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