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Re[2]: Lagosuchus



Am I trying to swim uphill, or what?  (That's not the right cliche, is 
it?)

To my query about _Lagosuchus_, Tom Holtz replied (4/18/96; 11:54a):

>_Lagosuchus_ (or _Marasuchus_) is not a dinosaur because the most
>parsimonious distribution of derived character states yields a
>position outside of the node joining birds and _Triceratops_ (or
>_Megalosaurus_ and _Iguanodon_).  It is very close to that group,
>but has not yet been demonstrated to lie within that clade (and
>hence is not a dinosaur).

This brings a couple of questions to mind, some of which are tangential 
to the original point, but may provide food for thought:

I assume _Lagosuchus_ has been renamed because the type material is 
non-diagnostic at the generic level.  I seem to recall that rediagnosis, 
accompanied by designation of new type specimens (there are typological 
terms for that) is allowable in taxonomy, No?  Why wasn't that done in 
this case?  Was the situation similar for _Anatosaurus_ (now 
_Anatotitan_) and _Coelophysis_ (now _Rioarribasaurus_), and maybe 
others?  

A partially parallel situation may be that many bryozoan genera were 
established years before bryozoans were studied in thin section.  The 
original names were often kept, however, with new diagnoses established 
on the basis of the new technique.  For example, the well-known genus 
_Constellaria_ was established in 1846.  The diagnosis appearing in the 
Treatise (Part G, 1953) was:  "Erect fronds with surface marked by 
depressed stellate clusters of mesopores."  This diagnosis applies to 
MANY currently recognized genera, although none of these could be 
identified by that diagnosis alone, yet the genus has been retained.  The 
diagnosis of _Constellaria_ appearing in the Treatise revision (Part G, 
Vol. 1, 1983) is:  "Zoarium encrusting, ramose, or frondose.  Monticles 
stellate to subcircular; primary plus secondary rays of zooecia flush or 
elevated; monticular center and interrays of vessicles depressed, flush, 
or elevated.  Autozooecia larger, with irregular polygonal cross section 
in exozone; generally isolated by vessicles in intermonticular areas; 
lunarium lacking but with some autozooecia with thicker proximal wall. 
Walls indistinctly and transversely laminated......"  I'm quitting here, 
but you get the idea.  This is about half of the current diagnosis.  You 
could not identify what we now understand _Constellaria_ to be, based 
upon the original diagnosis.  We now know that the oringinal diagnosis 
was not diagnostic, but the name was not suppressed, much to 
"everybody's" satisfaction.

I understand that replacing the name _Coelophysis_ by _Rioarribasaurus_ 
for virtually all specimens formerly referred to as _Coelophysis_, due to 
inadequacy of the type material(???) has been appealed to the ICZN.  I, 
for one, can certainly see why.  Changing the name has compounded a 
situation that didn't have to be (compounded).  Why not just say that 
since the original types and/or diagnoses were inadequate, here are some 
revisions?  When you follow cladistic rules to the letter, absurdities 
may result.  When you follow ICZN rules to the letter, absurdities may 
also result.  The rules may indeed ALLOW the renaming, but is that always 
WISE?

I am interested in the logic of definitions and diagnoses of these 
critters.  It initially strikes me as odd that a presumably REAL group of 
animals should be defined on the basis of a node in a cladogram, since 
cladograms AREN'T real things sitting out there in the real world.  I 
know that higher taxa are mental constructs also, but they are something 
fundamentally different than the graphic representations of their 
critical features.  I'm just not a good enough logician or philosopher of 
science to put my finger on what's wrong here.

It also interests me that this explanation seems to be correct in form 
and complete, yet includes almost nothing of use to me.  After reading 
this, I still don't KNOW why _Lagosuchus_ isn't a dinosaur.  I can parrot 
back that explanation, but I (ME, MYSELF, etc.) still don't think I KNOW 
why it's not a dinosaur.  Perhaps I'm just like a cave man trying to 
understand an explanation of television.

I have the list of characters published by Sereno et al. in the short 
paper in Nature (1993) on _Eoraptor_, so I guess I know the suite of 
derived character states that make something a dinosaur.  But I'm not 
sure where the concept of parsimony can be applied to that list.  I 
infer, therefore, that I need something else than just that list.  A 
cladogram, perhaps?  (I hope not, since cladograms are not aspects of 
animals.)  

Of course, we all know that each of these characters presumably evolved 
independently ("mosaic evolution"), so, if we had all of the fossils, 
there would indeed have to be just one character that would be critical 
for identifying a dinosaur.  We're getting off easy due to the 
incompleteness of the fossil record.  Why don't we just admit it, and 
decide right now what character it should be?  Then my question would 
have the answer I'm looking for.  We wouldn't have to keep revising the 
list of animals that qualify to be dinosaurs, or redefine the nodes 
because we have already decided on some other basis what we want to be a 
dinosaur.

What if nature is not parsimonious?  

I'm rambling.


*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu