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

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.


--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