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bauplan convergence and stratigraphic data (was:100% goose-free responses)



John,
I've been following your idea for a while now, and I'm finding it more intriging than my dino-chauvinist outlook would expect. I do have a problem with two of your claims though:


1)>Why, if the dinosaur body form is so successful, doesn't it
reestablish itself as the dominant form in the large-animal niche? My
answer to that is that it should. I would say T. rex would kick >Serengeti butt if brought back to life. So, why hasn't it, or, >rather, something like it, been brought back to life? I think you >must be able to at least suggest an hypothesis for that lack of >phenomenon--or, scrap the notion of convergence!

It's always seemed pretty apparent to me that true non-avian dinosaurimorphs never evolved after the K/T boundary because birds had derived too specialized a bauplan (has "bauplan" finally been put in the OED, by the way?)at the time of their extinction. Not only the lack of teeth, but confluence of the orbit and lateral temporal fenestra, snout kinesis, and even increased encephalization combine to reduce jaw strength (important in large predators) as well as reduce the space available for muscle volume. The lack of well developed digits probably didn't help, and nearly precludes a sauropod or large ornithiscian morphology.
I'm not one to put too many constraints on evolution, but several ecological and deveopmental factors probably conspire to make sure teeth and non-avian forelimbs are gone for good (yes, I know some phororsurachids had "hands," but they're a poor excuse for a non-avian maniraptoran manus). First of all, while enough of the structural genes for teeth are present to grow transplanted tooth buds under the correct stimuli, the regulator genes that control the expression of those structural genes are either a)gone, b)extremely reduced, or c)co-opted to control the expression of a different structural element. Since "teeth" are useful enough that there are tooth-like crenulations known from some Cenzoic birds, but no examples of actual tooth reaquirement, I suspect that either a or c is most likely. This would most likely preclude tooth evolution in post K/T birds.
Avian limb evolution is probably also largely constrained, although by ecological factors in this case. Flying birds use their limbs for locomotion, of course, but because of such they always have (and probably always will) utilize their hindlimbs for terrestrial locomotion. As terretrial locomotion becomes more important, and flight is lost, there will be a strong selective pressure to reduce the huge muscle and bone complex that supports flight, which would otherwise be a terribly wasteful portion of bodymass (and potentially degrade cusorial locomotion as well).
Of course this kind of constraint depends entirely upon prevailing ecological pressures, so isn't absolute, ergo phororsurachids with "hands." But as arms are reduced, some of their regulator genes may also wander off or apply for different jobs, further limiting future limb evolution. I suspect that the only way functional grasping nonavian theropod hands could re-evolve is in situation where the forlimbs are important in non-flight locomotion while they are still useful for flight (e.g. a hoatzin). And a quick gander at avian natural history (pun intended) shows you how often that happens.
Maybe future hoatzin descendents will be entirely arboreal, and further develop their hands, while flight related structures are reduced. I wish I could see it if it happens. So even if the entire non-avian dinosaur extinction was caused by omlette loving mammals, the lack of extant dinosaurmorphs probably has nothing to do with egg predation.


2)Well, I used up more space with #1 then I had originally intended. So briefly: I was going to object to using our current sample of K\T ecosystems as even preliminary evidence to support the idea of a non-catastropic extinction event. I don't think we really have enough data to say one way or another yet, unless we are going to propose that dinosaurs nearly went extinct in the Middle Jurassic.

Alas, I would dearly love to see a tyrannosaur in the Serengeti...does anyone have ILM's number?

Scott
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