Re: polyphyodonty in dinosaurs

[znc14@TTACS.TTU.EDU]

        In response the query about whether dinosaurs ever developed
multi-cusped teeth for grinding:
        Well, the classic explanation I have heard is that diphodonty
(replacing your teeth only once) and tooth complexity are correlated. This
makes a little sense, anthropomorphically, in that no one would go
to the trouble to develop a complex cusp morphology and then throw it out
every couple months. Seriously, I am not well versed in the mammal side of
this issue, but the dinosaur side is relatively straightforward.
        There is one group of dinosaurs (at least) which developed a
complex "grinding" tooth structure: the hadrosaurs. Rather than developing
individual grinding teeth, however, hadrosaurs developed a complex dental
battery. The occlusal surface of this batter is an irregular surface of
long enamel ridges flanked by dentine surfaces. These ridges are formed by
the combined enamelled faces of the teeth, and were arranged
longitudinally, and were somewhat curved due to the curvature of the jaw
itself. The presence of enamel on only one side of the teeth (inner on the
lower jaws, outer on the upper) resulted in uneven wear which maintained a
slightly inclined occlusal surface, as well keeping the ridges raised in
order to provide an irregular grinding surface.
        Because hadrosaurs are polyphodont, this surface was continuously
renewed in the same location on the animal, rather than requiring the
sequential eruption of molars to provide a fresh grinding surface when the
old one is worn out. One consequence of this is that it permits hadrosaurs
to attain great body size without the necessity of fast growth (although
this is still possible), because, unlike an aged elephant, they are never 
left toothless and decrepid. 
        The design of the reptilian jaw imposed constraints which
favored a slightly different method of chewing in hadrosaurs than in
mammals. Mammals can acheive precise dental occlusion on only one side of
their jaws at a time (unless, like your truly, they have had extensive
dental work). This condition, termed isognathy, is correlated with the
solid cranium and mandible of mammals, and necessitates a more flexible
jaw articulation which allows transverse motion for effective chewing.
While the reptilian jaw is not adversely suited to propalinial
(for-and-aft) motion (indeed, I have read this is a common component of
tetrapod jaw mechanics, and Ostrom suggested it as a primary element of
mastication in hadrosaurs).
        However, instead of developing a strengthened
mandible capable of such traverse motion, hadrosaurs apparently developed
a kinetic skull which drew the quadrates (and therefore the jaw joint and
lower jaw) out and back while the maxilla rotated inward about its long
axis. This method provides a transverse component to chewing (of an
overall oblique occlusion) which drags the bolus over the ridged surfaces
of the tooth as it grinds it between the flat surfaces.
        Details of this system may be found in the many papers of
Weishampel and Ostrom on cranial kinesis in ornithopods.

        Hope this helps.

        Wagner