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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
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.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280 0181