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Re: Triassic dinosaur evolution



>In essence, the fundamental question is this:  would dinosaurs
> have "taken over" if not for the TJ extinction?  Classic  ideas would
> say yes: dinosaurs were "superior" and preordained for success, and 
> showed that superiority over millions of years of tooth-and-claw 
> competition.  We say no: there were no signs that the dinosaurs were
> doing  anything "better" than the crurotarsans over the 30 million years
> they  overlapped in the Triassic.  Dinosaurs were lucky in the sense
> that there  was a very rapid mass extinction that changed the rules
> upside down.  What  had been the norm for 30 million years was no longer
> the norm.  Dinosaurs  coped, crurotarsans did not.  Dinosaurs were
> "lucky" in that they had the  adaptations to cope with a sudden and 
> unexpected mass  dying.

So basically...
Dinosaurs became dominant in a similar way that mammals became dominant*

Likely Diapsids weren't outcompeting Synapsids until the End-permian extinction 
event.
Dinosaurs weren't outcompeting other archosaurs until a Triassic extinction 
event.
Mammals weren't outcompeting Dinosaurs* until the KT extinction event

Thus throught extinction events, things have come full circle, and synapsids 
are again dominating as they did so long ago.

* This assumes it can be said that Mammals are the dominant form of life on 
land - Diapsid/Dinosaur/Bird diversity is immense, but mammals occupy the top 
of the food chain/role as large land predator in almost all ecosystems (and 
they are the top predator of the sea in many places as well- currently synapsid 
mammals certainly fair better at sea than the diapsids).

If in the triassic you wouldn't assume Dinosaurs would become dominant without 
a fundamental change, based on diversity and disparity, you certainly wouldn't 
have predicted Synapsids would become as dominant as we are now on the basis of 
some fundamental superiority.

So my related question is: Is there a fundamental superiority to Mammalian 
quadruped predators that resulted in them outcompeting the Biped "terror birds" 
(really the closest thing after the KT to a classical dinosaur), or was it an 
accident of geographhy?
Mammalian hearing bones are more advanced, no? is the quadruped bauplan of a 
cat (tigers, sabercats, etc) better than a biped predator?
It allows a lower profile when sneaking up on prey, cheetahs show us it can be 
every bit as fast or faster than a biped (cheetah > ostrich), so aside from 
freeing the forelimbs (which humans and birds made good use of), what good is a 
bipedal stance?
This should also apply to the raisuchians, as they were mainly quadrupeds - why 
would a biped therepod have any advantage? and what of the sauropods? (for that 
matter, what was the stem dinosaur before the Therepod+ sauropod and 
ornithischia really like, is Eoraptor really a good model for a stem dinosaur? 
it seems awfully therepod like...)

Would there be some advantage in a "1 on 1" fight? 
Would a biped typically be taller than a quad(bipeds would need a longer 
stride, thus longer legs?) 
Would height give an advantage in a fight? striking from above, and keeping the 
body off the ground so the raisuchian oponent has to rear up to get to it? 
Is a biped better onl in the absence of a quad predator that can jump well? 
(such as a tiger that can leap from the grass and knock a man off the top of an 
elephant, a small higher difference wouldn't make much difference I'd think).

Unless you are limited to 4 limbs and want to make tools or have wings, I don't 
see why Bipedalism is desirable.

Im surprised the Dinosaurs did as well as they did - size constraints suggest a 
quad predator could become larger than a biped predator, would be more "sure 
footed", and wouldn't have a large portion of its mass serving only as a 
counter balance.
Imagine a Tyrannosaur sized bear, Wolf, Tiger, or any Raisuchian - It seems to 
me the Tyrannosaur would probably lose.