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Re: Cladistics question



At 02:47 PM 3/12/97 -0500, Rob Meyerson wrote:
>   Let's say we have hypothetical clades A and B, with B nested within A.
>Let's pretend for a moment that green and blue buttons are legitmate
>characters that could be used for diagnosing these clades.  Clade B is
>comprised of blue buttoned animals.  Everything else outside of clade B in
>clade A is comprised as green buttoned animals.  Clade B can be diagnosed
>using the blue button character because all Bs have blues, yes?  Can Clade A
>be diagnosed using green buttons even though Bs have blues (assuming all
>animals outside of A have red buttons)?

Yes.  A great example.

A real life example: Tetrapoda can be diagnosed among living vertebrates by
the possession of four limbs with digits (as opposed to flippers), even
though several subclades have subsequently lost this state (caecillians,
snakes and other legless lizards, cetaceans, etc.)

>   Also, someone recently mentioned a stem vs. a node based clade.  Wot's
>the difference?

[You know, there are times that I think I must be wasting my time typing to
the net.  Does anyone read these postings?  I know I generally don't talk
about things like collecting on private land and other matters that are of
more interest to the list than phylogenetic taxonomy, but we have been down
this route many, many times before!!]

Stem-based clade definition format: Taxon A and all taxa sharing a more
recent common ancestor with Taxon A than Taxon B.  Examples: Synapsida (with
"Mammalia" as Taxon A and "Reptilia" as Taxon B); Carnosauria (with
"Allosaurus" as Taxon A and "birds" as Taxon B).

Node-based clade definition format: All descendants of the most recent
common ancestor of Taxa A and B.  Examples: Mammalia (with "Monotremata" as
Taxon A and "Theria" as Taxon B); Allosauroidea (with "Allosaurus" as Taxon
A and "Sinraptor" as Taxon B).

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661