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© 2010 Dinosauria International Ten Sleep Report Series No. 1
in part Senter’s (2007) discussion of neck elongation in
sauropods, which may have been driven by sexual selection,
and not by feeding competition. The height, bifurcation of
neural spines, and ventral width of centra between males
and females is another sexual dimorphic feature seen in the
cervical vertebrae. The narrower width of the neural spines
on the cervical vertebrae in DQ-SB, DQ-BS, and DQ-TY are
quite slender in comparison to DQ-PE. In other words, the
width of the cervical across the ribs in Prince allows it to
fit over the slender female necks. Moreover, a broadened
neck may also have played a role in courtship display and
in male rivalry. The courtship displays may have had a
cobra-like effect in their swaying heads and necks helping
to appear much larger. Intraspecific combat, territorial
defense, and dominance could have been achieved among
males by pushing or slamming with their necks, in the
same way male giraffes compete with each other using their
necks and heads. The small size or absence of pleurocoel
fossae in much of the vertebrae, particularly in the anterior
caudals, may be a developmental male response to acquire
the necessary weight needed for dominance in rivalry “push
off’ battles for example. Many of the features regarded as
gender specific can also be seen in young individuals. It is
possible that the extreme morphological features observed
in males are retained neotenic traits. For example, the long
cervical ribs, short axial skeleton, and absent pleurocoels
are neotenic traits that can be seen in SMA 0009 from Howe
Quarry. All these features also recall the male characteristics
we see in the Dana sample.
Morrison diplodocids may have occupied a
proboscidean-like niche with a gregarious population of
males and females analogous to African elephant herds.
We speculate that Apatosaurus males with larger and more
massive bodies would have powerful broad necks for
courtship displays and subduing females. African elephants
vary greatly, but males are generally much more massive than
same age adult females. In addition to overall greater size and
stature, male elephants may be twice the weight of females.
Principle courtship display in male elephants is their larger
tusks. Amphicoelias females may have formed herds to help
rear calves, which were led by a dominating harem keeping
male. Males may have lived a solitary existence away from
the herd appearing only during courtship. This may explain
why, in the best fossil sites, Amphicoelias/Diplodocus is found
in multiples while Apatosaurus, only as occasional isolated
occurrences. One notable exception is in Dinosaur National
Monument where three Apatosaurus specimens have been
reported, but these individuals may have been washed in
from further upstream.
The morphological distances and occurrence
between Camarasaurus and Brachiosaurus are similar in many
ways to what exists in the Dana sample, thus the possible
need to reconsider them as possible gender based taxa.
Ontogeny
Great morphological variation exists in one
documented Saurischian population, the mass mortality
collection of Coelophysis, from Ghost Ranch New Mexico
(Colbert 1989). Colbert (1989) reports on the proportions
of limb to cervical lengths, in Coelophysis, which can vary
greatly, as much as 20% or more, demonstrating that axial
vs. appendicular development is not equally synchronized.
As exemplified in two immature specimens, DQ-TY and
SMA 0009, the same ontogenetic condition appears to exist
in both Amphicoelias and Coelophysis. The recognition of
ontogenetic features is of great taxonomic importance not
only for diplodocids but other sauropods. In addition to
sexual dimorphism and individual variation, ontogeny helps
to indentify character polarity in a clear biological manner.
Fig 32 Comparison of illia belonging to DQ-TY and SMA
0009 illustrating the elongated pubic peduncle in juveniles of
Amphicoelias. Bottom row: adult Apatosaurus and Diplodocus illia for
comparison. See also Fig 7C, DQ-SB. (Illustrations: Schwarz 2007,
Hatcher 1903)
44
The baby sauropod skeleton “Toni”, described by
Schwarz, et al (2007) (SMA 0009, Howe Stephens Quarry,
Shell, Wyoming), belongs in the genus Amphicoelias and
probably represents A. “brontodiplodocus”. Toni’s clavicle is
the same boomerang-like shape as in DQ-SB and DQ-TY.
Compared with the fragment described by Harris (2007)
as a possible clavicle belonging to Suuwassea, the Dana
Quarry clavicles are more bent and without rugosities on