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I've been mulling over this line of thought for a long time now, and I
was wondering if someone could comment on it. It's not easy to explain
in a small amount of space, and I hope I get the idea across. The basic
question, though, is: could small to medium sized ornithopods have been
in competition with at least partially herbivorous coelurosaurians in the
Cretaceous, leading to a decline in numbers and diversity of the former?
I've noticed that in the Morrison, there are small, "medium"
(dryosaurid-size), and large ornithopods, along with the Wealden.
However, in the well-sampled North American and Asian faunae of the Late
Cretaceous, the medium category appears to be mostly gone (except for
*Thescelosaurus*), and in Asia so is the small category. I don't know if
hadrosaurids would have pressured out medium-sized herbivores; in the
Wealden dryosaurids lived alongside hadrosaur-sized iguanodonts, for
Meanwhile, coelurosaurian dinosaurs seem to go through a great increase
in diversity in the individual lineages in the Cretaceous (although the
amount of bias in the fossil record is uncertain), including the
oviraptorids, caenagnathids, ornithomimids, and therizinosaurians. Each
of these groups has been, at one time or another, considered to be at
least omnivorous, if not at least completely herbivorous. The typical
known sizes of the members of these groups also fits in well with the
sizes of the absent ornithopods. For example, the Mongolian faunae
appear to have no ornithopods besides hadrosaurids, but also have the
greatest known diversity of oviraptorids, ornithomimids, and
therizinosaurians (although they didn't all live at the same time).
Could this diversity be a result of their taking over or assuming the
roles previously held by small to medium-sized ornithopods?
So, what do you think? I know this is the sort of thing no one can
expect to prove short of time travel, and it rests on a couple of
intermediate steps, namely that the oviraptorids et al. were at least
partially herbivorous, and that the fossil record in the relatively
well-sampled formations alluded to isn't hiding anything like a
"Thescelosaurus mongoliensis" or a "Sinoparksosaurus" (and we all know
the story of the Morrison ankylosaurians). Feel free to fire away with
Justin Tweet, *Thescelosaurus*
See "Thescelosaurus!": http://personal2.stthomas.edu/jstweet/index.htm
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