Re: Discovery tonight ... 8 pm

[Mike Habib <habib@jhmi.edu>]

On Dec 7, 2009, at 2:17 PM, Michael Erickson wrote:

> I don't know what this tyrannosaur cranial kinesis hating is really  
> all about, maybe that one Witmer paper? But I still beleive that it  
> occurred. Apparrently so does Pete Larson. One single little paper  
> (yes, I've read it top to bottom, and I do find it rather poor, but  
> that's another story) isn't going to convince me that all of the  
> previous work done by too many workers to count since the early  
> 1900s is completely wrong. Now you can attack me for this. And then  
> attack me for expecting to be attacked. (For the record, I'm only  
> half-joking.)

Part of the concern with that particular sequence in the show is that  
the kinesis that was shown involves a full disarticulation of the jaw  
joint.  There are a number of issues with this, and it seems to have  
its origins (as a common speculation) in the rampant misconception  
that snakes do something like that.  In reality, the only squamate  
joint that "disarticulates" (though not really) is the contact between  
the two mandibles - they don't have a fused symphasis, and the  
ligament there is elastic, so the bones do spread apart.  The  
articulation of the lower jaw with the skull proper *looks* like it  
separates in snakes, but that's actually a widely swinging quadrate,  
that effectively produces a two-joint system for depression of the  
lower jaw.  The bones do not lose contact.

This is a common theme: in animals with kinetic skulls (scleroglossans  
- snakes included, birds, teleost fish, etc) the motion occurs  
primarily because joints are mobile and/or numerous - not so much  
because the joints come apart.  Snakes have hyper-kinetic skulls  
because they have a whole lotta joints in there.  If we are to suppose  
a kinetic skull for tyrannosaurids, we need to find some joints.

So, in short, if we are to suppose kinesis in the tyrannosaurid skull,  
we have to ask: where does it happen?  There is an intramandibular  
joint in some theropods that can add kinetic aspects, but it is  
apparently not very mobile in tyrannosaurids.  Same goes for skull  
roof, palatal elements, etc.  In really kinetic skulls, the elements  
essentially hang from the braincase - but tyrannosaurs are basically  
the opposite.  That leaves a lot of skepticism regarding the ability  
of the tyrannosaur skull to undergo kinetic deformations, because  
there just aren't any joints for it to happen at.

Cheers,

--Mike H.


Michael Habib
Assistant Professor of Biology
Chatham University
Woodland Road, Pittsburgh PA  15232
Buhl Hall, Room 226A
mhabib@chatham.edu
(443) 280-0181