Re: [asa] Did dinosaurs become chickens?

> A naive question. How can dinosaurs evolve into chickens if the
> extinction of dinosaurs occurred suddenly?

Two different questions, I will respond to the one above first.
Taxonomy is the procedure for assigning names to species and to groups
of species. The field of systematics involves developing the criteria
by which organisms are grouped into a heirarchy of categories. There
are different competing methodologies for how this is done. The most
influential systematics being used today is cladistics. Using
cladistic proceedures, birds (all birds) are a subset of the dinosaur
group. In this classification, birds ARE dinosaurs. Therefore,
dinosaurs as a group did not go extinct at the end of the Cretaceous.
However, if you used the older "evolutionary" classification system or
the still older Linnean classification system , birds were grouped as a
separate class. Using that classification, dinosaurs did go extinct.
It is important to understand that classifying birds in a separate
group does not imply that they are unrelated to dinosaurs.

In response to the question in your subject line: The fossil evidence
for the evolution of birds from the maniraptoran group of dinosaurs
(Oviraptors, Troodontids, Therizinosaurs, Dromaeosaurids) is very
extensive and rapidly increasing. There are now numerous dinosaur
specimens that show transitional characters with the earliest birds.
Various maniraptoran dinosaurs (especially the Dromaeosaurids)exhibit
nearly all of the diagnostic skeletal features of the earliest
unquestioned birds. As with other cases in which there is a good
fossil record, there are specimens whose proper classification is in
question. Paleontologists talk of avian and non-avian dinosaurs - the
the distinctions are sometimes very subtle and disputed. The origin of
birds from maniraptoran dinosaurs has been given increased support with
the discovery of non-avian dinosaurs with various filamentous and
feather-like body coverings. Some even have true feathers, and a few
(eg Microraptor) may have been capable of gliding flight.

I discuss some of this in my book chapter on transitional forms in
"Perspectives on an Evolving Creation." However, even since the
publication of that book in 2004, there have been many new fossil
discoveries that have further filled in and illuminated the
maniraptoran - bird transition.

Keith

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Received on Fri Jan 30 11:42:01 2009