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re: mesosaurs - what are they?



Jaime wrote:

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).

>>>>  My analysis places turtles outside of the diapsida, between
Procolophon and Anthodon. So now we're stuck with a potentially
problematic clade list.

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.

>>>>> I don't think the tabular, big or small, or lack thereof, as been
seen in Mesosaurus. If wrong, please ref. Absent or fused in
Sauropterygia.

 * 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).

>>>>  If we allow that the ectopterygoid fused to the pterygoid in
turtles, as it appears, then this character is homologous and present
whether you call it a palatine fenestra or a suborbital fenestra. Either
way, present in basal sauropterygia and mesosaurs (the palate is on the
Internet, but I can send you the evidence).

 * 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.

>>>>  I'm going to need more information on this one. By the
description, it sounds like an internal braincase structure and I have
no data one way or the other to understand  or see this. The temporal
regions of nearly all Mesosaurs are pretty well shattered, and I haven't
seen a good reconstuction of it anywhere, so I eagerly look forward to
seeing and understanding what this is all about.

 * 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."

>>>>>  As in Claudiosaurus, the last in the lineage to retain an
identifiable PP,  so this character is present in Sauropterygia.

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

>>>>> And here is the fault with all prior cladistic analyses! All of
the above are obscure cranial characters. Cladistic analysis, in its
glory, espouses a holistic approach, employing all the characters from
every part of the body.

In my analysis, Mesosaurs share with either Pachypleurosaurus or
Lariosaurus  or both, 128 out of 139 characters
except characters relating to:

Skull elongatation, rostrum length, orbit in anterior half of skull
Skull width vs. height
Metacarpal length and alignment (here it is midway between the two)
The number of phalanges in mdigit IV (both have four, Mesosaurus has
taken it one step further and has three)
A fenestratated tarsus (but shares that with Claudiosaurus)
The longest metatarsal is V, and Lariosaurus has an elongated mt V just
shorter than IV.
Likewise, pedal digit V is the longest.

And I'm not even counting pachystotic ribs.

Hardly superficial.

Best,

David Peters




  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making
leaps