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

RE: Differences between *Vancleavea* and



Tim Williams wrote-
 
> Hi Mickey
> 
> Can you re-send in plain text?
 
Oops.  Forgot to switch to that.  Ironically, my message was something Jaime 
wrote that he couldn't send to the DML due to connection issues.  I have no 
side in the present case, as my knowledge of diapsid systematics and 
thallatosaur anatomy is poor.  I will note that I don't have an issue 
philosophically with someone challenging even a detailed paper by experts when 
it comes to phylogeny, as people do poor phylogenetic work more often than 
realized.  I also think that diapsid phylogeny is comparatively untested, where 
people mostly use the same characters (and always leave out many taxa) which 
are designed to support the 'known' clades.  On the other hand, Peters has 
shown that he doesn't know how to do a proper phylogenetic analysis (ordering, 
correlated characters, etc.), so even ignoring his sketchy Photoshop method I 
don't trust his analysis at all.  The way he initially presented it, as a mere 
statement that the authors were wrong, is ineffective regardless.  Lately it's 
been improving, as people are discussing individual characters and how they 
vary.  And on to Jaime's original message-
 
 
 
 
Pardon me for my previous comments on *Askeptosaurus*. I was mistaken on a few 
features, and misidentified a couple of aspects in differentiating it from 
*Vancleavea* (and from the archosauriform/morph diagnostic characters).
 
1. *Askeptosaurus* lacks femoral sigmoidality, and the femur is straight in 
lateral and cranial views.
 
2. The femur lacks an intertrochanteric fossa, and the internal trochanter is 
positions so high on the head that it is only slightly differentiable; I had 
mistaken this for the femoral caput, and this also indicates that:
 
3. The femoral head is not inflected medially, and there is no caput; the 
proximal femur articulates directed into the acetabulum, and is apparently 
oriented at a right angle to the body wall (as in other highly aquatic diapsids 
including some nothosaurs, pachypleurosaurs, ichthyosaurs and mosasaurs. 
Futhermore, *Askeptosaurus* possesses a fenestra between the parietal, 
squamosal and postorbital, resulting in a "intromittent" surpatemporal 
fenestra, which reflects on the potential for closure of the fenestra within 
Thalattosauria and not prior to the thalattosaurian ancestor.
 
One could also discuss a laundry-list of features by which thalattosaurians and 
*Vancleavea* differ, but I will expound primarily on the most apparent:
 
1. The quadrate of *Vancleavea* possesses a long, triangular pterygoid process, 
which is short or absent in *Miodentosaurus* and *Askeptosaurus*.
 
2. As David Marjanovic noted, *Vancleavea* appears to lack a pineal foramen, 
and this is notable because despite the separation of the parietals from one 
another in the Ghost Ranch specimen, no channel for the foramen is apparent on 
the medial margins of the exposed parietals. This differs from both 
*Askeptosaurus* and especially *Miodentosaurus* as in the latter the foramen is 
particularly large and placed between the frontals and parietals.
 
3. *Vancleavea*, unlike either *Askeptosaurus* or *Miodentosaurus*, preserves a 
large, ossified laterosphenoid. I omitted this feature previously (faulty 
memory!) but it would be interesting to note its relationship to 
archosauromorph phylogeny if the element preserved anterior to the cranial 
nerve V opening in basal turtles is indeed a laterosphenoid (and that this does 
not provide more evidence for them of being archosauromorphs themselves).
 
4. Caudal ribs are present on anterior caudals in thalattosaurians, especially 
*Askeptosaurus* and *Miodentosaurus*, but absent in *Vancleavea*.
 
5. The scapula of thalattosaurians is short, rounded and crescentic in aspect, 
rather than elongated and rod-like as in *Vancleavea*. The scapula is 
particularly interesting as it possesses a distinct acromion, expanded and 
crescentic distal expansion, and an offset glenoid which is not offset to the 
scapular shaft in posterior view (thalattosaurians thus articulated their 
humerus laterally, while *Vancleavea* articulated its humerus posteriorly).
 
6. *Askeptosaurus* and *Miodentosaurus* have an elongated, spur-like aspect to 
the proximal (dorsal) portion of the ilium, where the sacral ribs articulate. 
This portion of the bone is horizontally aligned, and does not terminate in a 
broad
 
7. The calcaneum in *Askeptosaurus* is short, blocky, and angular (it is not 
preserved in *Miodentosaurus*), as in *Vancleavea*, but in the latter, there is 
an elongated process that is perhaps analogous if not directly homologous to 
the calcaneal tuber of various terrestrial archosauriforms (and absent in 
proterochampsids, drepanosaurs, etc.).
 
It seems that *Vancleavea* (despite an earlier post claiming otherwise) is not, 
in fact, *Miodentosaurus* as a direct comparison from the literature alone 
resolves this concern. Nesbitt et al. may be faulted for at most one large 
thing in their study, and that is the brevity of the phylogenetic analysis they 
performed. This analysis, along with Parker and Barton's, is particularly 
small, although the authors considered as likely the question of *Vancleavea* 
relative to nonarchosauromorphs well tested by including index taxa that are 
unlikely to be archosauromorphs in the study. Other taxa that shift around 
appear absent. It would be interesting to actually see a published analysis 
attempting to sample a very broad range of diapsids, but at this point none 
exists. When this study is produced, *Vancleavea* (and likely *Doswellia*) 
would serve as interesting inclusions.
 
Jaime Headden