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I think there are a lot of problems with the idea that there weren't any
birdlike theropods in the Late Jurassic because we haven't found any.
First is the assumption that the sampling is at all good. The only Late
Jurassic small non-flying theropods I can think of are two Compsognathus, two
specimens of Coelurus and a possible third (the hand long referred to
Ornitholestes), and the type of Ornitholestes. That's pretty miserable.
Consider that at any given point there were probably hundreds of thousands if
not millions of individuals of any of these species runnning around, as well
as individuals of many species we haven't found. Then consider that each
species probably was around for millions of years, and it works out that there
were probably, er, billions and billions of theropods we're not finding. And
then there are the six I can think of that we *have* found. Maybe seven if
that little thing Jensen described isn't a bird, and maybe some more I'm
missing. At any rate, the math doesn't have to be too on target to still tell
you that we're missing one heck of a lot.
Second, some of the things we do have are, in fact, pretty birdlike.
Compsognathids look to be very primitive by their tail morphology (they never
get skidlike chevrons like in maniraptoriformes, they have transverse
processes way the heck out the tail, and they have way more than the typical
maniraptoriforme complement of 40 or fewer caudals), but Ornitholestes and
Coelurus were originally thought to be part of Maniraptora by Gauthier.
Makovicky and Sues recently found the same thing, and I think they are quite
likely right. Coelurus, for example, had a semilunate carpal block and bowed
ulna, while Ornitholestes shows hypapophyses on the cervicodorsals, and the
back end of the ilium curves down instead of being squared off.
Third, you get all these lineages popping up pretty well-formed in the
Cretaceous. So you've now got definite therizinosaurs from the earliest
Cretaceous (China) along with a possible oviraptor relative (Caudipteryx).
Dromaeosaurs are very early arrivals too; the troodont Sinornithoides is Early
Cretaceous, and then Pelicanimimus is Early Cretaceous too. It seems to me
that it probably (but not necessarily) took a while for all these lineages to
diverge as much as they had by the start of the Cretaceous, putting their
origin sometime in the Jurassic.
Fourth, there is the tooth stuff. I haven't read the Koparion paper, but
there's a new one out recently in the April 1998 _Palaontologische
Zeitschrift_ called "Small theropod teeth from the Upper Jurassic coal mine of
Guimarota (Portugal)", and they claim that "Six further morphotypes of
theropod teeth are also described, which are closely related to Cretaceous
theropods such as dromaeosaurids, troodontids, tyrannosaurids,
Richardoestesia, and Paronychodon. A Late Jurassic origin of these groups of
theropods, which is very often postulated, is discussed."
Oh, yeah, and isn't Stokesosaurus (late Jurassic) sometimes thought to
related to tyrannosaurs?
I think in principle stratigraphic data is a great way to test some of
evolutionary hypotheses. In practice... well maybe, at least for now, at least
for this problem, it's best left to people like the invertebrate
paleontologists who have enough specimens to where they don't count them on
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