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Re: Tetrapod note (was: Re: [Re: Resting Sauropods])



At 10:47 PM 7/17/98 -0400, Jonathon Woolf wrote:
>"Recently, Tetrapoda was formally defined as a crown-group (Gauthier et al.,
>1989).A crown-group is a clade that includes the last common ancestor of two or
>more extant taxa, and all its descendants. In this case, Tetrapoda was defined
>as the clade that includes the last common ancestor of lissamphibians and
>amniotes, and all its descendants."
        This definition is specifically *not* the definition I was referring
to. Hopefully this hasn't confused any more people out there too much. Let
me try to seperate the Tetrapod thing out for everyone:

        Connotations/definitions/invocations of "tetrapod":
a)      "A four legged animal" or "an animal that walks on four feet"
b)      "All animals which share a four limbed heritage" [my wording]
c)      "The most recent common ancestor of Lissamphibia and Amniota and all of
         its descendants."

Definition (a) is the "common sense" definition, and I'm not sure that it is
commonly used in science, since tetrapod (lower case "t") seems to carry the
strong taxonomic and/or evolutionary connotation. That connotation is most
liberally summed up by definition (b). Definition (c) is an explicit
definition of a phylogenetic taxon (the "cladistic" connotation others have
referred to). Do not confuse (b) and (c).
        There is a taxonomic quandry here. Tetrapoda is commonly used as a
taxon. In this case, "tetrapod" refers to members of that taxon. Those who
do not subscribe to phylogenetic taxonomy, or who eschew crown clades will
not appreciate definition (c) as a formal TAXON definition, and will perhaps
want definition (b) as the TAXON definition. Therefore, the phylogenetic
taxonomist (who recognizes the taxon Tetrapoda as defined in (c)) will only
refer to descendants of the MRCA of Lissamphibia and Amniota as "tetrapods".
        Others may or may not recognize a taxon Tetrapoda, but will probably
use definition (b) for the term "tetrapod". In a sense, (b) is the "jargon
definition" of "tetrapod", in sort of the same way that the "jargon
definition" of "carnivore" is a member of the Carnivora, not necessarily any
animal that eats meat (although I gather this is not so strict). Indeed,
even as a rampant cladist (tm), if someone says "tetrapod" to me, I usually
assume definition (b). The point of my earlier posting was that I do not
believe that (nor have I ever experienced a situation in which) scientists
EVER apply (a). Application of (c) ammounts, perhaps unfortunately, to a
special case, rather than a general rule.
        Let's worry about (c) after people understand that (b) is used over (a).
        :) Wagner
----------------------------------------------------------------------------
    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
                    "...To fight legends." - Kosh Naranek