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Re: Ratite Evolution ?



Roy Nash wrote:
>is it not time to re-define "birdness" ?
        Many people seem to agree with this. Some of them choose to define
"birdness" by the ancestry of the animals. The general gist is that "birds"
consist of the common ancestor of a certain set of animals and all of its
descendants (all animals more sharing a more recent common ancestor with
modern birds than with _Deinonychus_, according to Gauthier). This removes
the need to use "characteristics" to define what a bird is, since, as you
allude in your post, those characteristics have a habit of changing with new
data.

>Is Aves still valid ?
        The folks I refer to above (who define groups by common ancestry and
descent) would say it is, based on the fact that you will always have a
group of animals fitting the pattern of ancestry and descent you describe
(now if only there were agreement on which ancestor and descendants should
be called Aves!). Everyone else would say it is too, on the assertion that
"everybody knows what a bird is", even if they have trouble isolating
characters unique to all birds.

>Is it a polyphyletic grouping resulting from different birdlike dinosaur
>ancestors?
        Under the ancestry and descent definition, nothing can be
polyphyletic. What would happen in your example is that the group would
simply extend to include more animals before the ancestors you describe.
        In any case, I haven't heard of anyone who has recently suggested
this. There are apparently characters which link ratites to other modern
birds within Dinosauria.

>Do birds have a common ancester ?
        Any two organisms you care to pick share a common ancestor,
according to theory. You, me, a slime mold, an apple, _Paramecium_,
_Streptococcus_... the list goes on. The important questions are what else
descended from their most recent common ancestor, and who shares common
ancestors which aren't shared with the others?

>Are ratites, and kiwis actually birds at all ?   
        I believe so, yes.

Matthew Troutman wrote:
<<Do birds have a common ancester?>>
>Yes.  The common ancestor is _Archaeopteryx_.
        AFAIK, this has *NOT* been demonstrated (not that it can be). There
have been arguments as to whether _Archaeopteryx_ *could* be ancestral to
later birds. The latest view I read (Sereno 1997) stated it was possible. As
recently discussed on this list, there are criteria for hypothesizing that
an animal is ancestral to others. AFAIK, these criteria are debated in the
case of _Archaeopteryx_. Even if the criteria are met, we have no way of
knowing that it is indeed THE (most recent) common ancestor, much less A
common ancestor, of later birds. Has this changed?

>No matter which basal 
>avian phylogeny that you read (whether it supports Sauriurae or 
>Ornithothoraces) they all stick by the conclusion that _Archaeopteryx_ 
>is the closest thing to an avian common ancestor.
        Closest is a difficult concept. "Avian ancestor" is as well. If Aves
is defined as == {_Archaeopteryx_ + Neornithes}, by definition, no bird can
be more or less closely related phylogenetically to the common ancestor than
is _Archaeopteryx_. If Aves == {Ratitae + Tinamoui + Neognathae}, in which
case _Archaeopteryx_ isn't even an avian, many many birds are closer to the
common ancestor than is _Archaeopteryx_.
        As far as the morphology of _Archaeopteryx_, it has been stated by
some authors that nothing precludes it from being *AN* ancestor of all later
birds. Others disagree. Suffice it to say, however, we must first know what
common ancestor we refer to, then define what we mean by "closely". Even
then, we are left with a situation which is much more vague than Matt suggests.
        Just so long as no-one interprets the "basal" position of
_Archaeopteryx_ as indicating its "primitiveness", I'm happy.

>Other possible birds (_Unenlagia_) which may be more "primitive" than
>_Archaeopteryx_ are simply too fragmentary to make a detailed case for their
>position in Aves.
        No phylogenetic definition of Aves allows the inclusion of
_Unenlagia_ within that taxon (given current phylogenetic hypotheses). Given
the criteria of Gauthier, we may refer to _Unenlagia_ as a "bird", but I
prefer to restrict "bird" to a dinosaur which flies or is descended from
flying dinosaurs (ok, ok, George, we know how you would apply this... :).
        Wagner
----------------------------------------------------------------------------
    Jonathan R. Wagner, Dept. of Geosciences, TTU, Lubbock, TX 79409-1053
                    "...To fight legends." - Kosh Naranek