Stegodon (from the Ancient Greek στέγω (stégō), meaning "to cover", and ὀδούς (odoús), meaning "tooth", named for 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 mammutids. Fossils of the genus are known from Africa and across much of Asia, as far southeast as Timor (with a single record in southeast Europe). 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. While the genus became extinct in Africa during the Pliocene, Stegodon persisted in South, Southeast and Eastern Asia into the Late Pleistocene.
Morphology The skull of Stegodon is relatively tall top to bottom but short front to back, and similar in many respects to those of living elephants. The lower jaw in comparison to early elephantimorphs and its ancestor Stegolophodon 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), the numbers of ridges are greater in later species.van der Made, J. The Evolution of the Elephants and Their Relatives in the Context of Changing Climate and Geography. In Elefantentreich—Eine Fossilwelt in Europa; Verlag Beier & Beran: Langenweißbach, Germany, 2010; pp. 340–360. . Members of the genus lack permanent premolars. 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 long, with an estimated mass of approximately , substantially larger than the largest recorded modern elephant tusk. 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.
Size
The Chinese S. zdanskyi is suggested to be the largest species, and 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. The Indian S. ganesa is suggested to have a shoulder height of about , and a body mass of around . The Javanese species S. trigonocephalus is suggested to have been around tall, with a body mass of around . S. orientalis was around the size of an Asian elephant (Elephas maximus).
Dwarf species
Similar to modern-day elephants, stegodonts were likely good swimmers,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 from http://www.jstor.org/stable/986523 allowing them to disperse to remote islands in Indonesia, the Philippines and Japan. Once present on the islands, due to the process of insular 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 from Sumba in Indonesia, is estimated at around 8% of the size of mainland Stegodon species, with a body mass of . Sometimes the same island was colonised multiple times by Stegodon, as in Flores, where the Early Pleistocene strongly dwarfed species Stegodon sondaari, which was tall at the shoulder and weighed about , was replaced by the species Stegodon florensis during the 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 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 , and a body mass of about .Puspaningrum, Mika; Van Den Bergh, Gerrit; Chivas, Allan; Setiabudi, Erick; Kurniawan, Iwan; Brumm, Adam; and Sutikna, Thomas, "Preliminary results of dietary and environmental reconstructions of Early to Middle Pleistocene Stegodons from the So'a Basin of Flores, Indonesia, based on enamel stable isotope records" (2014). Faculty of Science, Medicine and Health – Papers: part A. 2035.
During Pliocene-Early Pleistocene (from around 4–1 million years ago), a succession of endemic dwarf species of Stegodon, probably representing a single lineage lived in the Japanese archipelago, probably derived from the mainland Chinese S. zdanskyi. In chronological succession these species are Stegodon miensis (4–3 million years ago) Stegodon protoaurorae (3–2 million years ago) and Stegodon 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. The latest and smallest species S. aurorae is estimated to be 25% the size of its mainland ancestor with a body mass of around . S. aurorae also shows morphological traits associated with dwarfism, like shortened limbs.
Palaeobiology
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. Stegodon populations from the Late Pliocene of the India (including Stegodon insignis) are suggested to have been variable mixed feeders, while those from the earliest Pleistocene (including Stegodon ganesa) of the same region are suggested to have been nearly pure grazers based on isotopic analysis. Based on dental microwear analysis, populations of Stegodon from the Pleistocene of China (Stegodon orientalis and Stegodon huananensis) and mainland southeast Asia (S. orientalis) were found to be browsers, with clear niche differentiation from sympatric Elephas populations, which tended towards mixed feeding (both browsing and grazing), though isotopic analysis of Stegodon cf. orientalis specimens from the late Middle Pleistocene of Thailand suggests that these individuals were mixed feeders that consumed a significant amount of C4 grass. 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. 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. Specimens of Stegodon kaiesensis from the Pliocene of East Africa were found to be browsers to mixed feeders, based on mesowear analysis.
The East-Southeast Asian species Stegodon orientalis is thought to have primarily inhabited densely forested environments though it may have also inhabited somewhat more open forested habitats, while the Javanese Stegodon trigonocephalus is thought to have primarily lived in a mix of open savannah woodland and forest.
On Flores, where dwarf Stegodon species were the only large herbivores, they were likely the main prey of the Komodo dragon.Diamond, Jared M. (1987). "Did Komodo dragons evolve to eat pygmy elephants?". Nature. 326 (6116): 832. Members of Stegodon florensis and perhaps other Stegodon species may have deliberately entered caves to mourn the dead or for water and minerals.
In the Siwalik Hills assemblage, the Pliocene S. insignis shows a relatively low frequency of enamel hypoplasia, likely due to the relative stability of regional climate during this epoch. The Pleistocene S. ganesha, on the other had, had high rates of enamel hypoplasia that have been attributed to a highly unstable environmental dynamics during the epoch.
Ichnofossils of a group of Stegodon from the Late Pliocene of Japan suggest that like modern elephants, Stegodon were highly social animals and lived together in herds.
Taxonomy
Stegodon was named by British palaeontologist Hugh Falconer in 1857, originally as a subgenus of Elephas, the genus which today contains the Asian elephant.Falconer, H. 1857. On the species of mastodon and elephant occurring in the fossil state in Great Britain. Part I. Mastodon. Quarterly Journal of the Geological Society of London, 13:307–360. Among the first described remains of the genus by Falconer were from the Siwalik Hills of the northern Indian subcontinent, which he assigned to the species Stegodon insignis and Stegodon ganesa, which have since become the subject of taxonomic confusion because of the lack of clearly assigned type specimens and the uncertain stratigraphic provenance of Falconer's collected material. 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, with transitional fossils between the two genera known from the Late Miocene of Southeast Asia and Yunnan in South China, with Stegodon and Stegolophodon collectively placed into the family Stegodontidae. Stegodontidae is placed within the clade Elephantimorpha within Proboscidea, with its exact position in the group being uncertain. Many studies have supported stegodontids as more closely related to modern elephants than to true mastodons of the family Mammutidae, though some studies have found other positions, with one 2025 study finding (albeit with weak support) stegodontids more closely related to Mammutidae than to elephants. Stegodontids are believed to have derived from gomphotheres, with Gomphotherium annectens having been suggested to be the ancestor of Stegolophodon by some authors.
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, Taiwan 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, Indonesia Stegodon florensis Middle – Late Pleistocene, Flores, Indonesia Stegodon luzonensis Middle Pleistocene, Luzon, Philippines Stegodon trigonocephalus late Early Pleistocene – early Late Pleistocene, Java, Indonesia Stegodon sompoensis Late Pliocene – Early Pleistocene, Sulawesi, Indonesia Stegodon sumbaensis Middle – Late Pleistocene, Sumba, Indonesia Stegodon timorensis Middle Pleistocene, Timor, Indonesia Stegodon mindanensis Pleistocene Mindanao, Philippines An indeterminate Stegodon molar of an uncertain locality and age is known from Greece, representing the only record of the genus in Europe. Indeterminate remains are also known from the Early Pleistocene and early Middle Pleistocene of Israel.
Relationship with humans