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Pleistocene Sunda was home to the small to medium size Stegodons, with size evolving dramatically with time. The Late Neogene elephantoid-bearing faunas of Indonesia and their palaeozoogeographic implications by Gert D. van den Bergh examines these species and give us body mass estimates.
He used 4 equations to estimate body mass:
1) Mass (kg) = 3.790 × 10-4 × FC (mm)2.827 (5.5 mm < FC < 413 mm)
2) Mass (kg) = 9.448 × 10-4 × HC (mm)2.611 (4.9 mm < HC < 459 mm)
3) Mass (kg) = 1.774 × 10-5 × FL (mm)2.654 (6.0 mm < FL < 980 mm)
4) Mass (kg) = 2.767 × 10-5 × HL (mm)2.675 (5.7 mm < HL < 830 mm)
The specimens:
S. sondaari of Flores, Indonesia:
A fragmentary femoral piece with missing the proximal extremity. Bone length was estimated but circumference was able to be measured. The length equation produced an estimate of 207 kg and the circumference equation an estimate of 553 kg.
S. sompoensis of Sompe, Indonesia:
A single complete humerus. Estimated to be 350 kg on length and 950 kg on circumference.
S. t. trigonocephalus from Trinil HK and Kedung Brubus, Java:
Four femurs were examined, but there were two largest specimen candidates. The largest femoral length estimate was 1713 kg and said to be the second largest specimen, but the circumference estimate for this femur was not given in the text, and I can't find the supplementary tables yet. The largest specimen was fragmentary, but the circumference estimate gave an estimate of 6287 kg.
S. florensis of Flores, Indonesia:
A single humerus taken from Hooijer (1972) measuring 630 cm in length, producing an estimate of 852 kg. This fossil is lost, thus circumference could not be taken. Another fragmentary humerus of circumference c. 280 mm was estimated at 2317 kg.
All specimens are said to be adults.
The author stated that the length estimates were probably more accurate while the circumference estimates were too high, citing that increase in bone width may be due to gait and not weight bearing. I subsequently tested the aforementioned equation on two elephants from Campione and Evans (2012) and found the following percent errors (+ indicates overestimate, - underestimate).
As we can see, the length based estimates are extreme underestimates. Femur circumference is a significant over estimate, thus correcting the estimates by around mass/1.25 may provide the most accurate estimate on its own. Humerus circumference are probably accurate enough.
Thus, from the Sunda stegodons, the largest specimen, of S. t. trigonocephalus, would weight around 5030 kg, around the size of an Asian elephant. Although, these stegodons were much more robust (we see this trend in the tiger of the area as well, go figure), and likely shorter even though the masses were similar.
S. t. ngandongensis of Ngandong may have been somewhat larger than S. t. trigonocephalus based on its larger dentition, but dentitions are very inconclusive.
