ABSTRACT
The mode of occurrence and the stratigraphical record of fossil freshwater molluscs of the Middle Miocene to Pliocene Churia Group in Nepal (mostly equivalent to the Siwalik Group in Pakistan) provide much information about the paleoenvrionmental change related to the upheaval of the Himalayas. In the Churia sedimentary sequence, freshwater molluscan fossils are found from about 10.5 Ma to 3.5 Ma with faunal enrichment marked about 7 Ma [7]. Before 7 Ma, the fauna is rather simple and is mainly composed of typical Indian subcontinent unionid bivalves. After 7 Ma, the diversity of the fauna becomes high by addition of some species regarded as palearctic and Southeast Asian elements. The diversity rapidly increased after 4 Ma.
On the other hand, the first phase of the upheaval of the Himalayas reached to the acme from 9 to 11 Ma, and from 7 to 8 Ma became to have a great influence on the development of the monsoonal condition [1; 10]. The first occurrence of freshwater molluscs coincides with the development of flood deposits of the meandering river system caused by the seasonal heavy rain of the monsoon [8]. The introduction and enrichment of palearctic and Southeast Asian elements in the malacofauna after 7 Ma can be considered as transported by mammals and birds, which has been provoked by the formation of conspicuous climatic zonation in the Eurasian Continent. The major vegetational change at 6 to 7 Ma in the flood plain from forest to grassland, as indicated by the stable isotopic data of pedogenic carbonate, also provides the preferable condition of animal migration along the rivers.
