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Re: Aquatic dinosaurs



In a message dated 96-06-26 11:34:17 EDT, Tompaleo@aol.com writes:

>Taking your logic to it's fullest then we could say that the
>cretaceous forms _Ichthyornis_ and _Hesperorinis_ to name a couple
>were aquatic dinosaurs since they were aquatic birds. But I think
>that what is really meant is that there are no known _nonavian_ ( in
>the cladistic sense) aquatic _ dinosaurs . Or something like that
>;-)

Although most taxonomists would currently take the position that only clades
may serve as valid taxa, and that when one clade is included in a larger
clade it is thereby made a subtaxon of the larger taxon, I have a broader
idea of what constitutes a taxon. For example, when you have a well defined
clade A and another well defined clade B that is included within A, you may
construct an equally well defined group A minus B: those organisms that are
in A but not in B. Occasionally it is very useful to give a formal name to
such a paraphyletic group and to treat it as a real taxon (I call such groups
"parataxa," short for "paraphyletic taxa," in _Mesozoic Meanderings_ #2).

In particular, the clade Dinosauria contains the clade Aves (as far as we
know), but since in paleontology we are more accustomed to dealing with
non-avian dinosaurs, we should consider attaching a formal name to the clade
Dinosauria minus Aves, if only to avoid repeating the cumbersome term
"non-avian dinosaurs." I'd like to suggest Eudinosauria ("good dinosaurs")
for the name of this parataxon.

Unfortunately, I use the name Aves for all dinosaurs closer to _Megalosaurus_
than to _Iguanodon_ (after all, the birds came first) and Avialae for the
clade consisting of the common ancestor of _Archaeopteryx_ and modern birds
(which most people call Aves). So I would use Eudinosauria for Dinosauria
minus Avialae. (The parataxon Dinosauria minus Aves in my system would be the
clade Phytodinosauria plus the common ancestor of both Phytodinosauria and
Aves: too much like Phytodinosauria to deserve a separate name of its own.)

What we really need is an organization similar to the ICZN (if not identical
with it) to codify and prioritize definitions of clades and so forth at
levels above those of the family, so we can dispense with this nonsense about
"my system" versus "their system," etc. As long as this situation persists,
it's difficult for me to harken to all the different definitions for all the
different dinosaurian clades and subclades that are being passed around these
days. This is why I ignore a lot of that stuff and simply create my own
terminology.

Also, the present ICZN code, as well as the revised one coming up for
approval, does not deal with families, genera, species, etc. as clades. If
taxonomists are _really serious_ about insisting that _only_ clades be
considered valid taxa, then I'd like to see some provision made for
invalidating (or revising) a family or genus that is found not to be a clade.
For example, in Tom Lehman's recent paper on the El Picacho ceratopian, his
cladogram contains two paragenera: _Chasmosaurus_ (ancestral to but not
including all the other ceratopine ceratopids) and _Torosaurus_ (ancestral to
but not including _Triceratops_).

A genus should be reserved for a clade of species that are all more closely
related to one another than to any other species. Likewise, a family should
be reserved for a clade of genera that are all more closely related to one
another than to any other genera. Again: I have no problem with paragenera
and parafamilies and would use them when appropriate, but such parataxa are
anathema to cladists.