[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: DinoMorph Strikes Back!... or does it? (long!)
Scott, Greg, and Kent make good points, about reasonable articulations and
actual articulations. I felt a few additional comments, some of what has been
said before, would be worth repeating.
Scott brings up how the WDC *Camarasaurus* is mounted with some large
disarticulations to conform to the horizontal model. I note that in the images
presented in this discussion, the ball-shaped anterior central condyle is
almost entirely removed from the cup-shaped cotyle of the preceding vertebra
when viewed laterally. This would increase the zygapophyseal disarticulation in
horizontal view. If we pulled the condyle into the cotyle, rather than using a
10% distance which dislocates the centra, the zygapophyses would become more in
alignment and the gap a thigh's width would diminish.
Kent brings up that wedge-shaped centra are not present in *Mamenchisaurus*
as GSP has reconstructed it, and not in *Camarasaurus* also so reconstructed.
To their benefit, I note that the upward orientation of the more anterior
cervicals from the base of the neck is largely provided by partial removal of
the condyle from the cotyle, and not always just the shape of the centrum
itself, which seems have the marginal rim of the condyle and the rium of the
cotyle largely parallel, but the dorsal orientation and the anterior position
of the prezygapophyses will effect this position as well; that is, the
relatively caudal position of the anterior zygapophyses will create an anterior
plane of the vertebra, rather than the centrum, that will affect the preceding
vertebra's orientation relative to it if the posterior zygapophyses are not
also caudally elongated to compensate.
GSP brings this latter point up, and argues that there is a movement to adopt
the neutral position, and I would argue ... why not? Why pull the vertebrae out
of typical alignment so as to effect a habitual posture in most skeletons, and
most life restorations performed? Perhaps such upward extensions were viable,
but I think not continuous, and the animals, for large portions of their life,
would have neutral cervical attitude, and as Kent brought up, the giraffe
itself does appear to possess a wedge-shaped basal cervicodorsal region that
contributes to the habitual elevation of the neck. I don't see why we should
argue that an animal will put its muscles and tendons under constant tension
and pressure during most of its life? A normalistic posture would be best
explained by a neutral neck position, and Kent has never said that any other
posture is not possible, just not likely to be habitual.
Scott considered the energy deficient model of a long neck sweeping back and
forth as less viable than a shorter neck and faster legs, which is reasonable.
However, other exapted reasons than diet may have contributed to neck length,
such as display, increase in mobility during rigidification of individual
cervical joints, increase in pneumatic diverticy perhaps for respiration, in
which airsac inflation or deflation would have contributed to neck movement
rather than just muscles as implied by Tsuihiji in his paper on the anatomy of
the rhea neck in last years' _JVP_.
Finally, GSP brings up the death-pose of the juvenile *Camarasaurus* at the
Carnegie: that the cervical regions seems intact and is in dorsal flexion shows
it is possible. However, death poses should never be used as models for
possible neck movement, as during rigor a bone can become disarticulated, not
to mention the burial process itself and geologic forces acting to distort the
skeleton as the sediments coalesce and thus contract. In fact the CM
*Camarasaurus* has the cervicodorsal region so strongly disarticulated that the
neural spines are abutting one another, which I think is highly unlikely. This
would argue, conta some claims, that the base of the neck was not so upwardly
inclined in life, but was much more moderately level in posture, even if not
vertical or 45 degrees out as GSP as restored *Mamenchisaurus* and *Euhelopus*.
Kent has drawn from the original plates and photos themselves, as well as
personal handling of many fossils, to pay special attention to the
cervicodorsal region. How should we dismiss this information to favor a theory
we hold near, the swan-necked or giraffe-necked sauropods? Old theories do not
neccessarily mean they are wise or logical, or based on fact, as the tail
dragging of dinosaurs and their sluggish physiognomy theories have been
Jaime A. Headden
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
Yahoo! DSL ? Something to write home about.
Just $16.99/mo. or less.