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Re: Quick cladistics question

In a message dated 97-03-30 20:02:38 EST, jpoling@dinosauria.com (Jeff
Poling) writes:

<< Dinosauria is defined as the most recent common ancestor of _Triceratops_
 and modern birds.
    What happens if birds were to, for example, be found to have evolved from
 a crocodile?  Would clade Dinosauria be moved down the line to be
 synonomized with what appears as the clade uniting ornithodira and
 crurotarsi in Weishampel's book?  Would the Dinosauria be redifined?  Would
 it be dumped as a polyphyletic group? >>

If the phylogenetic definition given above of Dinosauria is accepted, then it
will always stand as a monophyletic group, by definition. Only the contents
and diagnosis may change as new information becomes available. If, for
example, crocs were found to be more closely related to birds than, say,
theropod dinosaurs are, then crocs and at least some croc relatives would
have to be moved into Dinosauria. Perhaps Archosauria might become a junior
synonym of Dinosauria.

>Defining< Dinosauria as above >makes< birds dinosaurs, period. The question
of whether birds and dinosaurs are related thus goes beyond debate, and the
central question becomes, "With birds and _Triceratops_ dinosaurs by fiat,
what other archosaurs must fall into the clade Dinosauria so defined?"

In a message dated 97-03-30 20:57:47 EST, jwoolf@erinet.com (Jonathon Woolf)

<< Is this actually the way the Dinosauria is defined, cladistically?  That
 looks like a crown-based definition (I think that's the right term),
 which makes no sense for a group like Ornithischia which exists only as
 fossils.  I would have thought that Dinosauria was defined as the first
 animal to have the identifying synapomorphies for Dinosauria (whatever
 those are) and all its descendants.   >>

Cladists are presently trying to move definitions of taxonomic groups away
from characters ("character-based" definitions) and toward definitions based
on relationship ("node-based" and "stem-based" definitions). Key characters
may change with the acquisition of new information and the discovery of new
fossils, whereas relationships are more stable against such discoveries. This
is, amazingly enough, not an entirely unhealthy situation, provided the
definitions are constructed with some regard for well-established groups and
with common sense.