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Re: Rigid dinosaurs
What about _Rhea_, as figured and discussed by Tsuihiji 2004? Are you
saying that you don't consider that to be "nuchal" because it's
derived from the Lig. elasticum interspinale rather than
Lig. elasticum supraspinale?
_Rhea_'s an odd duck (ahem). Certainly, it's elastic ligament system
comes as close to being a mammal-style nuchal ligament as any bird has
gotten, but mammalian nuchal ligaments connect to the axis (in dogs) or the
occipital surface of the skull (everything else that has one), whereas the
structure in _Rhea_ doesn't seem to make it cranial to Cv8. Even Tsuihiji
doesn't call it a nuchal ligament; he only uses the term in quotes to refer
to the sheath that surrounds this peculiar elastic ligament system.
Moreover, the system in _Rhea_ appears to be singular; in most* mammals, the
funicular nuchal ligament is paired (although the two parts often fuse)
(*the situation has been reported both ways in dogs). Yes, this doesn't do
all that much for the supraspinous ligament derivation hypothesis, but then
again, I've not read any detailed histological treatments of the
supraspinous ligament to see if it is also paired.
Homologous? No. Functionally similar? At least partly, although it's
peculiar why no other bird seems to need one, including other ratites, which
leads me to suspect that its evolution in _Rhea_ was driven by something
other than functional pressures, although I haven't a clue what they were.
An argument could be made to give them the same name ("nuchal ligament")
despite evident analogy, rather than homology, similar to the situations
with the "patellae" of birds and mammals and other cases I discussed in my
paper in _Anatomical Record A_, but since they only partly occupy the same
physiographic space, this may not be the best idea.
Of course, "nuchal" just means "neck," and in a purely descriptive
sense, _any_ neck ligaments could be called "nuchal ligaments." But I
rarely see the term used in this way; pretty much all readings seem to come
with the underlying assumption that they're talking about the mammal-style
thing, or at least something identical in terms of structure, origin and
insertion points, elastin content, and function.
Language is a stupid form of communication.
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