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Re: Fossil Discovery Threatens Theory of Birds' Evolution
bipedalism is known from some non-dinosaurian thecodonts and Triassic crocs.
Bipedalism has evolved a minimum of FIVE times among mammals.<<
Those events were all for a specific purpose, though: if you live in a desert or
grassland, bipedalism is useful. Kangaroos, kangaroo rats, jaboras, and humans
all evolved in places where standing up was useful (well, humans may have had
some useful pre-adaptation in this direction from our more distant arboreal
ancestors, but anyway...). By and large, however, mammals are quadripedal, by
default. Dinosaurs, however, were BIPEDAL by default. They only descended to
all fours when it was absolutely necessary (the plant eaters' gut became too
large and it threw their center of balance off). My pet theory is that
evolved from some little kangaroo rat-like ancestor that lived in deserts (where
bipedalism is an advantage) and then retained the trait thereafter.
>>How do you know that Caudipteryx is a dinosaur? It could just be a bird that
is primitively filghtless.<<
That is a problem. I say that a flightless bird _is_ a dinosaur, but other
people don't. What we need is a diagnostic feature that we can find and say
"yes! That's a dinosaur". We can look at mammals' jawbones (or teeth) and
seperate them from mammal-like reptiles, and that has made things much easier.
Convergence is quite powerful, but it can only work with what it has. Dolphins
look a great deal like fish, but their tails move up/down not side/side because
the up/down movement is how mammals' spines move when they run. If birds had
evolved from dinosaurs, but from some crocodile-like ancestor, they would never
have displayed all of those dinosaur-like traits by mere convergence. If
Longuisquama was quadripedal (which seems to be the consensus, although, to be
fair, the hindlimbs aren't there so we don't know), then it would have given
to a race of quadripedal birds. Sound crazy, well quadripedal mammals gave rise
to quadripedal bats, didn't they?