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Re: Arctometatarsalia



On Tue, 26 Feb 2002, Jaime A. Headden wrote:

> 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.

A phylogenetic hypothesis will tell you whether a trait is synapomorphic
for a given clade. An example would be alvarezsaurids. The studies placing
them more advanced than _Archaeopteryx_ would consider the keeled sterna
homologous with those of more advanced birds. Sereno's phylogeny, placing
them as ornithomimosaurs, would not.

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.....)

>   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?

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.

>   ... 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.

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

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