Re: Dinosaurs and birds

["T. Michael Keesey" <keesey@gmail.com>]

On 4/3/07, Tim Williams <twilliams_alpha@hotmail.com> wrote:
> Stephen V. Cole wrote:
>
> >Do you real VPs ((I mean, other than Olshevsky) no longer believe birds
> >descend from Archy and his cousins?

To show that any given fossil organism is a direct ancestor to modern
organisms, you'd need an unbroken lineage of parents and children.
Since this will not be happening anytime soon for organisms from deep
time, paleontologists have shifted from searching for actual ancestors
to searching for possible ancestors or ancestor models (i.e.,
organisms that, if not themselves an ancestor, have not diverged much
from an ancestor).

_Archaeopteryx_ is probably not ancestral to modern birds, but it's
probably fairly similar to some actual ancestor in many ways. In most
ways it's a good ancestor model.

> Do you have a reference/citation for Olshevsky?  Last I heard, he believed
> that birds existed as early as the Permian-Triassic, and that dinosaurs
> evolved from these birds (BCF = Birds Came First).

Yes, but (last I heard) he also calls any arboreal, volant
dinosauromorph a "bird", and his argument is almost more ecological
than phylogenetic. I.e., he doesn't disagree radically with "normal"
phylogenies (well, maybe in a few areas, but that's not entirely
relevant to the basic idea), but he thinks the lineage leading to
modern birds was arboreal and volant for a long time before
_Archaeopteryx_, and that most Mesozoic dinosaurs we know are
secondarily terrestrial and nonvolant (as well as a few Cenozoic ones
of course!).

This argument hasn't found a lot of support, although some think
flight may have originated somewhere in basal _Maniraptora_, at least.
(Then again, some don't even think _Archaeopteryx_ was volant.)
--
Mike Keesey