Bears of the Pleistocene

[tigerluver]

Arctodus simus and Arctotherium angustidens

The North and South Ameican short faced bears, as more commonly referred to. Undoubtedly, these two species are at the pinnacle of the size pyramid of mammal carnivores. The North American form initially held the title of the largest bear, until the South American form came to light, specifically by Soilbenzon and Schubert in 2011. They packaged several different equations from difference sources and came up with estimates, and proposed a largest mean mass of 1,591 kg. A few months later, Figeorido et al. (2011) was published, and they proposed a value of 957 kg. In my opinion, these two values aren't on a level playing field for comparison. 

The South American form was estimated from many equations, some which look to be discussing a universal scaling factor rather than a genus specific. The North American form was from one equation set, focused solely on the bear genus. So, for a better comparison, we could apply the Figuerido et al. equation to the largest South American form specimens. The equation to use is:
log(mass) = 2.77 * log(humerus length) - 4.68

One fossil of interest is a 620 mm humerus, equation a mass of 1135 kg. 

Fossil two, humerus length 615 mm, equation to a mass of 1110 kg.

This puts the south form at about 200 kg heavier than the north form. A glaring area for improvement would be comparing bone diameters. The humerus diameters cannot be compared at the moment, as Soilbenzon and Shubert (2011) took diameters at the midshaft rather than at 55% length of the bone, rendering Figuerido et al.'s humeral least shaft width incompatible. 

The case doesn't close here, as somewhere online there's a mention of an even longer North American short faced bear humerus. Once I find it I'll post some more.

Regarding equations. The Figuerido equation has one flaw, which could be major. The weights used in the regression are not of the individuals whose bone measurements were used, but rather literature values. In that way, Christiansen (1999) had a better database. Soilbenzon and Schubert also passed of the 2 metric ton weight of the South American form as an overestimate, which it may have been. But in itself, it isn't really reasonable to pass of an estimate based on objective mathematics just because one cannot fathom it, as has been done a bit too much. For all we know, the Figuerido estimates could be underestimates as the literature weights were less than the actual weights of the specimens where bone measurements were derived from.

The South American short faced bear's official report has been attached.  Reply