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Dwarfing on Indonesian Islands

The stegodontid elephants had their origin in the early Miocene (about 23 million years ago) of Asia. Species range from various islands of China and Japan to the islands of Indonesia. The Indonesian islands were accesible around 0.8 million years ago as a consequence of low sea level. However there would have still been a fast, strong current flowing from the Pacific to the Indian Ocean and so only strong swimmers could have made the crossing. The elephants may have also moved between islands, or ‘island-hopped’. The dwarfed elephantoid, Stegodon sondaari, existed on Flores, Timor and Sulawesi until the Middle Pleistocene when they were ‘replaced by new immigrants of large to intermediate size,’ (van den Bergh et al., 2001) Stegodon florensis, 850000 years ago. S. florensis had an estimated bodyweight of 850kg, and a much larger species found on Java, S. trigonocephalus weighed between 1017 and 1713kg, although these two species are not thought to be related (van den Bergh et al., 2008). Additionally a subspecies of Stegodon, S. florensis insularis has been found in Liang Bua, limestone caves of West Flores, that is intermediate in size between S. sondaari and S. florensis.

This site also has many stone artefacts and skeletons of Homo florensis. Bones of S. florensis insularis show cut marks, indicative of butchering by the hominins. This assemblage is thought to be the consequence of hominin activities, a site where meat was prepared for consumption. S. sondaari was found in a fossiliferous layer at Tangi Talo, Flores, dating to 900000 years ago, with the still extant Komodo dragon, Varanus komodoensis, and a giant tortoise. Thought to have weighed around 300 kg, S. sondaari is the smallest elephantoid discovered so far from the Indonesian region. The proposed ancestor of this species is ‘S. elephantoides, which has been recorded from Myanmar and an 1.2 Ma-old site on Java’ (van den Bergh et al., 2008). Although the enamel microstructure of the teeth of S. sondaari exhibit some primitive characteristics, derived characteristics are present ‘in the form of increased hypsodonty (high-crowned teeth and enamel that extend beyond the gum line) and relatively large molar wear surface’ (van den Bergh et el, 2001). This is an adaptation that has occurred only in island stegodonts, (mainland stegodonts have brachyodont (short teeth with well developed roots) molars), thought to assist foraging for food contaminated with sand and grit. The addition of tough grasses had to be included into their diets to survive the strong seasonal conditions experienced on Flores. Tusks found show heavy abrasion, presumably from the action of digging for water holes and edible roots. The sediment layer at Tangi Talo consists of tuff and pumice. Based on the composition of the layer, it has been extrapolated that the cause of death of the entire population of S. sondaari found at that site, was ‘a catastrophic volcanic eruption’ (van den Bergh et al., 2001). This volcanic demise is also true of the S. florensis insularis and Homo florensis. Both became extinct around 12000 years ago (van den Bergh et al., 2008). It is the last time remains of S. sondaari are found in the stratigraphy of Flores hence it can be assumed that the volcanic eruption was the ultimate demise of this species, 900000 years ago.

The origin of S. florensis may have been ‘from Java in the west via stepping stone islands Lombok and Sumbawa, or alternatively from Sulawesi in the north’ (van den Bergh et al., 2008). The assemblage in the Soa Basin in which S. Florensis was found had few juveniles. Due to the number of predatory komodo dragons with which this species was found, it is thought that juveniles may have been heavily predated upon. On Flores, S. florensis also eventually dwarfed. Their size reduced by 30% possibly in response to the lack in competition with other large herbivores (there were no others present on the island). However the ‘co-occurrence of large predatory animals may have prevented S. florensis from reaching minimal size associated with optimal energetic balance’ (van den Bergh et al., 2008).

References and Resources

  • van den Bergh, GD., de Vos, J., Aziz, F. and Morwood, MJ. 2001. Elephantoidea in the Indonesian region: new Stegodon findings from Flores. The world of elephants: International Congress, Rome. 623-627.
  • van den Bergh, GD. et al. 2008. The youngest stegodon remains in Southeast Asia from the Late Pleistocene archaeological site Liang Bua, Flores, Indonesia. Quaternary International 182, 16-48.

  • Authored by Emma Kerridge & Chris Rogers

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