Bulletin of the Geological Society of Malaysia, Volume 6, Jul, 1973, pp. 273 – 286
Universiti Kebangsaan Malaysia
Abstract: Geological and paleontological studies as well as our knowledge of Pleistocene vertebrates in Southeast Asia indicate that they originated in the Asian continent and migrated during the Pleistocene period into the island archipelago of Southeast Asia. It seemed that their migration followed two different routes, i.e. a western route which crossed the Sunda Land area, and an eastern route via Taiwan (Formosa) and the Philippines toward the Sunda Land area and Sulawesi (Celebes). In this last island both of those migration routes eventually met sometime in the Upper Pleistocene period.
From the Sunda Land area, the western migration route continued along the geanticlinal ridge of the Lesser Sunda island chain into Flores and from there it went on to arrive eventually in Timor. On the other hand, it seemed that there was no direct link between Sulawesi (Celebes) and the Lesser Sunda island chain; in other words the eastern migration route did not connect with the western one in the Lesser Sunda island chain.
So far, it seemed that the Pleistocene vertebrates had not reached Australia during the time when the Post-glacial sea submerged the areas of the Timor sea and the Sahul shelf. This is substantiated by the fact that up to now there have been no reports made of their discovery on that continent. However, the recent discoveries of Pithecanthropus-like hominids in Victoria (Australia) may suggest that a part of the Pleistocene vertebrate fauna actually did reach the Australian continent (Thorne 1971). The most obvious land-bridge connecting Timor with Australia could be via the Timor sea and the Sahul shelf.
There are no reports of Pleistocene vertebrate fossils from West Irian-Papua, Halmahera and Talaud islands, nor does our relatively little paleogeographic knowledge of those areas gives much idea as to whether there was a land-connection between Mindanau (Philippines) and West Irian-Papua via Talaud and Halmahera islands. The same is also true for the deep Banda sea, which is encircled by Sulawesi (Celebes), the Lesser Sunda islands, Sahul shelf, West Irian-Papua, and the Halmahera islands.
The migration of the Pleistocene vertebrates from the Asian continent to Southeast Asia was made possible by the interaction between orogenetic movements affecting many parts of Southeast Asia and the worldwide lowering of the sea-level during the Pleistocene period. As a result of this interaction many areas of this region emerged above sea-level to become dry land acting as land-bridges through which the above fauna could migrate from Asia toward Australia during the Pleistocene. The orogenetic movements caused block faulting and gave rise to graben and horst-like features.
During the Post-glacial period, the sea submerged the low-lying subsiding grabens. Consequently deep submarine basins and trenches as well as deep sea straits came into being between the numerous islands of Southeast Asia. These deep marine waters prevented the further migration of the Pleistocene vertebrates. Groups of this last fauna isolated on the numerous islands by the submergence of the Post-glacial sea afterward pursued an independent evolutionary trend, which resulted in specific differences among them until they became extinct after the Pleistocene period.