EVOLUTION OF ARVICOLIDAE IN SOUTH ASIA
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概要
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The records of south Asian fossil arvicolids are extremely poor except that the author has recently discovered this fauna in India from the lacustrine and glacio-fluvial sediments of the Kashmir Valley, northwestern Himalaya. The Indian arvicolids are significant for the reasons that : this is one of the most southern limits of fossil arvicolids in the Northern Hemisphere; the fossil localities are very near to what is considered as the centre of arvicolid evolution; and they have phylogenetic link with the European arvicolids. Based on detailed investigations of the Karewa (Kashmir) arvicolids by using the methods such as morphological variation within the population, sinumetric analysis and arrangement of enamel on the occlusal surface of the molars (Schmelzmuster), I suggest that the Karewa arvicolids have phylogenetic relationship with Mimomys lineage of Eurasia. It is assumed that they migrated across the rising Himalayas to populate the Himalayan belt of Baluchistan, Afghanistan and Kashmir. A suggested dispersal scheme envisages that it was European Cseria which migrated into Kashmir Valley prior to 2.4Ma in form of Kilarcola indicus and K. indicus sahnii. Kilarcola shares the basic morphological characters with the Mimomys group but forms an independent evolutionary lineage. It has primitive characters compared to contemporaneous forms of Mimomys group. The Kilarcola lineage in its geographical isolation south of the Himalayas is characterised by a comparatively slower rate of evolution. Unlike the European arvicolids, the Karewa fauna at 1.6Ma does not show highly advanced characters. However, this fauna (Kilarcola kashmiriensis) has more progressive linea sinuosa and more derived Schmelzmuster than that in K. indicus. It is suggested that K. indicus underwent endemic evolution to give rise to K. kashmiriensis at about 1.6Ma ago with comparatively derived characters. Kilarcola probably became extinct sometime in the Pleistocene since we do not find any extant arvicolid in the Himalayan region which might have been derived from Kilarcola. Another arvicolid dispersal in India is documented by the occurrence of Microtus deterrai in the Upper Pleistocene. It has all the derived characters and is as advanced as the European Microtus. However, it is large sized animal and may not have evolved into the present day Microtus of southern Asia.
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