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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
>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.
Well, there are lots of scales from hadrosaurs, and if I recall
from ceratopsians as well, and of course Carnotaurus, and osteoderms in
sauropods, thyreophorans, and Ceratosaurus, and polygonal scales in baby
sauropods (Luis Chiappe's beasties), and if I recall this list has posted
rumors of scaly psittacosaurs from Liaoning. I think it's just simpler to
say that insulation evolved within a subset of Theropoda (more
parsimonious), tempting as it might be to homologize feathers and pterosaur
fuzz and to invoke loss over the lifetime of the animal. Since
Sinosauropteryx looks to be more primitive than tyrannosaurs or
ornithomimids, maybe even further from birds than Allosaurus (it is one
primitive sucker), this would still be a considerable subset of the
Dinosauria, however. These days I draw anything closer to birds than
Ceratosaurus with hairy plumage.
As for bipedalism- do we know if it was obligate in the common
ancestor of Dinosauria? It seems to me that the relatively long forelimbs
of Herrerasaurus, which lack elongate distal phalanges typical of grasping
hands, might have allowed the animal to move on all fours kangaroo-style
(it struck me as odd at first, but hey, why not?). Obviously things like
Torvosaurus, Tyrannosaurus, and Carnotaurus weren't doing this. I'm having
trouble thinking of any obligate bipeds on the ornithischian side of the
tree (pachycephalosaurs?) and sauropodomorphs are all facultative or
obligate quadrupeds. Seems to me that facultative bipedalism would be
primitive, not obligate bipedalism.
Regarding some previous messages on trochanters, I have an abstract
in the SVP abstracts volume for this year that attempts to sort out their
homologies. Basically, all Maniraptora more derived than Coelurus have a
true trochanteric crest formed by complete fusion of greater and lesser
trochanters (this includes dromaeosaurs, troodonts, alvarezsaurs,
caudipteryx, oviraptoridae and caenagnathidae, and therizinosaurs, birds).
It probably articulates with a big antitrochanter. That "lesser trochanter"
you always see labelled? That's the *accessory* trochanter (heh, and you
thought things were complicated already. Not to mention that ornithomimids
do have posterior trochanters, they are just rather small. Avimimids have
big ones. >:). At least, that's the only thing that makes any sense to me.
(Accessory trochanters start out as small bumps on the tetanuran lesser
trochanter, then become well-defined in basal Maniraptoriformes, then get
very large in some Maniraptora).