Stegodons were small members of the Elephant order Proboscidea known from the Miocene-Pleistocene of Africa, Asia and North America. The last members of the group were found in Southeast Asia and the islands of Indonesia in the Late Pleistocene, where they appear to have coincided with the arrival of the earliest Humans in the area, raising the possibility that they may have died out as a result of human activity. However obtaining precise dates for these specimens has proved problematic, making it impossible to either make a direct connection with human activity or to rule it out.
In the 1960s archaeologist Theodore Verhoeven, who had previously located Human artifacts and Stegodon remains on the island of Flores, visited Timor with the hope of making similar discoveries. He returned with a variety of stone tools, as well as fragmentary remains from three Stegodons, though re-examination of the sites by later researchers has established that the tools came from different deposits to the Stegodon remains, making it hard to assess whether they date from the same time.
In a paper published in the journal Peer J on 10 March 2016, Julien Louys of the Department of Archaeology and Natural History at the Australian National University, Gilbert Price of the School of Earth Sciences at the University of Queensland and Sue O'Connor, also of the Department of Archaeology and Natural History at the Australian National University, apply uranium-thorium dating methods to one of Verhoeven's Stegodon's, and discuss the implications of the results.
Uranium-thorium dating works because uranium decays to thorium at a known rate, so that the ratio of the two elements in minerals that naturally incorporate uranium but not thorium can be used to establish a date for the minerals. Neither uranium nor thorium are typically found in organic tissues, but uranium can be absorbed into mineral skeletal elements, such as tooth and bone, after an animal is dead, creating the possibility for using uranium-thorium dating to at least establish a minimum age for such tissues; the uranium can potentially be absorbed at any point after death, not necessarily immediately post-mortem, so the method cannot be used to establish a maximum age.
Louys et al. examined a tusk of a Stegodon collected by Verhoeven from a landslide deposit at a site named Wéaiwé in West Timor. This tooth was split in two length ways prior to its discovery, enabling the extraction of a series of samples from across the specimen. These were found to contain similar levels of uranium across the tusk, but higher levels of thorium towards the rim. This suggests that uranium was absorbed from the outside of the tusk inwards, so that the outer layers formerly contained more uranium than the inner, and that this extra uranium has been lost, either back into the environment or through decay to thorium, making the similarity between the inner and outer uranium levels coincidental. However this is far from certain, so in order to obtain the most conservative estimate of the age of the specimen three samples from the centre of the tooth were used, all of which had similar thorium levels, and which yielded a minimum age of 130 000 years, making the tusk latest Middle Pleistocene in age.
Internal view of the refitted Stegodon tooth fragments drilled for the uranium series analysis. Louys et al. (2016).
Tool making Hominins are known to have reached Flores around 900 000 years ago, however there is no evidence of any such activity on Timor until the arrival of modern Humans around 42 000 years ago, suggesting that Stegodon died considerably before the arrival of the first Humans in the area. In itself this does not preclude Humans as a cause of the demise of Stegodons on the island, though numerous archaeological sites and faunal assemblages on the island dated to be less than 40 000 years old, leading Louys et al. to conclude that the species had died out before the arrival of Humans.
Palaeoenvironmental data is not currently available for the Pleistocene of Timor, but the Pleistocene environment of nearby Java is well understood, with a climate that varied from an open mixed woodland and savanna grassland environment in cooler drier periods associated with glaciations, and a humid forest environment during warmer, wetter interglacials. Around 130 000 years ago the world was in a glacial phase, suggesting that the Wéaiwé Stegodon lived in a savanna environment. However Stegodons are known to have survived a number of environmental shifts, which suggests that they were not likely to have been driven to extinction by such an event.
On Flores Stegodons dissapeared around 17 000 years ago (roughly the same time as the Hominid species Homo florensis), and notably are found in deposits directly below a layer of volcanic ash, but absent above, which strongly suggests that a volcanic eruption was responsible for their demise. The demise to Stegodons on Timor cannot be directly attributed to any such cause, but the island is volcanic, and it is possible that a combination of a geological event with a climate shift on Timor could have resulted in the extinction of the Stegodon population there.
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