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Birds (Re: WHAT I SAW TODAY AT NATIONAL GEOGRAPHIC)
George Olshevsky wrote:
>I prefer defining birds (Aves: stem taxon) as all dinosaurs closer to
>_Megalosaurus_ than to _Iguanodon_.
Problem with this in phylogenetic taxonomy (which I don't believe
you recognize, but anyway) would be that at least Cracraft and Gauthier have
published different definitions than this, definitions with priority over yours.
Also, aren't you succumbing to the same problem that exists with the
current definition of Dinosauria ("the most recent common ancestor of
_Triceratops_ and birds and all of its descendants")? This definition is
not, IMHO, a good one. As Dr. Holtz put it, this "makes birds dinosaurs by
definition rather than by discovery."
Especially given your rather novel theories about dinosaur
phylogeny, I should think you would want the likes of _Megalosaurus_ or
whomever be discovered to be a bird, rather than defined as such. Just a
>Peter Ax (1989) goes one further; he has (more or less) suggested defining
Aves >as all archosaurs closer to modern birds (e.g., robins, crows) than to
>crocodiles (thus keeping the extant crown groups separate).
Thank goodness whatever his paper was does not have priority. This
strikes me as somewhat neonto-centric...
As an aside, this is not the original intent of crown groups. As I
understand it, the crown group concept (using node-based taxa designed to
encompass all living members of the taxon, their most recent common ancestor
and all of its descendants) was intended at least in part as a "house
cleaning" measure. Neontologists make generalizartions about taxa based on
living forms. The idea was that, since a crown clade constitutes a
phylogenetic bracket (would you prefer a "DELTRANS" Dr. Brochu? :), these
statements could be assumed to be apllicable to fossil forms as well.
Therefore, statements such as "avians have air-sac lungs" or "reptiles
excrete uric acid" could be retained, rather than having to be restated as
"neornithines have air-sac lungs" or "members of the clade consisting of
diapsids and and chelonians excrete uric acid." But oh well...
Anyway, stem-based clades do not good crown clades make, for the
same resons that stem-based clades do not (in my limited understanding) make
handy phylogenetic brackets. The stems connecting crown clades were supposed
to called Total Groups, but I'm not sure how widespread such taxa have become.
As an aside, while I personally agree with the reasoning on both
sides of the crown clade argument, I have to "just say no" to them. Why, you
ask? Oh, you're too nice, you humor me... Anyway, if you think about it,
since we are not allowed to define a taxon as "The most recent common
ancestor of all living XXXX and all of its descendants" (if we did, we'd
succumb to Lucas' "extinction criterion", whatever the heck that is...), we
are forced to pick anchor taxa to define our crown clade. And just what,
pray, happens if a certain living animal falls outside the node formed by
the anchors we chose? I should think Dr. Orenstein would be most upset to
learn that seriemas [sic?] are no longer members of Aves!
>This would make >all< the dinosaurs (and pterosaurs, too)
And "bird" would lose ANY vestige of meaning, or relationship to its
former self. I'm cool with pushing things pretty far (Pseudosuchian
crocodiles being a personal favorite), but, like you say, no one is ready
Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
"...To fight legends." - Kosh Naranek