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Re: Re: FW: Re: FW: Help with Cladistics



At 9:06 AM 9/5/95, th81 wrote:
>>At 1:46 AM 8/31/95, James Shields wrote:
>>        As long as phylogenetic analysis utilizes derived characters (via
>>homology) to define the status of taxa, and as long as feathers are
>>considered a synapomorphy for birds, the feathers of Archaeopteryx include
>>it w/ in aves.  Other fossil birds, for which there are no feathers, are
>>included w/ in aves on the basis of their possession of osteological
>>synapomorphies for birds.
>
>Under cladistics, derived characters DO NOT define the status of a taxon.
>The tree(s) having the most parsimonious distribution of derived characters is
>the prefered phylogenetic hypothesis.  Archaeopteryx is considered a bird by
>certain defintions (i.e., "birds" [Avialae] = Archie & modern birds and all
>descendants of the most recent common ancestor of Archie & modern birds).

           Yes, the position of a taxon on a tree determines its status.
What determines the position of a taxon on a tree? SHARED DERIVED
CHARACTERS that are inherited via common ancestory.  You said so, yourself.
>
>Feathers remain a very weak synapomorphy of birds: since most bird outgroups
>have yet to be found in a Lagerstatten environment, we have no evidence with
>which to dismiss the hypothesis of feathered nonavian theropods.

        That is true, presence or absence of feathers are unknown for the
outgroup criterion.  We also have no evidence to support the hypothesis of
feathered nonavian theropods.  Is the lack of specimens found in
Lagerstatten envirnoments (while true), enough to say "They had 'em, we
just don't find 'em."?  If so, then I've got some nice property sitting on
Lippalian strata that I'm willing to sell, dirt cheap.

        All My Best,

>
>





Jason J. Head
Dept. of Geological Sciences
Southern Methodist University
Dallas, Tx. 75275