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Coelophysis/Rioarribasaurus



endocrin@desktop.com.au (Graeme Worth) wrote:

<<Every time there is a new posting on this subject everything seems to get
even more confused. Here is my understanding - please tell me if I am wrong
and why (no opinions, please - just facts and refs if necessary!!)
Re-examination of Coelophysis indicated that the type specimen was
undiagnostic.>>

So far, so good.

<<In a letter to Bull Soc Zool Nomenclature mid 1995 a formal
proposal was put to retain Coleophysis for the type material and re-name all
other referred Coelophysis material as Rioarribasaurus.>>

Not so. The genus _Rioarribasaurus_ was created in 1991 by Adrian Hunt and
Spencer Lucas, who challenged Colbert's identification of the Ghost Ranch
theropods as _Coelophysis_ on the grounds that _Coelophysis_ was based on
nondiagnostic material. They referred all the Ghost Ranch theropods to their
genus. ICZN had nothing to do with this. Hunt & Lucas's paper, however,
inspired Colbert and a group of other paleontologists to petition the ICZN to
remove the name _Coelophysis bauri_ from Cope's original material and attach
it to the type specimen of _Rioarribasaurus colberti_. (This--changing a type
specimen--cannot be done except by directly petitioning the commission.) They
thereby would sink _Rioarribasaurus colberti_ as a junior objective synonym
of _Coelophysis bauri_, since the names would be based on the same type
specimen.

<<10-15 submissions followed from various authors, about 50-50 in favour of
the proposal or a counter-proposal to retain Coelophysis. The last time I
checked through BSZN in Feb this year the ICZN committee had not yet
published a decision.
Subsequently it has been suggested on the basis of obdurator foramen
differences that there are probably 3 genera of small theropods at Ghost
Ranch (Coelophysis, Rioarribasaurus and Syntarsus).>>

As far as I know, _Coelophysis_ itself has not been identified at Ghost Ranch
(except by Colbert and those who agree with him, of course). But some new
small-theropod material has been found in the same strata whence Baldwin
unearthed the _Coelophysis bauri_ type material, and it is apparently _true_
_Coelophysis_ _and_ different from _Rioarribasaurus_. Bob Sullivan is working
on a new diagnosis for _Coelophysis_ that distinguishes it from the Ghost
Ranch theropods. He has also found evidence of _Syntarsus_ at Ghost Ranch
(meaning there is more than one genus of small theropods among the
skeletons).

<<I think the suggestion was that the Coelophysis referred to here was not
the type material but other referred Coelophysis that was different from
Rioarribasaurus.
If the above is correct, a number of possible outcomes result.
As the original proposal was to re-name referred Coelophysis material as
Rioarribasaurus, and if no decision has yet been made, use of
Rioarribasaurus as a generic name in general discussion would seem to be
jumping the gun, to put it mildly!>>

Presently, we are free to use either _Coelophysis_ or _Rioarribasaurus_ for
the Ghost Ranch theropods. I prefer the latter, since it is based on a
splendid skeleton rather than scrappy bone, and because it unambiguously
refers to the Ghost Ranch theropods. Please note that the genus _Coelophysis_
is _not_ being "renamed" _Rioarribasaurus_. It is the Ghost Ranch theropods
whose name is being changed from _Coelophysis_ to _Rioarribasaurus_.

<<If the obdurator foramen differences are real and Rioarribasaurus and
Coelophysis are different, then the usage of Rioarribasaurus is different
from that of the original proposal. In this case, if the initial objection to
the use of Coelophysis for other than the non-diagnostic type material was
valid, that situation still maintains and we need yet another name for the
referred "Coelophysis-but-not-Rioarribasaurus" material (and Coelophysis is a
nomen dubium).>>

No, this is exactly the usage intended by Hunt & Lucas. _Rioarribasaurus_ is
the Ghost Ranch theropod, unambiguously.

<<The question of usage of Rioarribasaurus ahead of other Coelophysis
synonyms
(? Longicollis ? - don't have the reference handy) also needs to be
addressed.>>

Hunt & Lucas also sank _Longosaurus longicollis_ as based on nondiagnostic
material. Welles, in coining _Longosaurus_, intended to use that genus for
the type material of _Coelophysis longicollis_, considering it a generically
different species from _Coelophysis bauri_. A mixup in specimen numbers
caused Welles to make referred material of _Coelophysis longicollis_ the type
of _Longosaurus longicollis_, not the actual type specimen. Thus we now have
_two_ distinct species names, _both_ based on nondiagnostic material (as Hunt
& Lucas pointed out): _Coelophysis longicollis_ and _Longosaurus
longicollis_(!). It is quite possible that they are both synonyms of
_Coelophysis bauri_ (but not if you ask Welles!). It is less likely that they
are synonyms of _Rioarribasaurus colberti_.

<<I presume that this whole thing came up because of a rule somewhere that
says a type specimen can not be re-named? Given the enormous amount of
Coelophysis material, the simplest solution otherwise would surely have been
to re-name the undiagnostic type and keep Coelophysis for the rest??!>>

That's the "lazy man's way out." It destroys the historical context of the
name _Coelophysis_ and legitimates an unfortunate nomenclatural error of some
44 years' standing. Plus it could add to the confusion if, were the ICZN to
shift the type specimen of _Coelophysis bauri_ to the type specimen of
_Rioarribasaurus colberti_, someone reexamined the original (and thus
unnamed) Cope _Coelophysis_ type specimen, found it to be diagnostically
different from the Ghost Ranch theropods, and gave it a new generic and
specific name, neither _Coelophysis bauri_ nor _Rioarribasaurus colberti_.

Of course, the ICZN can do whatever it wants in this situation. That's what
"plenary powers" mean. If they want to shift the name _Coelophysis bauri_
from Cope's original type specimen onto the type specimen of _Rioarribasaurus
colberti_, that's their prerogative.

<<If anyone can provide the definitive explanation for all this (if there is
one), I for one would be extremely grateful.>>

Check out my article, "Little Dinosaur, Big Controversy" on the Skullduggery
home page. There are illustrations by Tracy Ford of all the various type
specimens, too.