Freak Felids - A Discussion of History's Largest Felines

[tigerluver]

(05-29-2021, 06:33 AM)Acinonyx sp. Wrote: @tigerluver 
Shouldn't isometric scaling involving shaft width be much more accurate than isometric scaling involving only length since it should include all the meat that could be packed in the arm?

Isometric scaling means all dimensions grow equally, so a single type of measurement cannot be superior to another under this assumption. One would also have to decide which axis of shaft width to use as those differ between species/individuals.

Here's an example of why isometric scaling with midshaft widths isn't necessarily more accurate. You have an animal of bone length 100, midshaft width 50 in AP and 50 in LM directions. You have another animal with bone length 300, with midshaft with 30 in AP and 30 in LM. By midshaft width, isometry would say the former is larger by a lot (has more volume). However, let's calculate the volumes:

Animal (former): 100 x 50 x 50 = 250,000
Animal (latter): 300 x 30 x 30 = 320,000

Length has quite a significant effect on mass/volume, as would width dimensions. This is simply because length is a dimension in 3D of its own. As such, one cannot be superior over the other considering isometric estimations.

Modeling with multiple measurements is a different story as we don't assume isometry in its most literal sense (given we are averaging the measurements). In the end, long bone width dimensions have much less effect than measurements of the torso as the majority of the mass is in the torso. This is why GDI and other volumetric estimations that are used for dinosaurs are much better. For instance, if we see the humerus to ulna ratio of Smilodon (indicates of percent body height that is composed of torso), we would see it has much torso for its height, giving it a much higher weight for its long bone measurements (especially length). This is of course supported by its long bone widths. However, its long bones width (especially hindleg) are probably deflated per body as compared to extant large pantherines, an observation that cannot be detected from only long bone measurements. Reply