Stegodon (from theAncient Greek στέγω (stégō), meaning "to cover", and ὀδούς (odoús), meaning "tooth", named for the distinctive ridges on the animal'smolars) is an extinctgenus ofproboscidean, related toelephants. It was originally assigned to the familyElephantidae along with modern elephants but is now placed in the extinct familyStegodontidae. Like elephants,Stegodon had teeth with plate-likelophs that are different from those of more primitive proboscideans likegomphotheres andmammutids.[1] Fossils of the genus are known from Africa and across much of Asia, as far southeast asTimor (with a single record in southeast Europe). The oldest fossils of the genus are found in LateMiocene strata in Asia, likely originating from the more archaicStegolophodon, subsequently migrating into Africa.[2] While the genus became extinct in Africa during thePliocene,Stegodon persisted in South, Southeast and Eastern Asia into theLate Pleistocene.[3]
The skull ofStegodon is relatively tall but short,[1] and similar in many respects to those of living elephants.[4] The lower jaw in comparison to earlyelephantimorphs and its ancestorStegolophodon 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][5] the numbers of ridges are greater in later species.[6] Members of the genus lack permanent premolars.[7] The tusks are proportionally large, with those of the biggest species being among the largest known tusks in proboscideans, with a particularly large tusk ofS. 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.[8] These tusks have a slight upward curvature, and project forwards and parallel to each other, with the tusks often so close together that they are almost touching, such that the trunk would probably have had to rest on top of the tusks rather than be freely hanging between them as in living elephants.[4]
Skeletal restoration of a large maleS. zdanskyi with a shoulder height of around 3.87 metres
The ChineseS. zdanskyi is suggested to be the largest species, and is known from an old male (50-plus years old) from theYellow 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 ahumerus 1.21 m (4.0 ft) long, afemur 1.46 m (4.8 ft) long, and apelvis 2 m (6.6 ft) wide. The IndianS. 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 speciesS. 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).[9]S. orientalis was around the size of anAsian elephant (Elephasmaximus).[10]
Estimated sizes of dwarfStegodon species fromFlores compared to a human (Note: silhouette is based on much largerS. zdanskyi and is not meant to accurately depict the specific body proportions of these species)
Similar to modern-day elephants, stegodonts were likely good swimmers,[11][12] allowing them to disperse to remote islands in Indonesia, the Philippines and Japan. Once present on the islands, due to the process ofinsular dwarfism, as a result of decreased land area and the reduction of predation and competition pressure, they reduced in body size, with the degree of dwarfism varying between islands as the result of local conditions. One of the smallest species,Stegodon sumbaensis fromSumba in Indonesia, is estimated at around 8% of the size of mainlandStegodon species, with a body mass of 250 kilograms (550 lb).[13] Sometimes the same island was colonised multiple times byStegodon, as inFlores, where theEarly Pleistocene strongly dwarfed speciesStegodon sondaari, which was 120 cm (3.9 ft) tall at the shoulder and weighed about 350–400 kilograms (770–880 lb),[9] was replaced by the speciesStegodon florensis during theMiddle Pleistocene which was initially substantially larger, but progressively reduced in size over time, with the earlier subspeciesStegodon florensis florensis from the Middle Pleistocene estimated to be around 50% the size of mainlandStegodon species with a shoulder height of around 190 cm (6.2 ft) and a body mass of around 1.7 tons, while the laterStegodon florensis insularis from theLate Pleistocene is estimated to be around 17% the size of mainlandStegodon species, with a shoulder height of around 130 cm (4.3 ft), and a body mass of about 570 kilograms (1,260 lb).[13][14]
During Pliocene-Early Pleistocene (from around 4-1 million years ago), a succession of endemic dwarf species ofStegodon, probably representing a single lineage lived in theJapanese archipelago, probably derived from the mainland ChineseS. zdanskyi. In chronological succession these species areStegodon miensis (4-3 million years ago)Stegodon protoaurorae (3-2 million years ago) andStegodon aurorae, (2-1 million years ago) which show a progressive size reduction through time, possibly as a result of reducing land area of the Japanese archipelago.[15] The latest and smallest speciesS. aurorae is estimated to be 25% the size of its mainland ancestor with a body mass of around 2,122 kilograms (4,678 lb).[16]S. aurorae also shows morphological straits associated with dwarfism, like shortened limbs.[17]
Life restoration of the Indian speciesStegodon ganesha
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.[1]Stegodon populations from the Late Pliocene of the India (includingStegodon insignis) are suggested to have been variable mixed feeders, while those from the earliest Pleistocene (includingStegodon ganesa) of the same region are suggested to have been nearly pure grazers based on isotopic analysis.[18] Based on dental microwear analysis, populations ofStegodon from the Pleistocene of China (Stegodon orientalis andStegodon huananensis) and mainland southeast Asia (S. orientalis) were found to bebrowsers, with clearniche differentiation fromsympatricElephas populations, which tended towards mixed feeding (both browsing andgrazing),[19][20] though isotopic analysis ofStegodon cf. orientalis specimens from the late Middle Pleistocene of Thailand suggests that these individuals were mixed feeders that consumed a significant amount ofC4 grass.[21] Specimens ofStegodon trigonocephalus from the Early-Middle Pleistocene of Java were found to be mixed feeders to grazers, with a diet similar to that of sympatricElephas hysudrindicus.[22] The dwarf species from Flores,Stegodon sondaari andStegodon florensis, are suggested to have been mixed feeders and grazers, respectively, based on stable carbon isotopes.[14] Specimens ofStegodon kaiesensis from the Pliocene of East Africa were found to be browsers to mixed feeders, based onmesowear analysis.[23]
On Flores, where dwarfStegodon species were the only large herbivores, they were likely the main prey of theKomodo dragon.[24]
In the Siwalik Hills assemblage, the PlioceneS. insignis shows a relatively low frequency ofenamel hypoplasia, likely due to the relative stability of regional climate during this epoch. The PleistoceneS. ganesha, on the other had, had high rates of enamel hypoplasia that have been attributed to a highly unstable environmental dynamics during the epoch.[25]
Ichnofossils of a group ofStegodon from the Late Pliocene of Japan suggest that like modern elephants,Stegodon were highly social animals and lived together in herds.[26]
In the past, stegodonts were believed to be theancestors of the trueelephants andmammoths, but currently they are believed to have no modern descendants.Stegodon is likely derived fromStegolophodon, 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 tomastodons.[27] Like elephants, stegodontids are believed to have derived fromgomphotheres.[28]
The followingcladogram shows the placement of the genusStegodon among other proboscideans, based onhyoid characteristics, following Shoshani and Tassy (2005):[27]
An indeterminateStegodon molar of an uncertain locality and age is known from Greece, representing the only record of the genus in Europe.[29] Indeterminate remains are also known from the Early Pleistocene and early Middle Pleistocene of Israel.[30]
Remains at a number of sites suggest that humans (in a broad sense, includingarchaic humans) interacted withStegodon.[31] At a cave deposit on Gele Mountain nearChongqing in southwest China, a mandible ofStegodon orientalis was used to make ahandaxe, with dating suggesting the bone is around 170,000 years old.[32] At the late Middle Pleistocene Panxian Dadong cave site in southernGuizhou Province, southwest China, dating to around 300-190,000 years ago,[33] numerous remains of juvenile (0-12 years of age) and a much smaller number of adult remains of adultStegodon orientalis, representing a minimum of 12 individuals were found at the site in association with stone tools and human remains. It suggested thatStegodon remains were brought to the cave by humans though none of the elements show clear evidence of processing.[10][34] At the Xinlong Cave site in the Three Gorges area of Chongqing, suggested to date to around 200-130,000 years ago, twoStegodon cf.orientalis tusks have been found along with human remains. These tusks appear to have been delibrately engraved with patterns and are suggested to have been brought into the cave by humans.[35] At the Late Pleistocene Ma'anshan site also in Guizhou, remains ofStegodon orientalis including both adults and juveniles among other animals are found in two layers, the older dating to around 53,000 yearsBefore Present (BP), with the younger dates to around 19,295-31,155 years BP with the minimum number of individuals being 7 and 2 for the older and younger layers respectively, with the older layer containing adults and juveniles while in the younger later only juveniles are present. Bones at the site display cut marks indicating butchery, and are thought to have been accumulated at the site by people, likely by hunting or possibly scavenging in the case of the large adults found in the older layer.[36]
AtLiang Bua cave on Flores dating to around 80-50,000 years ago, remains of the dwarfStegodon speciesStegodon florensis are associated with stone tools produced by the dwarf archaic human speciesHomo floresiensis, with a small number of the bones bearing cut marks. The ambiguous circumstantial association between bones and stone tools, and the rarity of cut marks makes it unclear to what if to any degree, hunting ofStegodon was practiced byHomo floresiensis.[37][38]
The oldest fossils ofStegodon in Asia date to the Late Miocene, around 8-11 million years ago,[2] with the oldest fossils of the genus in Africa being around 7-6 million years old.[17]Stegodon became extinct in Africa during the late Pliocene, around 3 million years ago suggested to be the result of expansion of grassland habitats.[23]
The Javanese speciesStegodon trigonocephalus became extinct around 130-80,000 years ago during the latestMiddle Pleistocene-early Late Pleistocene (Marine Isotope Stage 5) following a change to more humid conditions, which may have reduced grazing habitat.[22] The last records ofStegodon florensis date to around 50,000 years ago, around the time of arrival of modern humans to Flores (the earliest evidence of which dates to 46,000 years ago), suggesting that effects of modern human activity were likely the cause of their extinction.[39]
Stegodon became extinct in the Indian subcontinent (Stegodon namadicus/Stegodon sp.), mainland Southeast Asia and China (S. orientalis) at some point during theLate Pleistocene epoch, while Asian elephants, which existed in sympatry withStegodon in these regions, are still extant. The precise timing of extinction is uncertain for these regions,[40][20][41][42] though in India records ofStegodon may date as recently as 35-30,000 years ago,[40] and to at least 30-20,000 years ago forS. orientalis in Guizhou, southwest China.[36] The survival of the Asian elephant as opposed toStegodon orientalis in Southeast Asia and South China has been suggested to be due to its more flexible diet in comparison toS. orientalis.[20] Although some authors have claimed a Holocene survival in China forS. orientalis,[43] these claims cannot be substantiated due to loss of specimens and issues regarding dating.[41]
^Simpson, G. (1977). "Too Many Lines; The Limits of the Oriental and Australian Zoogeographic Regions".Proceedings of the American Philosophical Society, 121(2), 107–120. Retrieved fromhttp://www.jstor.org/stable/986523
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