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RE: Tyrannosaurs with leathery skin, etc.
>. Regarding more primitive theropods, I like the idea that
>> bipedalism itself requires warm-bloodedness (was this a Bakkerism?), as a
>> biped would otherwise tend to slow down and fall over when cold. You can
>> your own conclusions, all the way back to Herrerasaurus, from there.
>> - Tom Hopp
>Some degree of bipedalism goes back as far as Euparkeria, a pretty basal
>archosaur (outside the crown group). There are even some reasonable
>for *obligate* bipedalism on the croc side of the family, assuming
>Ornithosuchus is on that side.
> --Toby White
At a more primitive level,more than bipedalism I would defend endothermy
for the fully erect posture (quadruped or not) evolved from a primitive
fully erect biped...and is assumed that all dinosaurs are just that:
descendants from fully erect biped ancestors. Euparkeria and other
primitive semi-erect facultative two legged runners or semi-erect
quadrupeds would have been ectotherms or incipient homeotherms with four
chambered hearts (like crocodilians have). The big change came as things
like Marasuchus et al. became more and more bipedal (You can call me a
Bakkerian here...I would also advocate George's
goin-up-coming-down-the-trees theory) became endotherms and changed (or
not) posture to quadrupedalism.
"Down" or 'dino-fuzz' must go as far back as the basal dinosauria. I'm
betting that ornithischia one day is going to show us the mixture of fibers
and scaly skin (and the loss of dino-fuzz) that the basal dinosauria must
have had. These animals could go either way: lose the dawn and use other
forms of insulation (like fat deposits... some small ornithischians may
have been fat like pigs... and there's going to be more information about
this one of these days!) or thermal strategies (like gigantothermy)... or
just keep the 'down' in different stages of development.
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