Re: Apatosaurus vs Brontosaurus

[Tim Williams <twilliams_alpha@hotmail.com>]

I would add that the taxonomy of the three _Apatosaurus_ species (_ajax_, 
_excelsus_, louisae_) is currently unresolved.

In the early 1990's, McIntosh distinguished _ajax_ from _excelsus_ based on 
overall size: _ajax_ has a larger adult body size than _excelsus_.  However, 
McIntosh also points out that, apart from "body size", the two species are 
indistinguishable based upon the described material.  Rather than splitting 
_ajax_ and _excelsus_ into two separate genera and thus resurrecting the 
genus _Brontosaurus_(which has _excelsus_ as its type species), it is more 
likely that the two would be combined into one species.

The third species, _A. louisae_ (for which a skull is known), was previously 
distinguished from the other two species based on vertebral characters 
(forward expansion on the cervical ribs in _ajax_ and _excelsus_ and the 
lack of this expansion in _ louisae_ ).  However, the diagnostic nature of 
this character has recently been called into question...

http://www.cmnh.org/dinoarch/2003May/msg00250.html
http://www.cmnh.org/dinoarch/2003Apr/msg00091.html

Wedel, M.J. and Sanders, R.K. (2002). Osteological correlates of cervical 
musculature in Aves and Sauropoda (Dinosauria: Saurischia), with comments on 
the cervical ribs of _Apatosaurus_.  PaleoBios 22(3): 1-6.

Thus, it may be that all three _Apatosaurus_ species could be combined into 
one species: _A. ajax_, with _excelsus_ and _louisae_ as junior synonyms.  
At the moment, _A. yahnapin looks safe in its own genus (_Eobrontosaurus_).  
As for _A. minimus_ (also from the Morrison Formation)... it as been mooted 
as a possible titanosaur based on the presence of six sacrals and the shape 
of the ilium.  Unless previous descriptions of _A. minimus_ are seriously 
awry, this species clearly does not belong in _Apatosaurus_.


Tim

_________________________________________________________________
Best Restaurant Giveaway Ever! Vote for your favorites for a chance to win 
$1 million! http://local.msn.com/special/giveaway.asp