Re: polyphyodonty in dinosaurs

["Jaime A. Headden" <qilongia@yahoo.com>]

Ken Kinman wrote:

<An on-topic and interesting question would be whether plant-eating 
dinosaurs ever develop heterodont teeth with mammal-like "molars" for
grinding. If they do, I assume they do not develop the same precise
kind of occlusion, since dinosaur identification is rarely, if ever,
based on dental characteristics (or am I wrong on that?).>

  Grinding teeth in mammals typically develop from bunodont teeth, and
mammal teeth themselves are specialized in their transverse
development, acquizition of a medial cusp instead of a mesiodistal
expansion, then reduce medial width and produce a fourth medial cusp.
These distinctions show that any presumption of similarity is very
questionable at best. few reptiles develop even similar dentition to
the bunodont condition, largely in the globidontine mosasaurs and
crocodiles, some iguanian lizards. Ornithischians do produce lineages
that have transversely derived teeth sets, but these form a battery
arrayed diagonally to the transverse axis, and are functional to a
rasp, or cheese grater, in effect.

  One other precise function is demonstrated in leptoceratopsid
dentition, which have a upper lingual "step" that would give the jaw a
grasp-and-pull ability, or the ability to tear laterally more precisely
than the primitive psittacosaur condition. No theropod has comparable
dentition, and this is strictly an ornithischian condition.

  However, ornithomimosaurs have an apparent tearing/shearing function
in the snout, and oviraptorids have an even more specialized condition
resembling the dicynodont propalinal palate grinder. This would be the
only possible similarity. I will be trying to prepare something to
present at next year's SVP to comment on the specialization of the jaws
in dicynodonts and oviraptorosaurs, so hold yer pants. The mechanics
are nothing but unique for archosaurs.

=====
Jaime "James" A. Headden

  Dinosaurs are horrible, terrible creatures! Even the
  fluffy ones, the snuggle-up-at-night-with ones. You think
  they're fun and sweet, but watch out for that stray tail
  spike! Down, gaston, down, boy! No, not on top of Momma!

__________________________________________________
Do You Yahoo!?
Yahoo! Messenger - Talk while you surf!  It's FREE.
http://im.yahoo.com/