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Re: Arctometatarsalia

Mike Keesey (tmk@dinosauricon.com) wrote:

<It doesn't have anything to do with whether the trait matches the
description perfectly. It has do with whether to trait is a synapomorphy
or a convergence. PhyloCode's method for defining apomorphy-based clades,
which is used by Gauthier & de Queiroz, does not allow groups which have
conergently acquired the feature to be classified in the apomorphy-based
clade. Otherwise, talpids, chrysochlorids, etc. would be carinates, too
(well, except for that "panavian" caveat.....)>

  The assumption here is that the definition is based on a structure,
where the structure is therein defined. This definition for the structure
is found in non-avians, specifically pterosaurs, where the phrase used --

  "The sternal keel is here defined as a bony crest extending from the
ventral midline of the sternum (e.g., *Concornis lacustris*; see
"Comments," below) for an hypertrophied pectoral (flight) musculature ( =
m. pectoralis and m. supracoracoideus in the crown; Baumel and Witmer

  -- is usable for pterosaurs as well. As the definition requires that the
condition of *Vultur* possess this, including taxa with which the
condition is homologous (synapomorphic) with that of _Aves_, this then
includes the condition in any other possible taxa which have such a
feature. Because in the operating definition, the inclusive taxa are
limited to Panavians (= homodefinitional junior synonym of Ornithosuchia).
In such a relationship that Ornithodira is contained within Archosauria,
rather than inclusive of Archosauria, and pterosaurs are closer to birds
than are crocodiles, then it becomes problematic to yet another degree,
and Carinatae in this sense becomes a _senior_ synonym for Ornithodira,
Ornithosuchia, _and_ Panaves.

  The statement refering to all the other phylogenies that show that
pterosaurs are not possibly close enough to dinosaurs to count is
irrelevant ... the definition does not exclude them to clarify condition
of evolution, only homology (which is not synonymous with "synapomorphy";
only condition of form and use) which is present. 

<Any clade definition requires a phylogenetic hypothesis to determine
membership, apart from the specifiers. This is no different, except that
it also requires certain anatomical parts to be known.>

  Any clade . . . except apomorphy-based definitions, which are based on
the condition of a feature in _one_ taxon and their first prescence and
all descendants of the first organism to demonstrate _that one feature_. 

<I think maybe you're using the word "homologous" differently than they
do. They are using it to mean "synapomorphic".>

  Exactly. The word is, as I understand it, in meaning "matching in
structure or position, form, etc." This is how my Webster's (Macmillan
Press, 1995) defines it, in any course.

Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

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