Freak Felids - A Discussion of History's Largest Felines

[tigerluver]

Smilodon populator - A new fossil and questions about bone robusticity to cursoriality, among other issues

Browsing through some older document, I found one of great insight to Smilodon fatalis and S. populator morphology, Relationships between North and South American Smilodon by Björn Kurtén and Lars Werdelin. The differences between the forms were analyzed by this work, and you can read up on it in the attachment. 

Postcranial anatomy interests me the most. For one, I found a record size humerus of 410 mm. Isometrically comparing to the bear humerus of 400.5 mm, this specimen would be about 470 kg (a post on p. 1 explains why bears may be better isometric basis for this species). This humerus puts S. populator back at the top of felid weights. But there's a caveat.

The same document found that "the forelimb of S. populator is somewhat longer, relative to the hindlimb, than in S. fatalis. Such a lengthening of the forelimb is a characteristic of the open plains."

An example of this observation is the fact that lion has a proportionally longer humerus and ulna compared to the hindlimb bones, being the only big cat living almost exclusively in the open plains. This morphological characteristic results in overestimation of mass from all bone measurements when comparing to a more average proportional individual. Bone length overestimates because the bone is disproportionately long, and width dimensions overestimate because the width is more for accommodating running stress than muscle in such cases.

The brown bear has much shorter frontlimbs than hindlimbs are compared to S. populator, and a bit shorter proportions compared to S. fatalis. In this form, S. fatalis is more robust and bear-like than S. populator, but neither were probably as muscular as a bear, but rather some of the bone width was more for running stress similarly to how lions bones have widened so greatly as compared to other cats. 

With that, the S. populator estimation using the brown bear as the base is probably an overestimate, or faulty at the least. S. fatalis reconstructions from a brown bear may be a bit less of an overestimate. Smilodon would lack the posterior weight the bear would in the this areas due to the FL/HL discrepancy, and thus the two species are not analogous, at least for humerus calculations. It is very possible the opposite effects of mass estimation would occur if a brown bear femur is being compared to the proportionately shorter Smilodon femur.

Smilodon's femur is proportionately much larger than its tibia compared to all pantherines by a long ways. Its humerus is also proportionately larger than its ulna, a ratio only matched by the very robust leopard and jaguar. The longer proximal bones is indicative of the fact that Smilodon is indeed much more heavily built than the lion and the tiger, and somewhat more heavyset than the leopard and jaguar.

From this, maybe the best route of Smilodon reconstruction would be one width dimensions and/or the length dimension of the bone, either allometrically or isometrically compared to only jaguars and leopards. The type of bone being used would also have to be taken into account to predict the accuracy of the estimation. Forelimb estimates may be overestimates somewhat, and vice versa for hindlimb estimates. Reply