[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
> 1.) If crocodiles aren't ancestral to dinosaurs, but instead a parallel
> branch from the thecodonts should we not presume their physiology
> (including metabolism) would be similar?
It depends. Thecodonts were a fairly diverse group that led to a
lot of things other then crocodilians, including (ultimately) endothermic
birds. Endothermic therefore evolved SOMEWHERE along the line; and
exactly where that is (in birds, theropods, the common ancestors of
dinosaurs, the immediate thecodont predecessors of dinosaurs) is open to
debate. The common ancestors of birds and crocodilians (which would have
been a thecodont) is generally presumed to have been ectothermic since
this is the norm among reptiles, but this does not mean that endothermy
could not have evolved in the ornithodirans or at some other point in what
would ultimately be the bird lineage after it split from the corcodilain
line. In that case, the answer to your question would be "some were, some
weren't", but there is no concensus.
> 2.) If dinosaurs are an intermediary step between thecodonts and birds,
> why did the first birds appear *way* before birdlike features became
> prevalent in dinosaurs?
A common, and perhaps most prevalent idea, is that maniraptorians
(the REALLY bird-like theropods) WERE present in the Jurassic; we just
haven't found the fossils yet.
Advocates of a non-dinosaurian origen for birds have accused this
idea as a means putting the dinosaur-bird connection on life-support; we
can always continue to beg off the absence of fossils by saying "they just
haven't been found yet", when in fact they probably didn't exist in the
Jurassic at all.
However, this accusation has a serious problem: there
aren't ANY good proto-Archaeopteryx type fossils from the Jurassic. No
we don't have maniraptorians, but on the other hand we don't have any bird
like crocodilians or thecodonts; in fact as far as anyone knows, NO
thecodonts (archosaurs other then crocodilians, dinosaurs, ansd
pterosaurs) made it out of the Triassic. The MOST BIRD-lIKE archosaurs we
HAVE from the Jurassic are theropods. Non-maniraptorian theropods, but
at least we know there were theropods. Compare your average Jurrassic
theropod with the most bird-like crocodilain you can find, and it comes
pretty clear the theropod-bird connection is still the most concrete. To
sum it all up; the immediate Jurassic predecessors of Archaeopteryx
HAVE NOT BEEN FOUND, period. Maniraptorians were present since at least
the Early Cretaceous, so extrapolating a Jurassic origen for them is
Greg Paul and George Olshevsky have proposed interesting
alternatives to the prevalant Jurassic-maniraptorian model, although the
fossil gap makes settling the debate difficult at this point. Further
discoveries will have to be made before the details of Archaeopteryx
origens are worked out.
"I'm fighting 100 years of dogma. Dinosaurs were characterized early on
as viscious lizards..."