New fossil and isotope evidence for the Pleistocene zoogeographic transition and hypothesized savanna corridor in peninsular Thailand

Abstract Despite the explanation that the present-day biogeographic boundary between the Indochinese and Sundaic subregions at the Kra Isthmus, the narrowest part of Peninsular Thailand, in relation to differences in faunal and floral composition is almost conventionally valid for many groups of organisms, the distribution limits of modern and Pleistocene mammals in this region remain unclear. Moreover, environmental factors driving the past distribution patterns in a Southeast Asian mammal community and vegetation types contributing to the controversial hypothesis of an equatorial savanna corridor in the Thai-Malay Peninsula have rarely been demonstrated due to the scarcity of data. Since the discovery of a new Pleistocene fossil site, Yai Ruak Cave in Krabi Province, mammal fossils have been recovered from infilling sediments within the Permian karsts. On the basis of our first excavation, four mammal taxa: Hystrix cf. brachyura, Crocuta crocuta ultima, Rhinoceros sondaicus, and Rusa unicolor are taxonomically identified. A preliminary biochronological age ranging from the late Middle to early Late Pleistocene for the fauna based on the presence of C. c. ultima is proposed. The occurrence of C. c. ultima in this cave also represents the southernmost record of its known distribution, thus suggesting a non-concordant southern range limit of the Indochinese species at the Isthmus of Kra, but somewhere south of Krabi, during the Pleistocene. A bulk analysis of stable carbon isotopes investigated from tooth enamel of these mammals exhibits a variety of habitats ranging from pure C3 to C4 ecosystems, implying the existence of an open grassland landscape. The serial δ18O data collected along the tooth crown heights of R. sondaicus possibly reflect no or little seasonal variation with high annual precipitation due to a major influence of both Southwest and Northeast monsoons in the region, similar to the modern climate. Unlike today, more open vegetation/forest-grassland mosaic was however dominant in Peninsular Thailand at that time. The southward distribution range of the spotted hyaena was likely limited by the discontinuous north-south savanna corridor, latitudinally separated by a transequatorial rainforest belt, starting around the northern part of Sundaland. The ecological distinction between closed and open canopy habitats in the Thai-Malay Peninsula remarkably played a major role in confining the southward distribution of some Indochinese species and in obstructing the migration of grassland-inhabiting taxa across the Sundaic subregion during the Pleistocene glacials.