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Re: mesosaurs



Norm King wrote:

>This is about extinct reptiles, although not dinosaurs:
>
>Most of what I have seen about mesosaurs has them grouped with anapsids.  
>However, I have also seen opinions expressed about them being synapids.  
>Then there are those who can't decide.  Apparently the temporal region of 
>the skulls are not well preserved.  Does anyone know if anything been 
>found recently to resolve this issue? 

Mesosaurs were often grouped within the anapsids, but on the basis of
primitive characters.  They lack the derived features of the skeleton shared
by true Anapsida (turtles, parieasaurs, procolophonids, etc.).  Gauthier's
most recent analysis (discussed in the Major Features in Vertebrate
Evolution Short Course volume from 1994) places them within Sauropsida
(closer to reptiles than to mammals), but outside Reptilia proper (the node
joining turtles, lepidosaurs, and archosaurs).

Although the names "anapsid", "synapsid", and "diapsid" once refered only to
skull conditions, the clades Anapsida, Synapsida, and Diapsida are diagnosed
from features throughout the skeleton, not just temporal fenestration.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"There are some who call me...  Tim."
-- Tim the Enchanter, "Monty Python and Quest for the Holy Grail"

"Tim?!?  They called me TIM?!?!"
-- me, on seeing the credits to "The Ultimate Guide to T. rex"  :-)