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Where to draw the line.

I've beaten this topic to death a million and a half
times on my own and to others, but new things keep
coming to my attention.

I've been comparing the three _Apatosaruus_ species
for about two years now (comparing their cervical
vertebrae). I do not compare their cervicals by
choice, it's just that I am sixteen, I do not attend a
university (and therefore it is more-or-less
impossible for me to find the exact paper(s) I'd need
to  finally solve everything out for myself. I compare
their cervicals only because John S. McIntosh was kind
enough to send me illustrations of selected cervicals
from the different species (they are all I have to go
by visually).

Illustrations I have are of

*_A. louisae_ (cervicals 1- 15)
*_A. excelsus_ mounted in Wyoming (cervicals 3-5,
7-10, 13, 15)
*Original _A. excelsus_ "Brontosaurus" (cervical 5?)
*YPM _A. Ajax_, not type specimen, (cervicals 9, 10)

Now, to get to the point, I've been trying to figure
out what the traits are that define _A. louisae_, _A.
excelsus_, and _A. ajax_. I originally contacted
Dr.(?) McIntosh pointing out differences in the
cervicals that I thought were odd (I was going by
mounts back then). McIntosh then sent me the
illustrations to show me the differences in cervical
structure from species to species. Since then I have
convinced myself that something is odd about those
illustrations. This is my most recent dillema...

     Dr. McIntosh pointed out to me that the presence
of a forward expansion on the cervical ribs in _A.
excelsus_ and the lack of the expansion in _A.
louisae_ is a main characteristic that separates the
two species. Since then I have become convinced that
this can no longer be used as a trait to separate the
two species. Mainly because in the YPM _A. excelsus_'s
cervicals some lack the forward expanion too. Dr.
McIntosh suggested that perhaps the cervical is
incomplete, however there is no sign of a break in the
bone. This leads me to believe the anterior expansion
is something that grows in over time. _A louisae_ in
some of its posterior cervicals shows signs of early
growth of an anterior expansion.

   This is where my last idea left off. Since then
I've been trying to find other differences in the
cervical vertebrae that could be used to separate the
species. However I have not found one difference that
couldn't be argued as being a gender, age, or
indivudual difference. The main difference I noted
(comparing the Wyoming _A. excelsus_ and Carnegie's
_A. louisae_) is that the individual postzygapophyses
of _A. excelsus_ are longer, not as thick , and hang
out over the posterior of the centrum quite a bit.
Cervical four stands alone as being the only cervical
with  shorter, thicker postzygapophyses, though it
still is extends past the posterior of the centrum. 
While in _A. louisae_ all postzygapophyses are
shorter,  thicker, and in most cervicals do not hang
out over the posterior of the centrum.

    I regarded this as a valid difference for awhile,
Until I looked back at the cervical of the Yale
specimen of _A. excelsus_. The figured cervical, I'm
assuming it is cervical 5 judging on its proportions
(it looks really small and cervicals 1-4 are missing)
contains  shorter, thicker postzygapophyses that do
not extend out over the posterior of the centrum. The
cervical also lacks the anterior expansion of the
cervical rib (all these characteristics are closer to
those in _A. louisae_ then those in the Wyoming _A.
excelsus_). However Dr. McIntosh has told me that some
of the unfigured cervicals of the Yale specimen do
contain the forward expansion (I do not know how the
postzygapophyses on the other cerivicals look for I
have never seen any of the other cervicals).

    In conclusion, every difference I have noted
between _A. louisae_ and _A. excelsus_ could be
regarded as just individual, gender, or age related
differences. Where do you draw the line between what
is a difference amongst species and what is a
difference amongst individuals? Now this is just going
by cervicals (I wish I could see the rest of the
skeletons) and I know that there is much more to each
animal than cervicals. Like I said I asked Mr.
McIntosh about the differences and he sent me the
illustrations but now looking at those, I do regard
any difference I spot between _A. louisae_ and _A.
excelsus_ as a valid means of separating the two.

What are the characteristics that paleontologists look
for to determine whether _Apatosaurus) material is
that of _A. excelsus_, _A. ajax_ or _A. louisae_?
(cervical differences or otherwise).

                                       -Michael Lima 

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