Bears of the Pleistocene

[tigerluver]

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. Reply