Stegodon

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Stegodon
Temporal range: Late
Ma
Stegodon skeleton at the Gansu Provincial Museum
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Proboscidea
Family: †Stegodontidae
Genus: †Stegodon
Falconer, 1847
Species
  • †S. aurorae (Matsumoto, 1918)
  • †S. elephantoides (Clift, 1828)
  • †S. florensis Hooijer, 1957
  • †S. ganesha
    (Faloner and Cautley, 1846)
  • †S. kaisensis Hopwood, 1939
  • †S. luzonensis
    von Koenigswald, 1956
  • †S. miensis (Matsumoto, 1941)
  • †S. mindanensis (Naumann, 1890)
  • †S. orientalis Owen, 1870
  • †S. protoaurorae (Aiba et al., 2010)
  • †S. sompoensis Hooijer, 1964
  • †S. sondaari van den Bergh, 1999
  • †S. trigonocephalus (Martin, 1887)
  • †S. zdanskyi Hopwood, 1935

Stegodon ("roofed tooth" from the

gomphotheres and mammutids.[1] The oldest fossils of the genus are found in Late Miocene strata in Asia, likely originating from the more archaic Stegolophodon, subsequently migrating into Africa.[2] While the genus became extinct in Africa during the Pliocene, Stegodon remained widespread in South, Southeast and East Asia until the end of the Pleistocene.[3]

Morphology

The skull of Stegodon is relatively tall but short.[1] The lower jaw in comparison to early elephantimorphs is shortened (brevirostrine), and lacks lower tusks/incisors. The molar teeth are superficially like those of elephants, consisting of parallel lamellae that form ridges but are generally relatively low crowned (brachydont),[1][4] the numbers of ridges are greater in later species.[5] Members of the genus lack permanent premolars.[6] The tusks are proportionally large, with those of the biggest species being among the largest known tusks in proboscideans, with a particularly large tusk of S. ganesa from the Early Pleistocene of India measured to be 3.89 metres (12.8 ft) long, with an estimated mass of approximately 140 kilograms (310 lb), substantially larger than the largest recorded modern elephant tusk.[7]

Size

Skeletal restoration of a large male S. zdanskyi with a shoulder height of around 3.87 metres

Some species of Stegodon were amongst the largest proboscideans. The Chinese species S. zdanskyi is known from an old male (50-plus years old) from the Yellow River that is 3.87 m (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 m (4.0 ft) long, a femur 1.46 m (4.8 ft) long, and a pelvis 2 m (6.6 ft) wide. The Indian S. ganesa is suggested to have a shoulder height of about 3.10 m (10.2 ft), and a body mass of around 6.5 tonnes (6.4 long tons; 7.2 short tons). The Javanese species S. trigonocephalus is suggested to have been around 2.75–2.8 m (9.0–9.2 ft) tall, with a body mass of around 5 tonnes (4.9 long tons; 5.5 short tons).[8]

Dwarfism

Main article: Dwarf_elephant § Indonesia_and_the_Philippines
Estimated sizes of dwarf Stegodon species from Flores compared to a human

Similar to modern-day elephants, stegodonts were likely good swimmers,

Middle Pleistocene which was initially substantially larger, but progressively reduced in size over time, with the earlier subspecies Stegodon florensis florensis from the Middle Pleistocene estimated to be around 50% the size of mainland Stegodon species with a shoulder height of around 190 cm (6.2 ft) and a body mass of around 1.7 tons, while the later Stegodon florensis insularis from the Late Pleistocene is estimated to be around 17% the size of mainland Stegodon species, with a shoulder height of around 130 cm (4.3 ft), and a body mass of about 570 kilograms (1,260 lb).[11][12]

Ecology

Life-sized models of Stegodon

Like modern elephants, but unlike more primitive proboscideans, Stegodon is thought to have chewed using a proal movement (a forward stroke from the back to the front) of the lower jaws. This jaw movement is thought to have evolved independently in elephants and stegodontids.

sympatric Elephas populations, which tended towards mixed feeding (both browsing and grazing).[14][15] In contrast, specimens of Stegodon trigonocephalus from the Early-Middle Pleistocene of Java were found to be mixed feeders to grazers, with a diet similar to that of sympatric Elephas hysudrindicus.[16] The dwarf species from Flores, Stegodon sondaari and Stegodon florensis, are suggested to have been mixed feeders and grazers, respectively, based on stable carbon isotopes.[12] Specimens of Stegodon kaiesensis from the Pliocene of East Africa were found to be browsers to mixed feeders, based on mesowear analysis.[17]

Tracks of a group of Stegodon from the Late Pliocene of Japan suggest that like modern elephants, Stegodon lived in social herds.[18]

On Flores, where dwarf Stegodon species were the only large herbivores, they were likely the main prey of the Komodo dragon.[19] Claims that Stegodon florensis was hunted by Homo floresiensis are based on ambiguous circumstantial association between bones and stone tools, and the rarity of cut marks makes it unclear to what if to any degree, hunting of Stegodon was actually practiced by Homo floresiensis.[20][21]

Taxonomy

Fossils of S. aurorae (left) and S. orientalis (right) at the National Museum of Nature and Science, Tokyo
Jaw fossil of S. sondaari at the Bandung Geological Museum
Skull of S. ganesha

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,[2] with transitional fossils between the two genera known from the Late Miocene of Southeast Asia and Yunnan in South China.[1] Stegodon is more closely related to elephants and mammoths than to mastodons.[22] Like elephants, stegodontids are believed to have derived from gomphotheres.[23]

Phylogeny

The following

hyoid characteristics:[22]

Mammut americanum (American mastodon)

Gomphotherium sp.

Stegodon zdanskyi

Loxodonta africana (African bush elephant)

Elephas maximus (Asian elephant)

Mammuthus columbi (Columbian mammoth)

List of species

An indeterminate Stegodon molar of an uncertain locality and age is known from Greece, representing the only record of the genus in Europe.[24] Indeterminate remains are also known from the Early Pleistocene and early Middle Pleistocene of Israel.[25]

Evolution and extinction

The oldest fossils of Stegodon in Asia date to the Late Miocene, around 8-11 million years ago,

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).[30] 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.[29]

References

Wikispecies has information related to Stegodon.
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  19. ^ Diamond, Jared M. (1987). "Did Komodo dragons evolve to eat pygmy elephants?". Nature. 326 (6116): 832.
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  30. ^ H. Saegusa, "Comparisons of Stegodon and Elephantid Abundances in the Late Pleistocene of Southern China" Archived 2006-05-08 at the Wayback Machine, The World of Elephants – Second International Congress, (Rome, 2001), 345–349.
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