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Bears of the Pleistocene
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I was just hypothesizing why the polar bear has a weaker bite force, outside of the obvious adaption of a slimmer head and neck which would constitute a weaker bite force since the area for muscle is less, I think the adaption of sharper canines helps to puncture prey with out the need of as massive of a bite force.
Angustidens appears to be a bit larger in the first two images, the humerus also appears to have a bit of a deformity, but may be it's the angle.
Bones can be tricky at times. Fractures can produce growth in strange ways... 
A. simus vs A. angustidens, the morphological comparison
The measurements of the two bones, respectively: A. simus  *This image is copyright of its original author A. angustidens  *This image is copyright of its original author Robusticity index ratios:  *This image is copyright of its original author I remeasured the pictures, and the epiphyses diameters seem to be compatible for comparison from both studies. Unfortunately the ulna measurements were not compatible but I could always get a few diameters from the photos later in time. To be continued... 
The South American version seems to be more robust in bone diameter/bone length ratios than the North American version. Is there any study relating to the enhanced running abilities of Arctodus simus over Arctotherium angustidens? In a documentary (about the extinct North American short-faced bear), I heard the narrator state that A. simus is relatively taller than A. angustidens at the same length, and that A. simus has relatively more muscular endurance than A. angustidens, allowing it to run horses down with ease.
According to a documentary about Arctodus simus, the theory was shot down about him running at race horse speed to catch fast moving prey. His long limbs are thought to be too gracile and would be too fragile considering his great bulk to make any sudden turns as would be needed while pursuing prey animals. In doing so the giant short-faced bear would likely break his own leg.
Arctodus simus may have been a long distance walker following a keen nose from one carcass to another. His great size alone would have been enough to displace predators from their kills. Like the grizzly, he may have been a part-time hunter and just about a full-time scavenger. As a bear, he was an omnivore but his diet probably leaned more towards meat and less vegetation than a grizzly. Of course, this is all theory judged on fossil remains.
Bear bones are extremely strong, even those of Arctodus. A. simus has more bone marrow/cortical bone ratio than bears of the Ursus genus, that is true. Yet, I seriously doubt their leg would break that easily simply by making sharp turns or trying to overrun. If these two things are done, it most likely would have resulted in a forward fall or sidewards roll instead of broken bones. Modern bears don't gallop as much, thus they can turn MUCH more efficiently than A. simus. Though, I'm not sure about running speed of both bears.
Consider though, a world filled with vast herds of mega-fauna and a wide variety of large predators. I view Arctodus simus as being the "God Father" of those predators. Why go hunting when he could simply take the spoils of another's hard work? Predator or scavenger?
Scavenging hasn't changed much in modern bears, particularly in brown and black bears. A. simus was both a scavenger and hunter, just like modern bears.
A. simus vs A. angustidens, Differences in Morphology II
Repost this for ease of reference:  *This image is copyright of its original author A. simus Humerus The A. simus specimem looks to be very normal for its species. The midshaft LM diameter/humeral length of 0.091 is in accord with Lynch (2012), where the majority of specimen had a ratio of 0.09. The distal width is above average, with this individual having a distal width/humeral length ratio of 0.27 as compared to the 0.25 average in Lynch (2012). Same goes for this specimen's proximal dimensions, which are superior to the species average of 0.20 and 0.23 in terms of LM and AP, respectively. All in all, this large specimen is a good representation of your somewhat above average built A. simus. As compared to modern ursids, I don't see the striking slenderness but indeed a significant amount of difference. In terms of the midshaft, essentially most modern bear species hover are 0.10 in midshaft LM diameter/length, 9% thicker. Proximally, modern bears average around 0.20 LM-wise and 0.24 AP-wise, essentially the same as the average A. simus and slightly inferior to this large specimen. Distally is where the most significant difference manifests, with modern bears averaging closer to 0.30 in distal width/length. This reduces the weight of the limb, evidence for relatively greater cursoriality in A. simus as compared to modern bears (Lynch 2012). Radius The midshaft is normally built, being a bit greater than the species average of 0.088 in terms of ML width but not the maximum, which is 0.10 in some specimens. The proximal LM is either average or slightly less for its species. The distal LM is essentially average. Thus, the radius represents a very average built specimen. The midshaft of A. simus, in both this specific specimen and the species average, is possibly slightly more robust than the black bear and grizzly, whose corresponding average ratios are 0.077 and 0.085 respectively. The proximal epiphyses robusicity varies quite a bit between bear species and even individuals. So much so that this value might not be of much help, but the proximal radius seem thinner than most modern bears. This is expected as the distal humerus, which articulates with the proximal radius, is thinner. Distally, A. simus looks to be a slightly inferior to most modern bears, namely the polar, grizzly, and black bears. I emphasize the slight, as there is a lot of variability intraspecifcally in this ratio. From the radius and humerus, it's probably safe to say A. simus was slightly to somewhat less robust than at least say, the grizzly. However, the thinner seems more oriented at the elbows and knees. Lynch (2012) put it well, A. simus is a bear in the early stages of evolution toward cursoriality (but not all the in the door). All in all, I mentioned the large humerus' dimensions in comparison to the species average to validate the image comparison. This humerus is a good poster bear for the species, thus the picture reliably shows the difference between A. simus and A. angustidens as well as our modern bears. A. angustidens It seems we only have one specimen of this species, so obviously, no discussion about how representative of its kind this specimen is will be conducted. Humerus The humerus distally is well superior to A. simus. The shaft does have a pathology near the midpoint on both of the humerii, but looking the images provided by Soilbenzon and Schubert and measuring the photo provided, it seems the pathology was somewhat excluded from measurements. Somewhat is not entirely, thus the midshaft thickness is falsely inflated. Measuring to the best of my judgment where the bone truly ends without the pathology, the midshaft/length ratio is find is around 0.12. Thus, still superior to A. simus. Proximally, A. angustidens is the same as A. simus in thickness. Perhaps this can be explained by proximal bone thickening in cursorial species that does not represent addition of heavy muscle but rather aids in running. I point to the difference between lion and tiger humerii as evidence, whereby the lion has significantly wider proximal epiphyses than the tiger, but the tiger catches up to and sometimes surpasses the lion distally. Different locomotive behavior caused these discrepancies. We can elaborate on this further in a later post. As compared to modern bears, the midshaft is very much thicker yet distally and proximally, there is not much difference between the A. angustidens and its modern relatives. Distally, the polar bear shows some specimens thicker in this region, moreso than the grizzly, yet shaft-wise no different from the grizzly. Thus, we'll have to attribute these differences back to lifestyle and return later. Radius Across the board. A. angustidens is more robust than A. simus here. The midshaft is superior to modern bears. Like the humerus, the proximal and distal ends do not show this marked increase in thickness as compared to modern bears. Not much else to say for this bone. Thus, the trends in humerus and radius flow into each other. The point for future discussion is that the shaft of A. angustidens is special, yet the epiphyses not so much. I'll have to refer back to other species for explanations. What can be said is that A. angustidens is likely not a cursorial bear, unlike A. simus. On a final note, I'd like to touch on sample sizes. A. simus is a well represented species. A. angustidens is not. We know the size limits of A. simus better due to the sample size, but when we talk about A. angustidens size, how do we approach the subject. There are two conflicting logical approaches. One, A. angustidens is simply so large in frame with an equally heavy bone structure that finding specimens any larger than this giant does not seem feasible. On the other hand, the likelihood of the only specimen on fossil record even being a large male for its kind is far from great. The mind boggling possibility is left that much larger specimens of the A. angustidens once existed, and what a productive ecosystem must have been present in the early Pleistocene to sustain a population of these behemoths. I reference Lynch (2012) often in this analysis, click here for the dissertation.
On the big cats, I see that lions possess more robust humeri than tigers at most cross-sectional areas, but in the tiger, both the ulna and radius corticalbone thicknesses are more increased within the tiger than the lion by a significant amount, assuming equal weights, of course. Same as in bears, with A. angustidens having more robust humeri than either A. simus or some modern bears. Am I reading your conclusion right?
There are various statements around the net stating that A. angustidens can reach maximum weights of up to 5000 pounds. Are any of these claims proven true?
@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?).
The Pleistocene bear called "the tyrant sea bear" was even larger than the big modern polar bears of today. Could this be because he was once a hunter of musk ox and perhaps some now extinct tundra herbivores and a scavenger of mammoths who then gradually learned to hunt seals?
I believe "tyrant sea bear" is referring to U. m. tyrannus as that is what the scientific name translates to. So refer back to a few posts ago for the estimates on size of the ulna. As a whole, ecosystem productivity (energy yield of the ecosystem) was much greater in the Pleistocene, thus plant mass was greater, leading to greater herbivore mass, which led to greater carnivore mass and allowed omnivores (i.e. the short faced bears) to become out of the world giants.
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