Re: Rigid dinosaurs

[Mike Taylor <mike@miketaylor.org.uk>]

> Date: Wed, 22 Feb 2006 15:08:40 -0700
> From: "Jerry D. Harris" <jharris@dixie.edu>
> 
>> 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  <mike@miketaylor.org.uk>  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.