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Phylogenetic definition of Aves (was Re: cladistics (not a rant:))



At 12:19 PM 1/21/97 -0500, Rob Meyerson wrote:
>
>>Therefore, he has
>>proposed to leave Aves as a more inclusive taxon (a node-based taxon,
>>Archaeopteryx + modern birds and all descendants of their common ancestor),
>>and use Neornithes for the node-based taxon joining all the extant lineages.
>
>Let's see how much trouble I can get into with this one.  Assuming for a
>moment that BCF is an accurate model for theropod evolution, wouldn't this
>make the new "Aves" paraphyletic, and therefore an invalid taxon?  If so,
>perhaps the previous definition would be better suited for taxonomy.

I) As its originator has pointed out before, the details of relationship
among the most advanced coelurosaurs (including extant birds) are not
radically different in BCF than in Holtz 1994.  No one has supported an idea
that sauropods, for example, or Triceratops, or even Eoraptor or Allosaurus
is closer to modern birds than is Archaeotperyx.  The question here is thus
whether some advanced coelurosaurs traditionally outside of birds are
members of the clade "Archaeopteryx + Neornithes" or not.

II) As defined by Chiappe, there is no way that Aves can be rendered
paraphyletic, assuming evolution (assuming independant creation, it doesn't
matter).  There will always be a clade comprised of Archaeopteryx,
Neornithes, and their most recent common ancestor.  Whether that clade
contains or excludes Mononykus, Deinonychus, Troodon, Oviraptor, and/or
Tyrannosaurus is irrelevant.  It is still a monophyletic group (i.e., a clade).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"To trace that life in its manifold changes through past ages to the present
is a ... difficult task, but one from which modern science does not shrink.
In this wide field, every earnest effort will meet with some degree of
success; every year will add new and important facts; and every generation
will bring to light some law, in accordance with which ancient life has been
changed into life as we see it around us to-day."
        --O.C. Marsh, Vice Presidential Address, AAAS, August 30, 1877