiformes), and mammals (Stegodontidae, Cervidae, Ursidae, and Rhinocerotidae) (Aoki, 2001, Hirayama, 2001, Kato, 2001, Matsui et al., 2001, Matsuoka, 2001, Nakajima and Kitabayashi, 2001, Takahashi and Kitabayashi, 2001, Watanabe and Kitabayashi, 2001). This fossil assemblage was named “Ajimu fauna” (Takahashi, 2001). Aoki (2001) suggested the presence of A. sinensis among the Ajimu fauna based on fragmentary cranial and postcranial elements but did not provide a thorough description and a taxonomic assessment. Because any fossil A. sinensis or its potential kin from the Cenozoic of East Asia is crucial in the study of Alligator biogeography, we here provide a detailed description and a firm basis for the taxonomic identification of the Ajimu Alligator and discuss its biogeographic significance.