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Re: Were some dinosaurs descended from birds?
There are two ways to identify potentially secondarily flightless theropods.
First, via the possessions of characters that may have evolved in flying
ancestors. Second, by the possession of characters that suggest the subject
was closer to birds than was Archaeopteryx, which probably had some flight
capability. The case is strengthened if both flight related and
post-Archaeopteryx characters are present.
The potentially secondarily flightless theropods are all Cretaceous -
dromaeosaurs, troodonts, oviraptors, therizinosaurs, alvarezsaurs, avimimids,
ornithomimids and tyannosaurs. The later only because it has been argued that
they are closely related to troodonts. Ornithomimids are also rather weak in
this regard because they do not possess any flight related characters, they
do have some avian characters not seen in Archaeopteryx. All the other
candidates have such bord-like characters too, too many to list here.
Dromaeosaurs, troodonts, oviraptors, therizinosaurs, and avimimids have
folding arms like birds, including the classic lunate carpal block.
Dromaeosaurs, troodonts and oviraptors also have avian shoulder girdles, with
horizontal strap scapula blades, vertical, enlarged coracoids whose outer
surface faces forwards, and articulated with long anterior grooves on a large
sternal plate. They are very similar to those of flightless birds in that the
angle between the scapula and coracoid is rather modest and the sternum is
not keeled. Like Triassic and Jurassic pterosaurs and Archaeopteryx that used
their long tails as aerodynamic controls, dromaeosaurs and troodonts have
highly vertically mobile tail bases and more elongated distal caudals. The
pterosaur and dromaeosaur tails also had ossified tail ligaments.
Some of these characters can be explained away, but that dromaeosaurs have
bird-like shoulder girdles AND folding arms AND a pterosaur-like tail without
a flying ancestory is pushing it. The most logical explanation is that this
complex evolved from a flying Archaeopteryx-like ancestor. This would have
been easy since the basal birds flew less well than later birds anyway, and
retained long, clawed fingers highly suitable for terrestrial purposes.
Of course I discuss some of this in Pred Dino World.