Bears of the Pleistocene

[tigerluver]

@Polar

I'm not aware of any cortical bone thickness (referring to inner bone density) studies in radius and ulna of pantherines, did you mean bone diameter (referring to external bone dimensions)? If not, please do share the paper, I'm only aware of Meachen-Samuels and Valkenburgh (2010). 

In terms of my humerus analogy, I think I was trying to point out the possibility of the biome causing convergent evolution in limb dimensions between the tiger and A. angustidens. In both, the proximal ends of the humerus are not greater than their cousins, but distally, the tigers seems to proportionally have wider elbows, . Christiansen's specimens show the opposite trend when looking at just the articular surface, but Peigne et al. (2005) shows that tigers have wider distal humeri than lions (perhaps because tigers have a shorter articulating surface on the distal epiphyses than lions, or sample size errors), despite the fact that lion are superior in other humeral dimensions. I infer that the the tiger's thicker proximal ulna as shown as Christiansen and Harris (2005) is correlated with this distal thickening trend. A. angustidens has this similar trend in bone diameter. Pleistocene South America and modern tiger habitat were similar, thus this bone characteristic may be explained by the shared habitat. 

Linking back to A. simus, Pleistocene North America was more open and lacked tree cover. Pleistocene South America was a rainforest. Different habitats need different locomotive behavior, thus the different bone dimensions. A. simus is also much younger (as in more recent) than A. angustidens, giving it plenty of time differentiate from the theoretical stem population of both these species. 

5000 lbs would be over twice the size of the giant A. angustidens specimen. We've found nothing larger and I doubt the species would get to that size unless this A. angustiden humerus was a female's (how likely would one think that would be?). Reply