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Cooper's chance for redemption- pachycephalosaur hearing

Sure, Cooper's taxonomic thinking and comprehension of the literature may be
lacking, but that's only half the thesis.  The other half involves her
thoughts on pachycephalosaur hearing as it relates to their cranial
structure.  Are the ideas set forth in this section good enough to let one
ignore the rest?  Do they show knowledge of amniote auditory systems,
logical correlations between osteology and hearing, and considerations of
pachycephalosaur anatomy?


No, they do not.

This was more difficult to evaluate than the last section, since I'm more
familiar with taxonomy than auditory systems.  If I made any errors, feel
free to point them out.

"The evolution of the otic system from the synapsid to the mammal, there is
a simple prootic; then the prootic fused to the opisthotic; then these two
structures occur along with the crista prootica (Palaeos, 2004). This triple
structure forms the paroccipital process, all with the function of
protecting the otic capsule and acting as signal transducer."

The prootic-opisthotic in mammals doesn't act as a signal transducer.
Instead it and the ectotympanic contain the tissues and elements which do
(e.g. incus, stapes, malleus, cochlea).

Cooper makes note of the fused prootic and opisthotic in Stygimoloch, after
the quote above.  Prootic-opisthotic fusion is common in dinosaurs, being
found in Tidwell and Carpenter's (2003) titanosauriform, Kerberosaurus,
Charonosaurus, Euoplocephalus, Irritator, Giganotosaurus and Citipati among
taxa for which I could search for the word "prootic" in their descriptive

"The stapes in synapsids is used in suspension of the jaw, and if used in
signal transduction, it must be by directly coupling surface vibration via
the mandible."

The stapes in synapsids is not used in jaw suspension, being a tiny element
between the incus and fenestra ovalis in mammals, and the primitive strut
between the tympanum and fenestra ovalis in more basal taxa (as in
dinosaurs).  It is used in signal transduction by transfering vibrations
from the incus to the cochlea (or tympanum to the cochlea), not by coupling
surface vibration via the mandible.

Cooper states more derived taxa with larger domes and nodes have larger and
better developed otic systems, but did not demonstrate what
prootic-opisthotic size has to do with auditory abilities, and never
describes any other way the otic system is more developed in Stygimoloch
than basal pachycephalosaurids.  She measured the area of external surface,
which isn't necessarily correlated with anything that might involve hearing-
fenestra ovalis size, stapes morphology, lagena morphology, etc.. Indeed,
none of the latter were mentioned at all.

"Crocodilians use the jaw, fused to the prootic, as a sound transducer
(Palaeos, 2004). ... The more primitive otic system in pachycephalosaurs
needed to use bony sound transducers much like the crocodilian. I will argue
that lacking the sophisticated enclosure of stapes and tympanic membrane,
pachycephalosaur grew vascularized, fibrolamellar bone to act as sound

Crocodilians use their mandible as a transducer, but only when in contact
with the ground or water, since sound waves are terrible at vibrating large
structures like mandibles when they come through the air. Pachycephalosaurs,
with their tiny arms, would not be able to stick their heads on the ground
whenever they needed to hear (nor were they aquatic). In fact, if
pachycephalosaurs wanted to increase their directional hearing, it would be
a bad idea to have transducing braincase elements. Thewissen describes how
directional hearing and bone conduction are incompatable, and how
terrestrial mammals strive to acoustically isolate the head.
So there goes the major part of Cooper's hypothesis.

"Pachycephalosaurs processed sound waves by means of bony skull structures."

Yes, the stapes.

"Lacking a tympanic membrane, the pre-mammalian hearing system utilized the
bony skull structures as a resonator. I extant alligators, the prootic is
fused to the jaw, which serves as the resonator."

The tympanic membrane is not a mammalian invention. Most reptiles have it,
including alligators. Indeed, they use it and their stapes to hear airborne

"The pachycephalosaur frontoparietal dome has been the subject of
histological analysis (Goodwin et al., 1998). This analysis concluded that
bone in this portion of the skull is highly vascularized. The fibrolamellar
bone radiates from the roof of the dome and perpendicular to the dome
surface. This arrangement of bone lamellae suggests that forces applied to
the dome would be directed toward the brain (Goodwin et al., 1998).
Histological evidence appears to support the notion that the dome serves as
a resonator."

Well, there's a non sequitur. Vascularization helps resonance how? Even
assuming the physical force discussed by Goodwin was analogous to sound
waves in the way the dome directs them, what good would it do to send them
to the brain?  The brain can't do a thing with sound waves. If it directed
them to the lagena, she might have a point...

"Barn owls acheive greater accuracy in echolocation by means of asymmetrical
left/right ears. ... I therefore placed left lateral versus right lateral
regions of interest in an overlay and found Stygimoloch to have an
asymmetrical ear design."

Unless it's postmortem distortion.

The only novel part of Cooper's thesis which seems possible is her theory
the dome and nodes act to delay sound waves getting from one side of the
head to the other, and thus aid in directional hearing.

Mickey Mortimer
Undergraduate, Earth and Space Sciences
University of Washington
The Theropod Database - http://students.washington.edu/eoraptor/Home.html