Re: rampant speculation

["Tim Williams" <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:

> But it assumes that coelurosaurs evolved herbivory in a world full of
> already herbivorous ornithopods which, at the beginning, must have been
> better adapted to that. Such things don't happen (competitive exclusion).

Ah, so incumbent taxa (e.g. ornithischians and sauropods, among dinosaurs) 
have a monopoly on herbivory, and their apparent success precludes any new 
adaptive radiations from "nudging" them out of their niches.  Unless, of 
course, the existing herbivores go extinct, then evolution goes back to the 
drawing board.

This "competitive exclusion" hypothesis is nothing more than that: a 
hypothesis.  It overlooks the possibility of either (a) new plants making an 
appearance in an ecosystem; and (b) novel herbivory strategies emerging 
among existing omnivorous or partially herbivorous lineages (many modern 
carnivores include plant matter in their diets).  You seem to be only 
allowing for a third scenario: (c) the introduction of hitherto 
geographically separated or isolated herbivores into a naive envronment, 
with the immmigrant species offering competition to the native faunas.

> The only possibility is that herbivorous coelurosaurs evolved somewhere
> where there were no ornithopods -- very difficult to disprove if 
> "somewhere"
> was, say, an island.
>         All this is ignoring the obviously herbivorous coelurosaurs, the
> segnosaurs.

Notwithstanding the "isotopic studies (mentioned onlist whenever)" which you 
mention, the manus of ornithomimosaurs does not seem to be very useful for 
seizing or handling prey.  Nicholls and Russell (1985) suggested that the 
ornithomimosaurian manus was best suited for "hooking and clamping" (such as 
tree branches), and I don't know of any study that has directly refuted this 
interpretation.

> Well, they seem to have occupied quite a different niche (when
> the ground sloths and chalicotheres died out that niche became empty and 
> has
> remained so, it seems) from any ornithopods, and if the famous Lufeng jaw 
> is
> really from a segnosaur, then they may have evolved directly after the Tr-J
> mass extinction in an empty world.

Try telling that to a prosauropod.


Tim


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