Dromaeosaurids' anatomy and features.

Dromaeosaurids, therizinosauroids, and birds are all theropods with a retroverted pubis. This is associated with the decoupling of the tail from locomotory use (the tail is a vital part of locomotion in most diapsids).

All these three groups are also characterized primitively by shortened, thicker tibiae and metatarsi (later reversed in many bird groups).

In therizinosauroids, I would support herbivory as a reason, but you never know..

All this points to a reorganization of the hindlimb system from the primitive system (with long M. caudofemoralis longus providing very powerful thrust for forward motion) to a more compact system. Some have suggested the new system was hopping-based. I would suggest, alternatively, that the new system sacrificed speed for agility (see the next point).

The long arm argument doesn't hold, since some theropods with very long arms (oviraptorosaurs, troodontids, ornithomimosaurs) have more primitive pubes.

> --Stiffened tail. Not uncommon among dinosaurs, but closely
>resembles that of Archies, and less so, Rhamphorhynchs. It's possible
>that this was evolved to serve as a feathered elevator/rudder.

The proximally mobile, distally stiffened tails of dromaeosaurids and birds are pretty rare among dinosaurs. Ostrom argued that the dromaeosaurid tail was a balancing pole during combat, allowing the raptor to balance on one foot while slashing with the other. I would add that this tail morphology would also allow dromaeosaurids and Archie to make rapid turns while running, much as cats do (see the cheetah in a recent car commercial, if nothing else).

> --Those funny bones that lie in rows on the dorsal side of the rib
>cage- "ulcinaries" Paul calls them. These might have served to stiffen the
>rib cage, against the contraction of powerful arm and shoulder muscles.
> Well developed on modern birds, these are absent in Archaeopterygians.

Unciates. They may have been cartilaginous in Archaeopteryx. Stiffening the rib cage, as you say, would be useful for grasping predators as well as for fliers.

> --the folding arms. Are we seriously supposed to buy that folding arms
>were more efficient for some running theropods, and coincidentally,
> a few million years later, they happen to be useful for birds trying
>to keep their flight feathers out of the way and out of harms way? Yeah,
>right.

Apparently, you are unfamiliar with a large body of literature on evolution (Darwin's 1859 book might be a good start). :-) Evolution works by exapting previously existing structures for new uses.

I would suggest that the arms of these theropods were NOT folded to increase speed, but were instead folded for use in a praying mantis-like grasping system.

> --big breastplate. I'll bet you the furcula is unusually large as
>well among the Dromaeosaurs. (George?)

A big breastplate only tells you that the forelimbs were powerfully muscled. I thing all of us agree that dromaeosaurids (and most coelurosaurs) had strong arms.

Furculae are not yet officially reported in dromaeosaurids, but their presence elsewhere in Tetanurae strongly suggests that they were there.

> --big arms. Theropods in general, from Tyrannosaurs to Carnotaurs,
>have evolved smaller arms. Why the big reversal in Dromaeosaurs?