Re: Do not misundreshtmate the king was Re: Evolution of tyrannosauroid bite power

[ptnorton <ptnorton@suscom-maine.net>]

Ken Carpenter wrote

> There is much new data in Lipkin and Carpenter's paper in the T rex book 
> due out soon. The evidence includes various pathologies (stress fractures, 
> fractures, muscle avulsions of the humerus) that it it clear that the 
> forelimbs are placed under a great deal of stress. Remarkably, the 
> incidence (as %) of these pathologies of JUST the forelimb bones 
> (including scapula, coracoid, and furcula) is highest of any theropod. 
> Although Farkes' caution is a good one, it is important to look at what 
> the entire "system" says, rather than one feature. Thus, while the 
> enlarged peroneal process (single feature) may not correspond to the 
> grasping power of the peroneus longus in some lemurs,the entire hindlimb 
> (a "system") clearly does show an adaptation for grasp while climbing. 
> Thus, the new evidences from the entire forelimb system of T rex show the 
> forelimb was used actively.<

A friend of mine recently suggested that T. rex used its arms to grasp and 
lift portions of a carcass at a kill site as a form of dominance display. 
Most animals use some form of dominance display behavior at various times, 
so the reasoning was that T. rex most likely did so as well. Establishing a 
pecking order among conspecifics at a kill site through displays of strength 
(such as carcass lifting) seemed like a reasonable conjecture to him.  It 
was interesting to me because it posited an explanation for arm function 
that was 1) less critical for survival than grappling with prey, 2) could 
explain the pathologies found in the arms of T. rex (trying to "lift" too 
much weight) equally as well as prey grappling and 3) was independent of the 
predator/scavenger argument.  Although behavior in extinct animals is 
impossible to test directly, I used the work of Carpenter and Hutchinson to 
informally look at the biomechanics of this, thinking that the upper limits 
of T. rex arm strength (how much the arms could grasp and hold) might be a 
function of the strength of the hip and leg extensor muscles (which would do 
the actual "work" of lifting by elevating the torso upwards). As it turns 
out, under those assumptions, the hips and legs of T. rex could easily lift 
much more weight than the arms could hold, so although this type of "carcass 
lifting" behavior was doable, the relationship between arm and hip/leg 
strength didn't seem to hold.  If however, as you suggest, the arm is looked 
at as a forelimb system (beyond just the M. bicep) you may find the arms to 
have been much stronger than originally estimated, perhaps more in line with 
the amount of weight the hip and leg muscles could actually lift.

PTJN