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fore limbs of dinosaurs



In the Mesozoic archosaur evolution has been characterized by
experiments with the forelimbs. One trend which is very prevalent is
that they have undergone shortening in a progressive fashion along some
lines. This was first seen in the rauisuchians and the ornithosuchians.
Subsequently in the theropod lines like the ceratosaurs,  allosaurids
and maniraptorans it occured independently. In many cases like the
tyrannosaurs and abelisaurs no apparent selective advantage is visible.
This may mean that the trait even though not selective remained in place
because the orgainsms derived huge advantages from other innovations
like teeth which over rode any dis advantage due to  the fore limbs. To
a molecular evolutionist it becomes apparent that the presence of
instability like - repetitive elements in the FGF/ hegdgehog response
elements of the hox gene of archosaurs could provide a predispositon for
limb changes. This may also explain the unusal evolution of the
alvarezsaurid. The avian and pterosaurian flight may also be some thing
to do with this. Another possibility which I am serious considering is
the Hox D13 genes poly A region. Deletions of some such regions causes
tridactyly in humans.    What do the paleotologists have to suggest?
Cann you think of any advantage in Carnotaurus or tyrranosaurid fore
limbs.  
Aravind