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Re: Archaeopteryx



In a message dated 6/16/03 11:26:43 AM EST, twilliams_alpha@hotmail.com 
writes:

<< I for one am very eager to see the cladogram that you have in mind.  BCF 
is 
 an interesting theory of avian evolution, but I have yet to see it 
 represented as an explicit or testable hypothesis.  Let's see where the BCF 
 cladogram puts birds relative to dromaeosaurs, ceratosaurs, herrerasaurs, 
 sauropodomorphs etc, then we'll talk. >>

BCF is a way of interpreting theropod-avian cladograms; it doesn't generate a 
cladogram on its own. Thus there's no "BCF cladogram" per se. I have 
(partially) constructed my _own_ cladogram of dinosaur-bird relationships, and 
I have 
interpreted it in a BCF way, but that's really a separate problem. (I should 
have it ready for The Dinosaur Catalogue. A lot of it already exists in the 
literature.)

BCF simply argues that the common ancestors of various theropod lineages and 
modern birds were small, arboreal (or acronomic--living in high places) 
animals. These are the animals at the branch nodes of the theropod-avian 
cladogram 
(whichever cladogram you like). A few descendants of some of these animals 
opted for a terrestrial lifestyle and their lineages adapted to it by shrinking 
the wings and chest, enlarging and elongating the legs, becoming fully bipedal, 
perhaps losing or vestigializing their feathers, and so forth.

So what we see as theropod lineages basically represent various stages of 
secondarily terrestrial and secondarily flightless forms, and many of their 
supposedly shared derived features (bipedality, shortened forelimbs, reduced 
chest) 
were actually repeated convergences toward the bipedal predator lifestyle--as 
continued in Cenozoic giant flightless birds, which because they descended 
from a much more modern birdlike ancestor are themselves much more birdlike in 
their anatomy. It seems a lot easier to evolve secondary flightlessness than 
flight; that's certainly true among modern birds and I see no reason it 
wouldn't 
also have been true of Mesozoic birds, which were functionally closer to 
secondary flightlessness (and raptorial forelimbs) than are modern birds.