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Re: Rigid dinosaurs
> Date: Wed, 22 Feb 2006 15:08:40 -0700
> From: "Jerry D. Harris" <email@example.com>
>> 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.
OK: "reaches to the skull or at least the axis" is about as good a
functional definition as I've seen for ligaments' qualification for
the adjective "nuchal". (From what I've seen in the speculative
sauropods-with-nuchal-ligaments literature, the term seems to be used
to indicate a single continuous ligament as distinct from a series of
short ligaments that span only one, two or three segments each.)
> 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.
(You should _so_ publish on this :-)
> 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.
Well, what else _is_ there? I don't see how a ligament embedded in
the cervical soft tissue can be a sexually selected (which is of
course the usual get-out clause for Inexplicable Mystery Morphology).
> 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.
Yup. It's a shame you didn't directly discuss ligaments in that
paper, then we wouldn't need to be having this conversation :-)
> Language is a stupid form of communication.
Yes indeed. Unfortunately, it seems to be the best we have.
/o ) \/ Mike Taylor <firstname.lastname@example.org> http://www.miketaylor.org.uk
)_v__/\ "Arboreality does not correlate with pelvis shape" is an
interesting conclusion, but it's hard to get a paper of more
than seven words out of it.