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