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Apatosaurus: Inter-species discrepancies in the cervical vertebrae



A few years back I took an interest in the cervical
vertebrae of the various Apatosaurus species, since
then I had lost interest in the subject until I
decided to look back at the copies of literature and
figured cervicals given to me kindly by Wedel and
McIntosh. Unfortunately over the years I've forgotten
several of the different types of laminae and so I
have to use the abbreviated labels that I have access
to. This is going to be pretty long and consists of
loosely related questions and observations so I've
broken it up into three parts.

I) Classification of AMNH 460
II) Distinguishing between A. ajax and A. excelsus
(with an observation about YPM 1980)
III) Concept of cervical rib projections in
Apatosaurus


I) Classification of AMNH 460

In 2000-2001 I recieved copies of the figured
cervicals of CM 3018 (A. louisae) and CM 563 (A.
excelsus). In the marginal notes of each McIntosh
explains that the absence or presence of of a forward
expansion of the cervical rib as a characteristic used
in discriminating between A. louisae and A. excelsus
or A. ajax. In the marginal notes on a picture of the
AMNH "A. excelsus" mount (AMNH 460) he points out that
this specimen too lacks forward expansions on its
cervical ribs (a characteristic of A. louisae). In
Wedel's 2002 paper "Osteological correlates of
cervical musculature in Aves and Sauropoda, with
comments on the cervical ribs of Apatosaurus," Wedel
refrences AMNH 460 as A. louisae (however, also
questions the significance of the presence of anterior
processes on the cervical ribs in  distinguishing
species). Ignoring this characteristic does AMNH 460
demonstrate any other distinguishing A. louisae
characteristics such as the significantly more robust
neck and limbs than A. excelsus? One distinguishing
difference I can note in the cervicals of AMNH 460 and
CM 3018 is that the neural spines and postzygapophyses
of AMNH 460 are level in many of the cervicals while
are not in any of CM 3018's. 




II) Distinguishing between A. ajax and A. excelsus
(with an observation about YPM 1980)

In his paper "Remarks on the North American Sauropad
Apatosaurus Marsh" (1995) McIntosh implies that other
than the adult form of A. ajax being 10% larger than
A. excelsus that there really are not any
distinguishing characteristics that separate the two.
Looking at the figures of the cervicals of 2
distinctions I can make are that A. excelsus has a
much longer neural spine that would have made the neck
longer vertically than laterally, while in A. ajax the
neck seems as though it is longer laterally than
vertically. Also something I found odd is that in the
figured cervicals of the Gilmore Monograph of CM 563 a
lamina (labeled "IpodL") runs laterally from the
centrum to the posterior of the diapophysis. The
"IpodL" also splits into a second lamina labeled "AL"
that rises out from the "IpodL" and intersects the
lamina running from the diapophysis to the
postzygapophysis labeled "DL." I do not know what
"IpodL, AL," or "DL" stand for. However, in the
figured cervicals of CM 3018 the abbreviations "AL"
and "IpodL" are applied to the same lamina that
doesn't split as in CM 563. However, this split also
isn't present in cervical 8 of YPM 1980 (A. excelsus
type speciment) which is the only cervical I've seen
of that individual. This characteristic, as far as I
can tell is only in CM 563.



III) Concept of cervical rib projections in
Apatosaurus

I agree with Wedel in the idea that the presence or
absence of an anterior projection on the cervical ribs
of Apatosaurus as some of the posterior cervicals of
CM  3018 seem to show signs of weakly developed
anterior expansions, as does cervical 8 of YPM 1980.
However, in the anterior cervicals of CM 3018 (A.
louisae) there are not anterior projections of any
sort just "ventrolateral processes" which Wedel
distinguishes from the "ventromedial processes" of A.
excelsus. Now I'm hesitant to challenge this idea (as
I have no real knowledge of the neck mechanics of
birds which was compared to that of Apatosaurus in the
paper) however, to distinguish the two doesn't seem
appropriate to me. In cervicals 3 and 4 of CM 563
which have weakly developed forward expansions of the
cervical ribs the projections seem to be
ventrolateral. Also the parapophyses and the (forgive
me if I get this wrong but I'm having difficulty
reading the hand writing on the labeled picture)
tubercle fine? of A. louisae seem to extend more
laterally than those of CM 3018 which has a tubercle
that runs more-or-less vertically. The projection
seems to continue off of where the parapophyses and
the tubercle, whether it is ventrolateral or
ventromedial is dependent on the angle of the two
parts. Looking at the posterior most cervicals of CM
563 which have tubercles that run vertically, the
origin of the weakly developed exapnsion is more
ventromedial than ventrolateral. Also from what I can
tell, if the ventromedial process is treated as if it
were the origin for an anterior projection, then from
looking at the cervical of YPM 1980 and some from CM
563 it looks as though if one were to cut the anterior
projection back so that bone did not extend past the
tubercle-diapophysis junction, that the origin would
be positioned relatively ventrolaterally. 




                
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