| Stegodon Temporal range: Late Miocene to Holocene |
|
|---|---|
| Skull | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Proboscidea |
| Family: | †Stegodontidae |
| Genus: | †Stegodon Falconer, 1847 |
Stegodon (meaning "roofed tooth" from the Greek words στέγειν stegein 'to cover' and ὀδούς odous 'tooth'), is a genus of the extinct subfamily Stegodontinae of the order Proboscidea. It was assigned to the family Elephantidae (Abel, 1919), but has also been placed in Stegodontidae (R. L. Carroll, 1988).[1] Stegodonts were present from 11.6 mya to 4,100 years ago. They lived in large parts of Asia during the Pliocene and Pleistocene epochs.[1][2]
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Some Stegodon species were among the largest of all Proboscidea, with adults being 4 metres (13 feet approximate) high at the shoulder, 8 metres (26 feet approximate) long, not including 3 metres (10 feet approximate) 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 proboscidean remains in southern China revealed Stegodon 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).[2] 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.
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 may be derived from 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 (other) 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 Bardia National Park in Nepal, there is a population of Indian elephants which, possibly due to inbreeding, exhibit many Stegodon-like morphological features. Some dismiss these primitive features as recent mutations rather than atavisms.[3]
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 land bridges 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 body weight of 300 kg,[4] smaller than a water buffalo. A medium to large sized stegodont, Stegodon florensis, with a body weight of about 850 kg, appeared about 850 000 years ago, and then also evolved into a dwarf form, Stegodon florensis insularis. The latter was contemporaneous with the hominin discovered in 2003, Homo floresiensis, and disappeared about 12 000 years ago.[4] Stegodon tetrabelodon syrticus was a spectacular late Miocene species with four tusks described from a partial cranium and jaws found in North Africa.[5]
There following are the species classified under Stegodon. The list may be incomplete or inaccurate.