STEGODON

Stegodon (meaning “roofed tooth” from the Greek words στέγειν stegein ‘to cover’ and ὀδούς odous ‘tooth’, because of the distinctive ridges on the animal’s molars) 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). Stegodonts were present from 11.6 mya to the late Pleistocene, with unconfirmed records of localized survival until 4,100 years ago. Fossils are found in Asian and African strata dating from the late Miocene; during the Pleistocene, they lived across large parts of Asia, East and Central Africa.

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 BCE (4,100 BP). However, Turvey et al. (2013) reported that one of the faunal assemblages including supposed fossils of Holocene Stegodon (from Gulin, Sichuan Province) is actually late Pleistocene in age; other supposed fossils of Holocene stegodonts were lost and their age cannot be verified. The authors concluded that the latest confirmed occurrences of Stegodon from China are from the late Pleistocene, and that its Holocene survival cannot be substantiated.

Stegodon was one of the largest proboscideans, along with more derived genera. S. zdansky is known from an old male (50+) from the Yellow River that is 3.87 metres (12.7 ft) tall and would have weighed approximately 12.7 tonnes (12.5 long tons; 14.0 short tons) in life. It had a humerus 1.21 metres (4.0 ft) long, a femur 1.46 metres (4.8 ft) long, and a pelvis 2 metres (6.6 ft) wide.

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 elephants and mammoths. Some taxonomists consider the stegodonts a subfamily of Elephantidae. Both Stegolophodon and primitive elephants were derived from the Gomphotheriidae. The most important difference between Stegodon and (other) Elephantidae can be observed in the molars. Stegodont molars consist of a series of low, roof-shaped ridges, whereas in elephants each ridge has become a high-crowned plate. Furthermore, stegodont skeletons are more robust and compact than those of elephants.

In Bardia National Park in Nepal, there is a population of Indian elephants that, possibly due to inbreeding, exhibit many Stegodon-like morphological features. These primitive features are considered recent mutations rather than atavisms.

Fossils of the small specialized stegodont Stegodon aurorae are found in the Osaka Plain, Japan, and date from approximately 2 Ma to 7 Ma. This species possibly evolved from Stegodon shinshuensis.