Siwalik Hills is a range of foothills of the Himalayas extending from northeast Pakistan through northern India to southwest Nepal, famous for its rich fossil beds containing extinct apes and other primates. The region has a long history of paleontological research and exploration, beginning in the early part of the 19th century. However, the most intensive research has been conducted since 1973 in the Potwar Plateau of Pakistan by David Pilbeam and his colleagues.
The fossiliferous sediments in the Siwalik Hills are very extensive, measuring several kilometers in thickness. During the Miocene (23-5 million years ago), active uplift of the Himalayas led to increased erosion, and this produced massive volumes of sediment that were deposited by floodplains and rivers. The sediments range in age from more than 22 million years old to less than 1 million years old and provide a remarkably complete fossil record by which to document the evolutionary history of mammals in the region. The sedimentary sequence in the Potwar Plateau has been subdivided into a series of geological formations: Kamlial Formation (18-14 million years old), Chinji Formation (14-10 million years old), Nagri Formation (10-8 million years old), Dhok Pathan Formation (8-5 million years old), and Soan Formation (5-1 million years old).
In addition to fossil apes (hominoids), several species of strepsirhines (sivaladapids and lorisids) and cercopithecoids (Old World monkeys) are known from the Siwalik Hills. In addition, a few isolated teeth from the middle Miocene Kamlial Formation (16 million years ago) belong to a small catarrhine primate (about 3-4 kg) that is quite similar dentally to contemporary species from China and East Africa. They appear to belong to a lineage of primitive catarrhines that originated prior to the divergence of the Old World monkeys and apes. Two groups of fossil strepsirhine primates are known from the Siwalik Hills. The sivaladapids, represented by Indraloris and Sivaladapis from the Chinji and Nagri Formations (14-9 million years ago), are the last surviving members of a diverse group of strepsirhines (the Adapiformes) that inhabited Europe during the Eocene (34-55 million years ago). They are medium-sized (4-7 kg), arboreal primates with specialized teeth for leaf eating. Slightly later in time, from the Dhok Pathan Formation (8 million years ago), is Nycticeboides, which appears to be closely related to the living slow loris from Southeast Asia. Old World monkeys or cercopithecoids first appear in the Siwalik Hills in the Dhok Pathan Formation and are the only fossil primates known from the Soan Formation. Presbytis sivalensis (or Semnopithecus sivalensis) from the Dhok Pathan Formation (7-8 million years ago) is a small colobine monkey related to the living Asian langurs. A baboon-like cercopithecine (Procynocephalus subhimalayanus) and a macaque (Macaca palaeindica) are recorded in the Soan Formation at 2.0-3.5 million years ago, while a gelada baboon, Theropithecus oswaldi delsoni, is know from somewhat younger deposits dating to 1.5-0.9 million years ago.
The most common and best-known fossil ape in the Siwalik Hills is Sivapithecus. Three species are represented: S. indicus (12.5-10.5 million years ago), S. parvada (10 million years ago), and S. sivalensis (8.5-9.5 million years ago). Smaller, female specimens of Sivapithecus were previously included in the genus Ramapithecus, once considered to be a human ancestor. However, with the recovery of additional specimens, especially a partial skull of S. sivalensis, it became clear that Sivapithecus was more closely related to the orangutan. Other fossil apes from the Siwalik Hills include “Sivapithecus” simonsi (10.5-11.5 million years ago), which is probably more closely related to Dryopithecus from the later Miocene of Europe than to Sivapithecus, and Gigantopithecus giganteus, the largest of the Siwalik apes (about the size of a modern gorilla), which is known from the Dhok Pathan Formation (6.5-7.0 million years ago).
References:
- Fleagle, J. G. (1999). Primate adaptation and evolution. San Diego: Academic Press.
- Hartwig, W. C. (Ed.). (2002). The primate fossil record. Cambridge: Cambridge University Press.
