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Re: Phyl tax misunderstandings



In a message dated 97-11-15 03:01:18 EST, th81@umail.umd.edu writes:

<< At 01:02 PM 11/14/97 -0500, Norm King wrote:
 >Tom Holtz said (11/14/97; 1:43p):
 >
 >(out of context)
 >
 >>Or, there will always be a clade (Maniraptora) comprised of birds 
 >>and all taxa sharing a more recent ancestor with birds than with
 >>Ornithomimidae.
 >
 >As I have ranted before, you cannot DEFINE this evolutionary path into 
 >existence...>>

The thing is that you're not "defining an evolutionary path" into existence;
you're simply giving a name to a natural group of organisms that _must_
exist. Which organisms are members of this natural group becomes the issue,
an issue to be decided by comparing characters, etc.
 
<< >I believe that is an hypothesis that has not yet been 
 >"proved."  Just ask George Olshevsky or Alan Feduccia whether that clade 
 >will always exist.
 
 To my knowledge, neither George nor Alan are creationists.  Accepting that
 evolution is real, George and Alan accept that there is a clade comprised on
 (to be clearer here) all modern birds and all taxa closer to it than to
 Ornithomimidae.  In George's cladogram, this would not be dramatically
 different than (for example) a Gauthier- or Holtz-style composition of
 "Maniraptora": where we differ is closer to the base of the tree.>>

I just read the article in _Science_ on the embryology of the avian wing, and
it's certainly the best argument yet on why the modern avian wing has digits
II-IV rather than I-III. If this article is correct (and I see no obvious
blunders), then Maniraptora may not include >any< of the known Mesozoic birds
(except perhaps a few groups of Cretaceous ornithurans, e.g., _Hesperornis_,
_Ichthyornis_), which along with theropods I'm positive had wings with digits
I-III. And Aves--defined conventionally as the common ancestor of
_Archaeopteryx_ and modern birds, and all of its descendants--may go back to
some feathered, volant four-digit-winged ceratosaurian dino-bird from which
birds with the two kinds of wings descended separately (_Protoavis_??).

We already know there were two major groups of Mesozoic birds
(enantiornithans and neornithans, or whatever they're called these days);
perhaps these can be differentiated by their wing digit counts as well as
their other already well known features. Then we can surely argue that any
similarities in wing and pectoral structure between the two groups arose as a
result of convergence for similar volant lifestyles acting on basically
similar forelimb anatomies. It's much easier to argue for convergence in two
fairly closely related groups sharing a similar volant lifestyle than it is
to argue for convergence between flying birds and cursorial dinosaurs, which
have such dissimilar lifestyles, the way the ornithologists have tried.

Tetanuran theropods (not a natural group but a bunch of convergent little
clades) would be cursorial members of the I-III-digit-winged clade. (Although
there might be some II-IV-digit-winged members too derived to distinguish.)

<< In Alan's "cladogram" (never published as such) the clade of taxa
containing
 birds and all taxa closer to birds would exclude the typical non-avian
 dinosaurs.  It would include things like Longisquama and Megalancosaurus.
 It might even include crocodilians.  However, that clade would still be
there.>>

Here Feduccia and I part company, at least over the inclusion of crocs and
exclusion of theropod dinos from that clade. In the phylogeny I advocate,
this clade excludes sauropodomorph and ornithischian dinosaurs, and
_Megalancosaurus_, too. This is the clade that I prefer to call Aves.