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Theropod "migrations"

Here`s a problem. Just read Don Lessem`s book "Dinosaurs Rediscovered". pgs
251-255 discuss Curries work in China and the subject of dinosaur
migrations. It is noted how the late Cretaceous herbivorous species seem
quite different between Asia and North America, wheras the Carnivorous
variety were "more alike across continents" .Here Currie explains that
"Hunters are more mobile than their prey, quicker to exploit new
areas....and....tolerate a wider range of enviornments..."
I don`t know if I follow that reasoning. Why wouldn`t the herbivores make
the journey as well (after all, it`s proposed that they were likely to have
undergone long distance seasonal migrations).
Also in a National Geographic article by Paul Sereno (June 1996), althought
(for some reason) I can`t seem to find it, I remember reading just a few
months ago that he also mentions an unusual "similarity" between the
Theropods found in the Sahara region, and those of the main continent of
Eurasia. (implying the lack thereof for herbivors). He proposes a possible
"land-bridge" for the migration of these theropods.
Again, I have to say, why not the herbivores also???....Unless, maybe there
wasn`t any connection between these land masses (which seems apparent from
the geological map shown at: http://vishnu.glg.nau.edu/rcb/Late_Cret.jpg).
The large separation at the time of the late Cretaceous of the land masses
involved would account for the divergent evolution of the herbivores, but
what about the Theropods similarity of form?? Well, BCF has an answer for
that,...the ancestors of these theropods just flew across these barriers,
and then became the secondarilly flightless predators that seem so "similar"
in their anatomical details.
........why not??