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RE: Differences between *Vancleavea* and thalattosaurs



Some specific discussion.

David Peters wrote:

<Let's take your points one at a time:

thecodont dentition - also in Miodentosaurus Askeptosaurus

> interdental plates -- also in Miodentosaurus Askeptosaurus>

Unfortunately, there is a distinction among diapsids between shallow pits on 
the dentigerous bones into which the tooth is situated into a cemented base or 
fused (acrodont) or on the lingual surface of the dentigerous bone and anchored 
(with cementation) to a subsidiary lingual shelf (pleurodont), or with a 
combination of a shallow socket on the medial surface of the dentigerous bones 
(pleurothecodont). This differs from the deep sockets in the thecodont 
condition.

Neither *Miodentosaurus* (using Wu et al. 2009) nor *Askeptosaurus* (using 
Müller's PhD thesis, 2002) have thecodont dentition, and neither of them have 
interdental plates (which is derived within thecodont jaws, rather than being 
present at the same time as thecodonty itself).

<lack of post-axial intercentra - also in Miodentosaurus Askeptosaurus>

  None are apparently ossified in either *Miodentosaurus* nor *Askeptosaurus*, 
however using Müller's thesis, one could only note the following:

  "The neural arches are often not firmly fused to the centra. Small, 
blade-like intercentra were
obviously present at least in the anterior cervical region." (pg.75)

  Intercentra are noted where centra do not fully articulate to one another or 
show a wedge-shaped aspect in lateral view or a facet on the ventral margin of 
the central face, providing an intercentral element between them. This appears 
to be the case in *Askeptosaurus*.

<femur with medially inflected head - from Nesbitt et al. 2009: "The
proximal head is poorly defined, being expanded but continuous with the
shaft.">

  Here, the terminology of an inflected head is the presence of a medial 
extension from the femoral shaft which articulates within the acetabulum. 
Inflection of the femoral head does not necessarily involve a femoral caput, in 
which a portion of the head becomes condylar to articulate with the acetabulum 
(and concordantly infers a deep acetabulum instead of a shallow trough onto 
which the femur articulates). In this case, *Askeptosaurus* does have an offset 
femoral head, but *Miodentosaurus* does not.

<sigmoidal femoral shaft - also in Miodentosaurus Askeptosaurus, but to your 
point, maybe not so much.>

  *Miodentosaurus* (Wu et al., 2009) lacks the sigmoidality you infer, while 
*Askeptosaurus* does possess this aspect.

<lack of intertrochanteric fossa on femur - also in Miodentosaurus 
Askeptosaurus>

  Unfortunately, as above, this seems to be a *Miodentosaurus* = no, 
*Askeptosaurus* = yes issue. In the former, the internal trochanter is places 
highly on the femoral shaft and forms a groove with the femoral head. There is 
no fossa. There does seem to be one in *Askeptosaurus*, however shallow it 
should be, but the details are not clear in the thesis.

<presence of osteoderms - a-ha! got me... except the outgroup includes 
placodonts, so there you go.>

  I really don't have much to say on this. True osteoderms in parareptilians 
have been suggested as features that associate them to archosauromorphs. I 
would like to think this would be a case of noting that the exception to a rule 
does not disprove the rule.

  Wu X.-c., Cheng Y.-n., Sato T., Shan H.-y. 2009. *Miodentosaurus brevis* 
Cheng et al., 2007 (Diapsida: Thalattosauria): Its postcranial skeleton and 
phylogenetic relationships. _Vertebrata Palasiatica_ 47(1):1-20.

  Müller, J. PhD Thesis. A revision of *Askeptosaurus italicus* and other 
thalattosaurs from the European Triassic, the interrelationships of 
thalattosaurs, and their phylogenetic position within diapsid reptiles 
(Amniota, Eureptilia). Fachbreich Geowissenschaften der Universität (Mainz). 
208pp.

Cheers,

Jaime A. Headden

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