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>Actually, this type of confusion goes on all the time, and in many cases it
>causes some problems (this is why I grudgingly accept cladistic
>information). The case of birds and ornithicians, is an example of
>convergent evolution, where two unrelated groups of animals develop similar
>forms. Usually, this is due to similar environmental pressures; but as I
>think about it, what kind of pressures would cause birds and ornithician
>dinosaurs to develop the same type of hip? I'm afraid my knowledge of
>theropod anatomy is hazy at best. Any ideas from the experts?
Actually, despite their names, 'ornithischians' do not have hips which are
very similar to that of birds. In both cases, the pubis (which normally
points forward in most critters) is turned backwards (in technical terms,
opisthopubic). Beyond that, however, there is little similarity.
In the case of Ornithischia, it has been argued that the back swing of the
pelvis, which would produce a larger gut, is associated with the herbivorous
diet of the beaked dinos. (Sauropods, of course, acheived a big gut by an
alternative method: they got huge!).
In the case of Maniraptora (in this case, dromaeosaurids and birds), the
back swing is associated with transformations of the hind limb and tail.
Steve Gatesy has argued (right on the button, in my opinion) that this is
due to a major transformation of the muscles involved in locomotion. As he
says, the tail of dromaeosaurids and birds are 'decoupled' from their
primitive propulsion purpose, and were freed for other purposes. I would
argue that one of these purposes was the transformation of the tail into a
dynamic stabilizer, increasing the ability of these theropods to turn while
running. Phil Currie thinks that the transformation reflects an increase in
hopping ability. I would agree that these are not mutally exclusive
(Parentheticaly, the opisthopubic pelvis was not the only reason
ornithischians have been associated with bird origins. The primitive bird
Hesperornis has a neomorph bone in front of, and joining, the two dentaries,
just as ornithischians have a predentary bone. However, since this structure
has not (to my knowledge) been found in any other Mesozoic bird, it is almost
certainly a convergence between birds and beaked dinos.)
Hope this helps,
Thomas R. Holtz, Jr.
Dept. of Geology
University of Maryland
College Park, MD 20742