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Re: Arctometatarsalia



Mike Keesey (tmk@dinosauricon.com) wrote:

<"'_Carinatae_' refers to the clade stemming from the first panavian with
a keeled sternum homologous (synapomorphic) with that of _Aves_ (_Vultur
gryphus_ Linnaeus 1758)."

"Note that although pterosaur sterna are also keeled, that keel is not
homologous to the keel of _Aves_, and therefore pterosaurs are not part of
_Carinatae_.">

  Okay, I will cut this off here, since this is the only real data we can
operate from. It is increasingly obvious to me that there is an assumption
in apomorphy-based definitions that phylogeny is used along with it to
verify when features become "homologous" or not. Definitions in the manner
seen above are explicitly stated in such a fashion as to provide a means
to apply the name. No where does the definiton above provide that the
definition only applies to a particular phylogeny.

  In this manner, the definiton stands on it's own feet ... otherwise, why
have 'em if they can't be so clear on their own, that they have to be
elaborated by a phylogenetic analysis to provide how so far the homology
can be extended?

  As for the nature of the homology, just because Gauthier and de Queiroz
(2001) state that the pterosaur and ornithurine keels are "not homologous"
ignores that fact that their definition of a carinate sternum applies to
pterosaurs as well, even if all other archosauromorphs between the two
taxa in the latest phylogenetic analysis lack it. This is the problem of
apomorphy-based definitions ... they only work if the phylogeny holds it
to be true, and not convergent, and you have to be absolutely _sure_ the
feature is part of a monophyletic group. One could then argue that an
apomorphy-based definition must be used in extension of a node- or
stem-based definition, in order to _set_ extent of the feature.

  One goes: Carinatae = {carinate sternum in *Vultur* where carinate keel
is thus ... and *Vultur* > *Pterodactylus*}

  ... so that the carinate sternum in *Pterodactylus* is excluded.
Otherwise, you have this problem. As I wrote before, the keel in
pterosaurs is directly homologous to the definition provided by Gauthier
and de Queiroz _for_ the carinate sternum, even if the phylogeny assumed
by its usage is not applicable.

=====
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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