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RE: Discovery tonight ... cranial kinesis



Not only:

the mandibular symphysis of snakes isn't a synovial joint (with smooth
cartilage surfaces and a joint capsule and all that stuff). It's just two
bones and a ligament that can stretch, 
something synovial joints are practically incapable of. [David M]

But also, in most extant snakes ('Macrostomata' except for a few presumed
reversals in Colubroidea, and except for the fact that this 'clade' has now
been partly busted by molecular evidence), the ligament no longer connects
the two mandibles directly, but only each one to the much more stretchy
skin. If pythons still had a continuous intermandibular ligament it would
have to stretch to several times the length of the head. Unfortunately there
doesn't seem to be any way to tell which state occurred in fossils, dammit.

And as well, joints don't have to be synovial; the intermandibular joint is
present in many tetrapods and, like others such as the streptostylic
(supratemporal-quadrate) joint in snakes, there's no joint capsule. A
classic paper is Scapino (1965), The third joint of the canine jaw (J.
Morphol. 116:23-50) but I don't have access to a copy right now.
 

-----------------------------------------------
Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa
riversleigh@outbackatisa.com.au
http://tinyurl.com/f2rby
 
"Get this $%#@* python off me!", said Tom laocoonically.


-----Original Message-----
From: David Marjanovic [mailto:david.marjanovic@gmx.at] 
Sent: 08 December, 2009 8:43 AM
To: DML
Subject: Re: Discovery tonight ... 8 pm

>  (they made the ever-so-popular error of indicating that snakes
>  "unhinge" their jaws - and they were not talking about the mandibular
>  symphasis).

As Skitt's Law would have it, it's _symphysis_...

But to repeat your point, the mandibular symphysis of snakes isn't a 
synovial joint (with smooth cartilage surfaces and a joint capsule and 
all that stuff). It's just two bones and a ligament that can stretch, 
something synovial joints are practically incapable of.