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 mammutids. 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, Stegodon remained widespread in South, Southeast and East Asia until the end of the Pleistocene.
Morphology
The skull of Stegodon is relatively tall but short. 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), the numbers of ridges are greater in later species. Members of the genus lack permanent premolars. The tusks are proportionally large.
Size
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 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 .
Dwarfism
Similar to modern-day elephants, stegodonts were likely good swimmers, 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 .
Ecology
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 Pliocene of the India are suggested to have been variable mixed feeders 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). 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. 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.
Tracks of a group of Stegodon from the Late Pliocene of Japan suggest that like modern elephants, Stegodon lived in social herds.
On Flores, where dwarf Stegodon species were the only large herbivores, they were likely the main prey of the Komodo dragon. 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.
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, with transitional fossils between the two genera known from the Late Miocene of Southeast Asia and Yunnan in South China. Stegodon is more closely related to elephants and mammoths than to mastodons. Like elephants, stegodontids are believed to have derived from gomphotheres.
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, 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.
Evolution and extinction
The oldest fossils of Stegodon in Asia date to the Late Miocene, around 8-11 million years ago, with the oldest fossils of the genus in Africa being around 7-6 million years old. 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 florensis became extinct around 50,000 years ago, around the time of arrival of modern humans to Flores. 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. 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. 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). 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
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