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more cladistics

>>>        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
>>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.

This is a point a lot of people miss.  In cladistic taxonomy, the DEFINITION
of a taxon is based on position: nothing more, and nothing less.  If, with
further discovery, a derived character previously used to *diagnose* (but
not define) a taxon is found to have a different distribution (e.g., if
feathers are found on Coelophysis), the definition of the taxon DOES NOT
CHANGE.  In the example above, the taxon Avialae will retain the same
defintion regardless of the discovery of feathers on Coelophysis.  The
character diagnosis will change, but the defintion will remain the same.

>>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.

You misunderstood what I said.  I did not say that nonavian theropods had
them (although some almost certainly had protofeathers).  I was simply
saying that the derived character "feathers present" is and will remain a
very weak synapomorphy of Avialae, because it is so rarely observed.  Other
derived, skeletal characters (see Gauthier, 1986, for the classic work) unite
Archaeopteryx and modern birds.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661