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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
Science Building
225 South 700 East
St. George, UT  84770   USA
Phone: (435) 652-7758
Fax: (435) 656-4022
E-mail: jharris@dixie.edu
and     dinogami@gmail.com
http://cactus.dixie.edu/jharris/

"Actually, it's a bacteria-run planet, but
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