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Re: Evolution of tyrannosauroid bite power

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.



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