Re: Evolution of tyrannosauroid bite power

[Michael Habib <mhabib5@jhmi.edu>]

> Abstract: "Although proportionately the forelimb is very small, the 
> mechanical advantage reveals an efficiently designed force-based 
> system (vs. a velocity-based system) used for securing its prey during 
> predation. In addition, the M. biceps is shown to be 3.5 times more 
> powerful than the same muscle in the human, the straight, columnar 
> humerus provides maximum strength to mass ratio to counter the 
> exertion of the M. biceps, and the thick cortical bone indicates bone 
> selected for ultimate strength. Such mechanical adaptations can only 
> indicate that the arms were not useless appendages, but were usted to 
> hold struggling prey while the teeth dispatched the animal. 
> _Tyrannosaurus rex_ was therefore an active predator and not a mere 
> scavenger, as has been suggested."

An interesting set of observations, but I see some possible flaws in 
their conclusions.  First and foremost, the standard assumptions 
regarding biomechanical adaptation may not hold well when long bones 
are greatly reduced (the calculations still hold, it is just the 
conclusions that are more uncertain).  For example, one reason that the 
small limbs have high bending strength is that their short length gives 
them a small moment arm.  In some animals, such shortening is an 
adaptive trend to increase bending resistance, but in this case it 
could be a side-effect of how non-avian theropods reduce forelimbs.  
There is a tendency for the length to reduce to a greater degree than 
the breadth, forming short, stout forelimbs.  Such stout limbs are 
strong, technically speaking, but it may be some kind of intrinsic 
effect.  Similarly, while thickened cortical bone often indicates 
selection for high ultimate strength, thickened cortical bone may also 
be a result of secondarily reduced limb breadth.  Ratites, for example, 
have relatively thick-walled humeri.

The second problem is a more kinematic one: as best I can tell, there 
is no way for a large tyrannosaur to hold struggling prey and engage 
the jaws at the same time.  The neck is simply not long or flexible 
enough.  Neither of these criticisms mean that the arms were actually 
useless, but it does give some pause (for me, at least) in making the 
assumption that the arms were very useful.

It is also worth noting (and I cannot take credit for this observation) 
that many of the taxa with very reduced forelimbs actually have quite 
large coracoids.

> So, the first question is, "why were the arms so little (1m)?" The 
> second question is, "why were they still useful (ie, not vestigial)?" 
> The third question "If they were useful, why weren't they bigger?"

The only caveat to throw in here is that the arms may, in fact, have 
been vestigial after all (see above).  Good questions, though, and mass 
re-allocation could indeed be an answer.  A similar solution is the 
re-allocation of muscle attachment area.

Cheers,

--Mike


Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181
habib@jhmi.edu