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Re: Sheesh

On 10/24/06, Tim Williams <twilliams_alpha@hotmail.com> wrote:

The definition I use is the node-based clade that includes _Archaeopteryx_ and modern birds.

I'd like to make the case that if you really want to capture the "traditional" (i.e., mid-20th century) paleontological usage of "Aves", David Marjanovic's definition works much better. He uses a branch-based (=stem-based) definition, specified internally by Passer domesticus and externally by Velociraptor mongoliensis, Oviraptor mongoliensis, Segnosaurus galbinensis, Troodon formosus, Sinornithoides youngi, Ornithomimus velox, Tyrannosaurus rex, Compsognathus longipes, Ornitholestes hermanni, Coelurus fragilis, Coelophysis bauri, Hypsilophodon foxii, Crocodylus niloticus, Sphenosuchus acutus, Euparkeria capensis, Megalancosaurus preonensis, and Sorex araneus. (I think the external specifier list could perhaps be trimmed a bit, but....)

This definition matches the traditional usage better because it does
not rely on Archaeopteryx' position, and that genus has at times been
excluded from Aves, especially where it has been considered closer to
deinonychosaurs. (See, for example, Predatory Dinosaurs of the World
[Paul, 1988] or A Field Guide to Dinosaurs [Lambert, 1983].) Under
most phylogenies, the known content is equivalent to that of Chiappe's
and Sereno's node-based definitions. But when archaeopterygids are
placed on the deinonychosaurian side, or outside Eumaniraptora, they
are excluded, and the rest of the content remains the same.

There are also reasons for liking Gauthier's crown group
definition--see Gauthier and de Queiroz' paper in the Ostrom Symposium
Volume (2002) for more on that. The main point is that it decreases
the number of unjustified inferences--we don't know if avialans
outside the crown group had certain soft-tissue or behavioral
characters of the crown group, or how early those characters appeared.

As mentioned earlier, more discussion (and a poll!) here:
T. Michael Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com