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Re: mesosaurs, what are they?



Dave Peters (davidrpeters@earthlink.net) wrote:

<What are mesosaurs?>

  If you're looking for another group to apply them to, you may be
swimming in different waters than the mesosaurs did. So far, phylogenies
have placed mesosaurids outside of reptiles, as basal non-reptilian
sauropsids, and which have at times been accompanied by procolophonids and
pariesaurs.

  Quoting Laurin and Gauthier ():

  "Reptilia is defined as "the most recent common ancestor of extant
   turtles and saurians, and all of its descendants" (Gauthier et al.,
   1988). Characters supporting Reptilia include:

 * Tabular small. The tabular (a bone on the posterolateral corner of the
   skull table) of early synapsids and diadectomorphs is large, but
   reptiles have only a small tabular, when it is present.
 * Suborbital foramen present (Fig. 1B-D). The suborbital foramen is a
   small hole near the lateral edge of the palate, between the pterygoid,
   palatine, and ectopterygoid (or jugal, when the ectopterygoid is
   absent). This structure was not found in early synapsids and
   diadectomorphs (Fig. 1A).
 * Supraoccipital anterior crista present. The supraoccipital of
mesosaurs,
   synapsids, and diadectomoprhs lacks anterior parasagittal flanges. The
   supraoccipital of reptiles has a paired anterior parasagittal flange
   called an anterior crista.
 * Supraoccipital plate narrow. The supraoccipital is a bone in the back
of
   the braincase. The supraoccipital plate of mesosaurs, synapsids, and
   diadectomorphs is broad and extends farther laterally than the
   postparietal."

  These are broad criteria, but supportive of cranial features that
distinguish a Reptilia--Mesosauridae separation. Any relation to
*Lariosaurus* would be superficial.

  Cheers,

=====
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.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
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