Re: Do not misunderestimate the king was Re: Evolution of tyrannosauroid

[Michael Habib <mhabib5@jhmi.edu>]

> Mike Habib's statement that "the arms were not under strong selection
> for increased resistance to load" erroneously assumes that evolution
> always creates the "perfect creature" - a very common misperception
> among people. The fact that there is a high incidence of pathologies
> shows that evolution had not yet kept pace to the factors causing those
> injuries, i.e., that T rex has not yet achieved that mythical peak of
> "evolutionary perfection."

My primary assumption is that safety factors within a given clade of 
vertebrates, with regards to limb bones, are fairly consistent.  This 
has been demonstrated for volant birds and bats (Kirkpatrick, 1994) and 
for non-volant mammals (Biewener, 2000).  Biewener has demonstrated 
that the mechanisms by which safety factors are maintained varies with 
size, and Blob and Biewener (1999) demonstrated that safety factor may 
differ markedly between clades and gaits.  Still, within a single clade 
with similar morphology and ecology, we can expect safety factors to be 
quite similar (even across a major size range, safety factors are often 
maintained, albeit partly by changes in gait and behavior).  Since 
tyrannosaurids apparently sustained repeated damage to their forelimbs 
(more so than other theropods), the safety factors for tyrannosaur 
forelimbs would appear to be lower than in other theropods.  Such a low 
safety factor could indicate low cost of damage, which suggests (but 
hardly clinches) limited function.  This does not assume complete 
optimization, but is merely based on the trends in bone strength (and 
rigidity) seen in other terrestrial vertebrates (including extant 
relatives of the clade in question).  This is further strengthened by 
the observation that bone strength can change very rapidly with major 
shifts in ecology and behavior at the species level, at least in avian 
taxa (Habib & Ruff, in press).

I see several plausible explanations for why the forelimbs sustained 
heavy damage and repeated pathology.  One is indeed that the forelimbs 
were simply poorly optimized, but this seems to be quite rare in 
tetrapods (for long bones, anyway).  Another is that the forelimbs were 
not of great importance to survival/reproduction, and that low safety 
factors were of negligible impact (low selection coefficient).  The one 
thing that can be said for certain is that the forelimbs were placed 
under substantial loads.  That, in itself, is very intriguing.  I 
remain skeptical that this clinches a particular feeding dynamic for a 
number of reasons, but the information is quite informative, regardless.

Cheers,

--Mike H.


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