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Re: Alamosaurus as biggest North American sauropod



Matthew Martyniuk <martyniuk@gmail.com> wrote:

> I think in these situations the correct thing to do is designate a
> neotype. If T. rex is non-diagnostic relative new new taxa A and B, as
> long as both are from roughly the same stratigraphic horizon and
> geographic location, then pick one--it doesn't really matter which.
> This is what the ICZN recommends be done for all nomina dubia and I
> can't think of any better alternatives. However, I have seldom ever
> seen this applied, except in the case of Coelophysis bauri. It's often
> mis-applied, as in the case of Iguanodon, where rather than assign a
> neotype for the species I. anglicus, a new type species was chosen,
> probably unrelated to the original, and from a different geographic
> location and stratigraphic horizon.


The original type material of _Coelophysis bauri_ appears to have come
from a a different stratigraphic horizon to the current holotype
(neotype, the specimen that was originally the holotype of
_Rioarribasaurus colberti_).


The _Iguanodon_ decision was probably the correct one under the
circumstances, at least insofar as designating a new type species was
concerned.


> In related cases like Titanosaurus, the new, better specimens from the
> same horizon, if they could not be differentiated from T. indicus,
> should have been made the neotype of that species, rather than become
> the basis of a new taxon which is almost certainly but unprovably its
> synonym. If, later, this is contradicted by evidence that the original
> material WAS distinct, or not really referable to the neotype, well
> then so be it (IIRC this happened with Ceoplophysis as well). As least
> the name is no longer attached to crap material cluttering up
> diversity counts.


With _T. indicus_ regarded as a nomen dubium, the name _T. indicus_ is
effectively excluded from lists of taxa.  So under this view, the
'Sauropod bed' at Bara Simla has only one valid sauropod species:
_Jainosaurus septentrionalis_.


If _Jainosaurus septentrionalis_ is instead referred to _T. indicus_
then there is still only one valid sauropod species from the 'Sauropod
bed' at Bara Simla: _T. indicus_, with _J. septentrionalis_ (=
_Antarctosaurus septentrionalis_) considered a subjective junior
synonym.


The reason why this is so tendentious is that we want to avoid
chimeric taxa.  If a named species is actually made up of two or more
taxa, then it gives us a misleading operational taxonomic unit (OTU)
for phylogenetic analyses.  This is the fear that drives these kind of
taxonomic decisions - the possibility that an OTU contains characters
states from multiple taxa.  In the case of _T. indicus_/_A.
septentrionalis_ I don't think this is a problem because, as you say,
the two are likely (but unprovably) synonymous.  Plus, _T. indicus_ is
based only on a pair of caudals that are morphologically similar to
caudals referred to _A. septentrionalis_.






Cheers

Tim