Diversity and evolution of small mammals in Thailand

Caves and fissure fillings are the most suitable places for the accumulation of vertebrate fossil material. However, in most cases, the fossil concentration of bones is the result of predator activity, either small (owls) or large (carnivores), and in these cases, the fossil communities show an association of species well representative of the corresponding ecosystem. Therefore, to reconstruct the history of biodiversity and the processes of the deposits, the study of karstic fissures and cave fillings is the best approach. They are numerous and usually yield very abundant fossil microvertebrates, expecially for the late Tertiary and the Quaternary periods. Additionally, preservation of bones is enhanced by the limited amount of weathering within caves and their alkaline environment. There are many caves in limestone mountains all over Thailand but Western Thailand especially the Kanchanaburi area provides an exceptionally high number of karstic fissures and caves. This area is also the most active area in Thailand for limestone mining, because of its proximity to Bangkok. Therefore, a detailed survey of limestone quarries of Kanchanaburi area provides the easiest way to find new localities rich in fossils. Small mammals are good indicators of the paleoenvironment and of plaeoclimatic conditions because they are extremely dependent on vegetation cover. They are also easy to identify at the species level with the help of their teeth and on the basis of a comparative study with extant species. The fossil communities can include extinct species, species which show more primitive characters than the extant ones, extant species which had a different geographic distribution in the past and also numerous extant species. Therefore the study of the changes in species composition through the Plio-Pleistocene will provide important information not only concerning the origin and evolution of extant micromammal communities, but also the paleoenvironmental changes which were driving these changes. The study of karstic and cave micromammals is the most promising way to understand the history of biodiversity. Seven localities were discovered in this study. Thiry-five species of rodent were found. Of these, 5 species are new. These fossils show that the environment and climate changed through time from grassland mixed with forest at 3-4 million years ago to forest in the present day. The radiation of rodents is related to the development of forest.