Stegodon

[steg-uh-don]

Stegodon is a genus of the extinct subfamily Stegodontinae of the order Proboscidea. Stegodonts lived in large parts of Asia during the Pliocene and Pleistocene epochs. Some Stegodon species were among the largest of all Proboscidea, with adults being 13 feet high at the shoulder, 26 feet long, not including 10 feet long nearly straight tusks. In some individuals the tusks were so close together that the trunk probably did not lie between them but instead draped over. A dwarf population survived until 12,000 years ago on the Island of Flores. A review of 130 papers written about 180 different sites with Proboscidea remains in southern China revealed Stegodons to have been more common than Asian Elephants; the papers gave many recent radiocarbon dates, the youngest being 2,150 B.C. (4,100 BP). The name Stegodon is derived from the Greek words στεγειν stegein ('to cover') and οδον odοn ('tooth') because of the distinctive ridges on the animal's molars.

Relationship

In the past, stegodonts were believed to be the ancestors of the true elephants and mammoths, but it is currently believed that they have no modern descendants. Stegodon is derived from the genus Stegolophodon, an extinct genus known from the Miocene of Asia. Stegodon is considered to be a sister group of the mammoth, as well as the elephants. Some taxonomists consider the stegodonts as a subfamily of the Elephantidae. Both Stegolophodon and primitive elephants were derived from the Gomphotheriidae. The most important difference between Stegodon and the Elephantidae can be observed in the molars. Molars of stegodonts consist of a series of low, roof-shaped ridges, whereas in elephants each ridge has become a high-crowned plate. Furthermore, the skeletons of stegodonts are more robust and compact than those of elephants.

In the Bardia National Park in Nepal, there is a population of Indian Elephants which, due to inbreeding are very similar to Stegodon and may retain many Stegodon features. Some dismiss these primitive features as recent mutations rather than atavisms.

Dwarfing on islands

Like elephants, stegodonts must have been good swimmers. Their fossils are frequently encountered on Asian islands, which even during periods of low sea-level (during the cold phases of the Pleistocene) were not connected by landbridges with the Asian continent (Sulawesi, Flores, Timor, Sumba in Indonesia, Luzon and Mindanao in the Philippines and in Taiwan and Japan). A general evolutionary trend in large mammals on islands is island dwarfing. The smallest dwarf species, Stegodon sondaari, known from 900,000 year old layers on the Indonesian island of Flores, had an estimated bodyweight comparable to a water buffalo. Another dwarf species lived on Flores more recently, and was contemporaneous with the hominin discovered in 2003, Homo floresiensis. Stegodon tetrabelodon syrticus was a spectacular late Miocene species with four tusks described from a partial cranium and jaws found in North Africa

There are twelve known species of Stegodon: