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Re: Rigid dinosaurs
Sorry to come into this so late, as many good comments on this thread have
already been made!
Scott Hartman wrote:
I wasn't there for Horner's presentation on his "microscopic analysis of
the dinosaur's vertebrae", but I assume he was referring to the
interspinalis ligament? Having >dissected a few ratites, I was already
convinced by the gross morphological similarity, not to mention that it's a
level one EPB inference AND you can track the >character's homology through
the fossil record to early birds.
Just to ensure that everyone on the list knows: interspinous ligaments
DO NOT EQUAL nuchal ligaments -- these are not the same thing. They're not
even homologous structures, as far as I can tell -- nuchal ligaments appear
to be derived from supraspinous ligaments. Only SOME mammals have a true
nuchal ligament; no non-mammalian animal, including birds, has one. While
it is certainly not impossible that some extinct animals, such as non-avian
dinosaurs, evolved a nuchal ligament convergently, the absence of such a
structure in birds and crocs suggests that the null hypothesis should always
be that dinosaurs DO NOT have one; they cannot be assumed to have been
present a priori.
I'm sure the interspinalis ligaments made T. rex necks just as "stiff" as
it does modern theropods, like ostriches.
I'll certainly defer to Scott on the nature of the ostrich cervical
ligamenture, given that I've not had a similar opportunity to dissect one,
but my understanding of bird necks is that they lack interspinal ligaments
_sensu stricto_ -- they have elastic ligaments (Lig. elasticum interspinale)
instead. These are probably homologous, but differ in morphology from the
laminar ligaments one typically sees in mammals (and other things with tall,
mediolaterally flattened spinous processes). Ventrally, there are other
ligaments, too, such as the interlaminar ligaments. In tandem, like in
anything else, they restrict movement by maintaining articulation between
elements, but I suspect that non-avian theropods (like tyrannosaurs) that
had tall spinous processes and true, laminar interspinous ligaments (as
evidenced by the rugose, bony sheets on the cranial and caudal margins of
the spines) and lacked heterocoelous vertebrae were somewhat less flexible
than modern birds -- tyrannosaurs weren't twisting their necks into bizarre
curves the way that flamingoes (for example) do today (e.g.,
http://www.jpbutler.com/philadelphia/zoo-flamingoes.jpg), not that their
necks were long enough, anyway. Amazing what a pain in the patookus spinous
processes can be...
Of course tyrannosaurs have fewer cervicals than extant birds, and they are
less elongate, but really, this is stupid; the ligament was likely elastic,
as it is in extant >theropods, and was there to reduce the load the dorsal
axial musculature had to support when the head and neck were in a fairl
neutral pose. It wouldn't have >constrained T. rex neck mobility any more
than the osteological articular range of movement allows.
I concur 100%. Interspinal ligaments in tyrannosaurs (and other
theropods) were almost certainly predominantly made of elastin and would
have probably been capable of stretching quite a bit (probably even to the
point of osteological disarticulation, which was probably prevented by
other, less elastic ligaments) -- if nothing else, they'd _have_ to be
elastic to some degree in order to effectively recoil when the neck flexes
during running -- inelastic ligaments would tear under repeated such strain.
And now I can look forward to 5 more years of kids asking "Is it true that
T. rex was just a scavenger?"
Me too. Joy, oh joy...
Jerry D. Harris
Director of Paleontology
Dixie State College
225 South 700 East
St. George, UT 84770 USA
Phone: (435) 652-7758
Fax: (435) 656-4022
"Actually, it's a bacteria-run planet, but
mammals are better at public relations."
-- Dave Unwin