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Re: Alamosaurus as biggest North American sauropod



Matthew Martyniuk <martyniuk@gmail.com> wrote:

> I don't see the issue with using parsimony. In fact I have kind of an
> issue when people don't use it. If the remains can't be differentiated
> from _A. sanjuanenis_, and there is no evidence to suggest multiple
> taxa were present in this environment, it should be considered _A.
> sanjuanenis_ until evidence to the contrary can be produced.


As a rule of thumb, this "rule" is unevenly applied.  For example,
Carpenter and Tidwell (2004) referred all sauropod material from the
Arundel Formation to a single taxon.  All the Arundel sauropod
material was consistent with the presence of a single species, which
took the name _Astrodon johnsoni_.  In the process, _Pleurocoelus
nanus_ and _P. altus_ were considered to be subjective junior synonyms
of _Astrodon johnsoni_, a taxon that was originally established on
material that is not diagnostic at the genus level (tooth + tooth
fragment).


On the other hand, Wilson and Upchurch (2003) did *not* combine
_Titanosaurus indicus_ and _Antarctosaurus septentrionalis_ into a
single taxon in their revision of the titanosaur material from the
'Sauropod bed' of the Lameta Formation at Bara Simla.  This was
despite the fact that this study found no reason to recognize more
than one species at Bara Simla.  The Conclusion makes this clear:


   "The discussion above, however, demonstrates that
    Huene & Matley’s (1933) decision to separate the
    ‘Sauropod bed’ specimens into several individuals
    from two taxa is not supported by the available
    evidence. It is possible that only one individual was
    present, as originally suggested by Matley (1921).
    Even if more than one individual is represented,
    two or more taxa cannot yet be reliably distinguished.
    However, although morphological and geological data
    do not argue against the presence of a single species
    at Bara Simla, there is simply no positive evidence
    supporting this hypothesis."


Because _T. indicus_ was based on material (two caudal vertebrae) that
Wilson and Upchurch (2003) found to be non-diagnostic at the genus
level, these authors declared _T. indicus_ to be a nomen dubium.  _A.
septentrionalis_ had been previously assigned its own genus
(_Jainosaurus_; Hunt et al., 1994), which was the name adopted by
later studies for the "valid" titanosaur from Bara Simla (e.g., Wilson
et al., 2009, 2011 - the latter study assigned additional titanosaur
material from a separate site [Chhota Simla] to _J. septentrionalis_
).  However, I'm confused as to why all the titanosaur material from
Bara Sima wasn't simply assigned to a single taxon that carried the
name _T. indicus_, with _Jainosaurus septentrionalis_
(=_Antarctosaurus septentrionalis_) as a junior synonym.







Cheers

Tim