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Re: Re: Dinosaur Discussion List Dictionary



>In a message dated 95-10-24 13:34:07 EDT, rowe@lepomis.psych.upenn.edu
>(Mickey Rowe) writes:
>
>>> OUTGROUP--n. A clade considered primitive to a group of organisms in
>>> a cladistic analysis of that group.
>>
>>I think this one is downright wrong because of the inclusion of the
>>word "primitive".  While it's true that you look for primitive
>>character states by comparisons with outgroups, it is a fallacy to
>>presume that the character state retained by an outgroup is the
>>primitive character state.  An outgroup is just a clade whose most
>>recent common ancestor with the ingroup lived before the common
>>ancestor that defines the ingroup as a monophyletic clade. 
>
>I don't say that the characters and character states are primitive in the
>outgroup, just that the outgroup itself is.

An outgroup is not necessarily more primitive than the ingroup,
unfortunately.  However, it is best to work with the most primitive members
of the outgroups, under the assumption that they are most likely to retain
the primitive features. To use Dinogeorge's phylogeny, Phytodinosauria as a
whole is the outgroup to the theropod clade, but is not necessarily more
primitive (i.e., Brachiosaurus and Triceratops, relative to Coelophysis).
In this example, it would be best to use the basal phytodinosaurs (such as
Lesothosaurus or Thecodontosaurus) for character polarization.


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
Phone:301-405-4084