Stegodon

The skull of Stegodon is relatively tall but short, the numbers of ridges are greater in later species. 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). 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 , 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 straits associated with dwarfism, like shortened limbs. == Palaeobiology ==

Palaeoecology Stegodon inhabited forested environments. 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. 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. On Flores, where dwarf Stegodon species were the only large herbivores, they were likely the main prey of the Komodo dragon. Palaeopathology 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. Social behaviour 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 ==

, Tokyo Stegodon was named by British palaeontologist Hugh Falconer in 1857, originally as a subgenus of Elephas, the genus which today contains the Asian elephant. 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, 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 ==

Remains at a number of sites suggest that humans (in a broad sense, including archaic humans) interacted with Stegodon. At a cave deposit on Gele Mountain near Chongqing in southwest China, a mandible of Stegodon orientalis was used to make a handaxe, with dating suggesting the bone is around 170,000 years old. At the late Middle Pleistocene Panxian Dadong cave site in southern Guizhou Province, southwest China, dating to around 300-190,000 years ago, numerous remains of juvenile (0-12 years of age) and a much smaller number of adult remains of adult Stegodon orientalis, representing a minimum of 12 individuals were found at the site in association with stone tools and human remains. It suggested that Stegodon remains were brought to the cave by humans though none of the elements show clear evidence of processing. At the Xinlong Cave site in the Three Gorges area of Chongqing, suggested to date to around 200-130,000 years ago, two Stegodon 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. At the Late Pleistocene Ma'anshan site also in Guizhou, remains of Stegodon orientalis including both adults and juveniles among other animals are found in two layers, the older dating to around 53,000 years Before 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. At Liang Bua cave on Flores dating to around 80-50,000 years ago, remains of the dwarf Stegodon species Stegodon florensis are associated with stone tools produced by the dwarf archaic human species Homo 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 of Stegodon was practiced by Homo floresiensis. == Evolution and extinction ==

The oldest fossils of Stegodon in Asia date to the Late Miocene, around 8-11 million years ago, Stegodon became extinct in Africa during the late Pliocene, around 3 million years ago suggested to be the result of expansion of grassland habitats. The Javanese species Stegodon trigonocephalus became extinct around 130-80,000 years ago during the latest Middle Pleistocene-early Late Pleistocene (Marine Isotope Stage 5) following a change to more humid conditions, which may have reduced grazing habitat. Stegodon became extinct in the Indian subcontinent (Stegodon namadicus/Stegodon sp.), mainland Southeast Asia and China (S. orientalis) at some point during the Late Pleistocene epoch, while Asian elephants, which existed in sympatry with Stegodon in these regions, are still extant. The precise timing of extinction is uncertain for these regions, though in India records of Stegodon may date as recently as 35-30,000 years ago, The survival of the Asian elephant as opposed to Stegodon orientalis in Southeast Asia and South China has been suggested to be due to its more flexible diet in comparison to S. orientalis. these claims cannot be substantiated due to loss of specimens and issues regarding dating. ==References==