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Re: Bakker's Brontosaurus and Late Cretaceous populations

The recent analysis of Upchurch et al. (2004) indicates that Apatosaurus
excelsus (the type species of Brontosaurus) is more closely related to A.
ajax (the type species of Apatosaurus) than either is to A. louisae.  So if
one were to split Apatosaurus and Brontosaurus, A. louisae would need its
own genus too.

The authors also found referred specimens of A. louisae did not cluster with
the holotype.  For instance, UWGM 15556 and the holotype of Elosaurus were
in a trichotomy with A. excelsus and A. ajax, and thus separated out as A.
parvus.  On the other hand, FMNH P25112 was found to be intermediate between
A. louisae and other species of Apatosaurus.  It may deserve its own
species.  So A. excelsus as usually conceptualized seems to be a
paraphyletic grade of specimens between A. louisae and A. ajax.

   |--A. louisae
   |  |--CMNH 3018
   |  `--CMNH 3378
   `--+--FMNH P25112
      `--+--A. excelsus (YPM 1980)
         |--A. parvus
         |  |--UWGM 15556
         |  `--CMNH 566
         `--A. ajax
            |--YPM 1840
            |--YPM 1860
            |--NSMT-PV 20375
            `--+--AMNH 460
               `--YPM 1861

Mickey Mortimer
Undergraduate, Earth and Space Sciences
University of Washington
The Theropod Database - http://students.washington.edu/eoraptor/Home.html