Stegodon

''Stegodon'' ("roofed tooth" from the Ancient Greek words , , 'to cover', + , , 'tooth' because of the distinctive ridges on the animal's molars) is an extinct genus of proboscidean, related to elephants. It was originally assigned to the family Elephantidae along with modern elephants but is now placed in the extinct family Stegodontidae. Like elephants, ''Stegodon'' had teeth with plate-like lophs that are different from those of more primitive proboscideans like gomphotheres and mastodons. The oldest fossils of the genus are found in Late Miocene strata in Asia, likely originating from the more archaic ''Stegolophodon,'' shortly afterwards migrating into Africa. While the genus became extinct in Africa during the Pliocene.

Morphology

Size

Some species of ''Stegodon'' were amongst the largest proboscideans. ''S. zdanskyi'' is known from an old male (50-plus years old) from the Yellow River that is tall and would have weighed approximately in life. It had a humerus long, a femur long, and a pelvis wide. Size varies across species, large stegodonts are comparable in size with modern elephants. Aside from ''S. zdanskyi,'' species like ''S. ganesha, S. miensis, S. orientalis, S. elephantoides'' and ''S. kaisensis'' are also relatively large bodied. Large stegodonts usually occur in the mainland. There also exist medium sized stegodonts present in large islands like those of Japan and Java. These stegodonts may include: ''S. aurorae, S. trigonocephalus, S. insignis'' and ''S. florensis florensis''. Stegodonts that live in smaller islands usually result in further dwarfism.

Dwarfism

''S. florensis insularis'' is an extinct subspecies of ''Stegodon'' endemic to the island of Flores, Indonesia, and an example of insular dwarfism. The direct ancestor of ''S. florensis insularis'' is the larger-bodied ''S. florensis florensis'', from Early Pleistocene and early Middle Pleistocene sites on Flores.Van Den Bergh, G. D., Aweb, R. D., Morwood, M. J., Sutiknab, T., Jatmikob and Saptomo, E. W. 2008. The youngest stegodon remains in Southeast Asia from the Late Pleistocene archaeological site Liang Bua, Flores, Indonesia. Quaternary International 182(1): 16–48. Remains of ''S. florensis insularis'' are known from the cave of Liang Bua.

Similar to modern-day elephants, stegodonts were likely good swimmers, as their fossils are frequently encountered on Asian islands (such as Sulawesi, Flores, Timor, Sumba in Indonesia; Luzon and Mindanao in the Philippines; Taiwan; and Japan), all locations not connected by land bridges with the Asian continent even during periods of low sea level (during the cold phases of the Pleistocene). A general evolutionary trend in large mammals on islands is island dwarfing. Many among the dwarfed species of Stegodonts came from the lineage of ''S. ganesha, S. zdanskyi'' and ''S. elephantoides''. The smallest dwarf species known is ''S. sumbaensis'' from Sumba, with an estimated body mass of 250 kg. The slightly larger ''S. sondaari'', known from Early Pleistocene layers on the Indonesian island of Flores, had an estimated body weight of between 355 and 650 kg. Another estimate gives a shoulder height of and a weight of . Philippine pygmy stegodonts also have a small stature bigger than or around the size of ''S. sondaari'' and ''S. sompoensis'' of Celebes, with ''S. mindanensis'' having a projected weight of 400'' ''kg. ''S. luzonensis'' and ''S. sompoensis'' have estimated masses of around 1,300 kg and 1,000 kg respectively. A medium- to large-sized stegodont, ''S. florensis'', with a body weight of about 1,700 kg, appeared about 850,000 years ago, and then also evolved into a dwarf form, ''S. f. insularis'', with an estimated body mass of about 570 kg. Another estimate gives a shoulder height of and a weight of . The latter was contemporaneous with, and hunted by, the dwarf hominin ''Homo floresiensis'', and disappeared about 49,600 years ago, earlier than initially thought. Dwarf stegodonts were believed to be the main prey of the still-extant Komodo dragon before modern humans introduced their modern main prey in its range, banded pig, rusa deer, and water buffalo.

Ecology

Like modern elephants, but unlike more primitive probscideans, ''Stegodon'' is thought to have chewed using a proal movement (a forward stroke from the back to the front) of the lower jaws. Based on dental microwear analysis, populations of ''Stegodon'' from the Pleistocene of China and Southeast Asia were found to be browsers, with clear niche differentiation from sympatric '' Elephas'' populations, which tended towards mixed feeding (both browsing and grazing).

Taxonomy

In the past, stegodonts were believed to be the ancestors of the true elephants and mammoths, but currently they are believed to have no modern descendants. ''Stegodon'' is likely derived from ''Stegolophodon'', an extinct genus known from the Miocene of Asia. ''Stegodon'' is generally considered to be more closely related to elephants and mammoths than either gomphotheres or mastodons. Some taxonomists consider the stegodonts a subfamily of the Elephantidae. 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, a population of Indian elephants, possibly due to inbreeding, exhibits many ''Stegodon''-like morphological features. These primitive features are considered recent mutations rather than atavisms.

Fossils of the small, specialized stegodont ''S. aurorae'' are found in the Osaka Plain, Japan, and date from around 2 million to 7 million years ago. This species possibly evolved from ''S. shinshuensis''.

Phylogeny

The following cladogram shows the placement of the genus ''Stegodon'' among other proboscideans, based on hyoid characteristics:

List of species

* ''Stegodon kaisensis'' Late Miocene – Pliocene, Africa
* ''Stegodon zdanskyi'' Late Miocene – Pliocene, China
* ''Stegodon huananensis'' Early Pleistocene, China
* ''Stegodon orientalis'' Middle – Late Pleistocene, China, Southeast Asia, Japan
* ''Stegodon namadicus/S. insignis/S. ganesa'' Pliocene – Late Pleistocene, India
* ''Stegodon miensis'' Pliocene, Japan
* ''Stegodon protoaurorae'' Late Pliocene – Early Pleistocene, Japan
* ''Stegodon aurorae'' Early Pleistocene – early Middle Pleistocene, Japan
* ''Stegodon sondaari'' Early Pleistocene, Flores
* ''Stegodon florensis'' Middle – Late Pleistocene, Flores
* ''Stegodon luzonensis'' Pleistocene, Philippines
* ''Stegodon trigonocephalus'' Pleistocene, Java
* ''Stegodon sompoensis'' Pleistocene, Sulawesi
* ''Stegodon sumbaensis'' Pleistocene, Sumbawa
* ''Stegodon timorensis'' Pleistocene, Timor

Extinction

''Stegodon'' became extinct in the Indian subcontinent, mainland Southeast Asia and China by the end of the Late Pleistocene epoch, while Asian elephants, which existed in sympatry with ''Stegodon'' in these regions, are still extant. The survival of the Asian elephant as opposed to ''Stegodon'' has been suggested to be due to its more flexible diet in comparison to ''Stegodon''. 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).H. Saegusa
"Comparisons of Stegodon and Elephantid Abundances in the Late Pleistocene of Southern China"
, The World of Elephants – Second International Congress, (Rome, 2001), 345–349. 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.

References

{{Taxonbar, from=Q772364

Stegodontidae
Miocene proboscideans
Miocene mammals of Asia
Miocene mammals of Africa
Pliocene mammals of Africa
Pliocene mammals of Asia
Pleistocene mammals of Africa
Pleistocene mammals of Asia
Pliocene proboscideans
Pleistocene proboscideans
Miocene genus first appearances
Prehistoric placental genera
Fossil taxa described in 1847
Taxa named by Hugh Falconer
Pleistocene genus extinctions