[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Beaks and Teeth

David Marjanovic (david.marjanovic@gmx.at) wrote:

<Wll, there's no evidence that Confuciusornithidae or Ornithurae -- or
Ornithomimosauria -- got gnawing teeth before they exchanged them for a
toothless beak. Makes sense only when *Incisivosaurus* represents the
basal state for oviraptorosaurs.>

  Makovicky's quote had nothing to do with gnawing. A condition I think
which is being taken too literally in light of rodents. There are no
opposing incisors, and the wear-facets are oriented almost vertically.
This indicates what few if any rodents have, a vertical shear. What
Makoviicky was pointing out, and which I think needs to be re-expressed,
is that the loss of teeth in birds cannot be correlated to the loss of
teeth in other animals. For example, a basal ornithomimosaur couples a
sharp tomial edge of the maxillary bone caudal to the dentigerous portion
of the jaw, showing a beak _with_ teeth, and lots of teeth. The suggestion
has been, in Perez-Moreno et al., that the gathering of teeth rostrally
effected a small beak-like bite rostrally, and the beak of later forms was
used to cover the loss of the dentition caudally. Here we have three
different cases of toothloss in theropods. The condition appears to have
been aquired on the premaxilla in ornithischians within stegosaurs,
ankylosaurs, ornithopods, and marginocephalians a whopping four times!
Similarly, only some edentates lack teeth, and those that show tooth
reduction belong to different groups where basal members have a greater
number than more advanced formed. Convergent evolution is a kicker, but
when it occurs, boy does it.


Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

Do you Yahoo!?
Faith Hill - Exclusive Performances, Videos & More