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Fwd: Non-serpentine lacertids (was RE:WHAT'S GOING ON?)
Matt Bonnan asked me to forward his response to my question to the entire
dinosaur list. So here it is.
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To me, a "natural"
group must include its common ancestor but need not include all the
descendant subgroups of this common ancestor (though it usually does). The
criteria for removing such a subgroup into an independent taxon are
qualitatively no different from the criteria that distinguish one species
from another. There is no difference in phyletic information content
the statements "birds are dinosaurs" and "birds descended from dinosaurs."
Why should one be preferred and the other rejected?
Probably about to show my cladistical ignorance, but I suppose the simplest
explanation would be a buckets in buckets idea. You have a group of
vertebrates, say the amniotes, which are characterized by the possession of
an amniotic egg as well as some other features. The amniotes comprise a
variety of groups (clades) including the synapsids, diapsids, "anapsids?",
archosaurs, ornithodirians, etc. These all fit in the amniote bucket
because they all share amniote characters. We can all agree that a
placental mammal and a varanid lizard are quite different from one another,
yet to remove either of these from the amniote bucket would suggest that
they are not amniotes or something different. In essence, it would make
amniotes an unnatural, arbitrary group because we removed a descedant that
shares all the characters of other amniotes.
It's a buckets in buckets idea, at least to me. You have the amniote
bucket. In that bucket you have the diapsid bucket, the synapsid bucket,
etc., and buckets within those, each holding various taxa that are united by
various shared characters (synapomorphies). Is a dinosaur an amniote,
diapsid, and archosaur? Yes to all three. Is a bird a dinosaur? Yes and an
archosaur, a diapsid, and so forth down the line. As Lance Grande has been
fond of saying, we are all fish.
So to answer George's question, a natural group, a bucket if you will, has
to include all the descendants because otherwise we are making an arbitrary
decision about what to exclude and what to keep. Although cladistics is far
from flawless, its emphasis on identifying natural groups helps to eliminate
some of the arbitrariness associated with previous phylogenetic frameworks.
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