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Ornithischian Metabolism (was "New Ref")



In respect to Ben Creisler's quote of  F. Seebacher's  Dinosaur body 
temperatures: the occurrence of endothermy and ectothermy. PALEOBIOLOGY 29 
(1): pp. 105-122 (abstract):

Although based on a line of inferential reasoning, I think we should expect 
that if an avian or mammalian-grade endothermic condition existed in 
coelurosaurs and basal theropods, that other small-medium (100 kg. or less) 
basal ornithischians and sauropodomorphs would have been the same, for two 
reasons:

1.  Ecologically, basal ornithischians and early sauropodomorphs were likely 
prey for at least some basal theropods, a stituation that would later have 
included hypsilophodontids, hadrosaurs, thyreophoreans, sauropodomorpha and 
ceratopsians. Any prey species must be at least as fast and/or agile as its 
predators, or go extinct. Basal ornithischians and sauropodomorphs would not 
have survived and radiated if their thermal physiology had not been on the 
same grade as that of coelurosaurs and even earlier saurischians. As a 
corollary, if some form of insular covering is required for a small/medium 
endothermic tetrapod to maintain this condition, we should expect to 
eventually find evidence of outer insulation in all basal Dinosauromorpha, 
with early forms of both saurischians AND ornithischians exhibiting a fluffy 
fur or featherlike covering.   

2.   If  basal Dinosauromorpha form a natural clade, as is supported by the 
evidence we now have, saurischia and ornithischia are sister groups that 
evolved from a common ancestor, and we should expect that the development of 
a  mammalian or avian-like endothermic condition (or one unique to dinosaurs) 
would have to persist and be retained among predominantly herbivore/prey taxa 
at least until a critical size and mass was reached among adults. After this 
point, true endothermy could be gradually replaced by gigantothermy or some 
other metabolism that would be functionally equivalent to endothermy. 
(Although untestable, I think endothermy would have persisted in many or 
possibly all large to gigantic dinosaurs-if not, why are/were megamammals 
like elephants and presumably indricotheres endothermic?) Hatchlings and 
juveniles would be obligatory endotherms in order to grow at the rates 
present findings suggest, and to escape predation from endothermic theropods. 
 An ectothermic juvenile ornithischian (or any other dinosaur) later 
developing into an endothermic adult does not make sense.  

If we are correct in hypothesizing a monophyletic origin for both Saurischia 
and Ornithischia, to me a reasonable hypothesis suggests itself: that the 
common ancestors of both subclasses also shared a common thermal physiology, 
and that this would have been a form of endothermy, with some form of insular 
body covering. Further, this condition would have persisted at least until a 
critical but as yet undetermined body mass was reached. Although as yet 
unknown, I predict we will someday find a small hypsilophodontid or other 
ornithischian with a fluffy, heat-conserving integument functionally similar 
to that of some theropods.

Mark Hallett