Re: [asa] Did dinosaurs become chickens?

Both the "birds are dinosaurs" and "birds are dinosaur cousins" sides
have made some arguments that aren't so good. (For example, the pose
of fossil dinosaurs on nests is similar to that of birds on nests, but
what other options are there for a tailed biped? Theropod arms are
usually shorter than theropod legs, unlike in birds, but there's
plenty of variation in limb length.) Also, "birds are dinosaurs",
"bird flight developed from running on the ground", and "modern bird
groups diversified in the Mesozoic, with several lineages surviving
through the end-Cretaceous" got treated as all part of one package, as
were the opposite positions ("birds are dinosaur cousins", "bird
flight developed from gliding between trees", "modern birds originate
from only a couple of lineages that made it through the extinction and
radiated rapidly in the Cenozoic"), when in fact each is independent
of each other. (Ironically, some advocates of a running origin of
flight have tried the argument that the known gliding arboreal
dinosaurs are later than Archaeopteryx, after previously arguing that
the fact that most known birdlike dinosaurs occurred after
Archaeopteryx wasn't a problem for the birds are dinosaurs idea.)
Another problem is the exact place where birds branch off from
theropods. It's possible, for example, that many of the meat-eating
dinosaurs derive from a more bird-like ancestor.

Thus, it's a fine example of a topic where egos have gotten unduly
involved. Nevertheless, a close relationship between meat-eating
dinosaurs and birds seems extremely well-supported at this point.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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