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RE: Differences between *Vancleavea* and thalattosaurs
Dave Peters wrote:
<Okay, my cards are on the table. Characters below shared with Askeptosaurus OR
Miodentosaurus or both (quite a variety between them!) and not shared with
proterochampsids AND erythrosuchids (the bounding taxa for Vancleavea, sensu
Nesbitt et al.)>
Thank you, Dave. I will be as thourough with this list now that you've supplied
<1. Skull shorter than cervical series>
I have never been fond of proportional characters as they are both horribly
prone to convergence and difficult to relate to skeletal systems outside of
functional features, and tend to group into suites of characters. You will
recall David Marjanovic's comments on that topic a short while ago?
Otherwise, this is an okay character but, unlike the others in the list I
would knock this out of the matrix simply because it scales to length of neck
and (in case you've not noticed it) aquatically-adapted animals seem to develop
a lot of the same types of proportions in their skeletons.
<2. Nasals separated by premaxilla ascending process (that's the "neomorph" in
Vancleavea and the related: Premaxilla contacts frontals>
According to Nesbitt et al., the neomorph is described as being tapered to a
point at both ends. This is despite the anterior tip apparently being "split"
by the posterior nasal processes of the premaxillae, and a crack seaprating one
arm of the "V" that results from the main body. Two features come to note here:
1) the neomorph is almost certainly continuous with the nasals, as the surface
texture is not only identical but it appears that a nutrient groove is present
on the nasal and continues over the neomorph; 2) the neomorph is separated from
the OTHER nasal by the same split in the mediolaterally compacted skull that
reveals the absence of a pineal foramen, and it is likely that this reveals the
anterior tip of the neomorph (could, but I do not "know"). The argument of
Nesbitt et al., who have examined the material first hand should be taken at
face value until and when it can be verified; you don't have to call it true.
This would render the premaxilla only as long on the dorsal surface of the
skull as are the maxillary processes.
<3. Frontals longer than nasals>
Shortened nasals also seem to be common in foreshortened skulls, and are
apparent in a host of facially-challened taxa, from bats to turtles to
oviraptorids. Foreshortening, followed by snout elongation MAY account for the
case in *Askeptosaurus,* but is *MIodentosaurus* is a basal thalattosaurian,
then it seems fairly safe to say that the snout elongation in the former is
secondary to a shorter, triangular snout in the latter, and ancestral for
<4. Antorbital fenestra absent>
Got me there. Closure of the fenestra can occur, but it seems particularly
rare. (Note, for example, that closure of the fenestra occurs early in
ornithopod dinosaurs and does not seem to reverse.)
<5. Upper temporal fenestra closed or slit-like and the related:
Postorbital-parietal contact long (extensive)>
Here is another one of those complex characters that is almost certainly an
[unordered] single character with a host of states. Interpolating a length of
parietal contact that must be scored as unknown for any fenestra-bearing taxon
must be pretty difficult, though. Note that the elongation of the squamosal
and a broad contact between it and the parietal as well as potentially the
front is also part of this.
<6. Parietal skull table broad>
Probably interrelated with the above. Seems important to note that there must
be some bony surface to pick up the slack for bearing the adductor musculature
originally borne by the now-sliverlike lower temporal bar.
<7. Surangular shelf (ridge) absent>
Correct me if I'm wrong, but it appears that not only does *Vancleavea*
possess this feature, so does *Miodentosaurus*. I am not sure about
*Askeptosaurus* at this point. Loss of the shelf seems coincidental with the
reduction of strong temporal adductors, as the jaw adduction becomes
pterygoid-based. I may have noted this before, but this is also convergent in
many aquatically-derived diapsids (*Dakosaurus* is a reversal to a strong
<8. Mandibular fenestra absent>
Such a fenestra seems likely to be present based on smooth, unbroken bone in
*Vancleavea*. Uncertain in *Miodentosaurus*.
<9. Cervicals decrease in size anteriorly>
With only a single exposed cervicals in GR 138, I find this difficult to code
for *Vancleavea*; other specimens include a single articulated pair (PEFO
33978; Parker and Barton, 2009) in which the anterior of the two is about 1mm
shorter in the length of the neural arch at the centrum contact [seeing as how
the ventral centrum is eroded in the posterior of the two, centrum length is
unknown but estimatable]. Other centra are disarticulated and their relative
ages and therefore sizes are unknown, as well as precise enough position to
project length relative to one another.
Also, convergent in aquatically-derived diapsids as it seems coincidental to
neck elongation and a reduced role in cranial support (contra the condition in
mammals; see sauropods, elasmosaurs, nothosaurs, long-necks mosasaurs, etc.).
And finally, with the exception of a few taxa, gradual increase of vertebrae
towards the sacrum is standard for a large number of terrestrial diapsids, and
pretty standard for aquatic ones. I would like to note which taxa you code as
lacking this condition.
<10. Metacarpal II is the longest metacarpal>
Seems pretty basic. I would note that this is the condition in most
archosaurs, however, with the exception of ectaxonal animals like
<11. M4.3+m4.4 fused>
I would like to know how you managed to note which of the mess of
disarticulated phalanges in *Miodentosaurus* AND *Vancleavea* (as well as
projecting fusion over elongation + loss) belongs to which digit and position
in the digit.
<12. Fourth trochanter absent>
Homology tests imply that the internal trochanter of some diapsids is the same
as the fourth trochanter in others, only the former is more proximally located
near the greater trochanter, and its separation results in the formation of the
intertrochanteric fossa. Different terminology for the difference between a
scar/pit/rugosity placed proximally (as in *Vancleavea*) versus a ridge or
crest placed distally (as in *Euparkeria*) as well as a rounded tuber or ridge
placed very proximally (as in *Miodentosaurus*) implies that the character
state transformation is both positional and shape-based. Attachment site
(position) and shape (morphology) of the M. caudofemoralis should be the
character here, and in this case, they differ.
<13. Metatarsals II-IV not shorter than half the tibia>
Seems fine. I've told you about my dislike for functional proportions,
however (see 1, above).
<14. Metatarsals I and V are wider than II, III and IV>
Oddly enough, while coded correctly, I think you mistake the condition for
almost all of diapsids if this is popping out as not apparent in some basal
<15. Pedal digit IV not narrower than III>
See my point about finding digits in a mess of dissarticulation at 11, above.
I appreciate the work put in towards matching taxonomic inclusion with a larger
character dataset, but I suspect the issue (as above) is with the characters,
not the taxa.
<I would be the first to agree with regard to scoring that when one considers
other possibilities, one sees other possibilities. An example would be the
neomorph bone of Nesbitt et al. which they could not identify because none of
their sister taxa have an extended premaxillary ascending process. All of my
sister taxa do, so identification was easy.>
Therein lies a problem. You shoehorned a feature under the assumption that it
could not have been unique in a group of long-premax taxa to possess a short
premax. I would suspect, though have not verified, that Nesbitt et al.
considered this element to be a portion of bone otherwise broken or separated
from another element. As I note above, it is closest in form and surface detail
to the nasal, not the premaxilla, and were it broken from any element, it would
be the former and not the latter. Not only does the premaxilla of *Vancleavea*
bear short dorsal processes, but they are also exceedingly thin, and one might
almost suspect based on the apparent subsumation of one half of the pair under
one of the nasals, that they were almsot certainly not exposed, unless barely,
on the cranial dorsal surface. This differs from ALL thalattosaurians, but it
would be in any extent unique to this taxon anyway and thus unimportant
<Second opinions may not always be right, but in some cases they do shed light
on alternate possibilities. I would love for Vancleavea to be a little armored
erythrosuchid. It LOOKS like one. When I first saw it several years ago, that's
all I could think that it was. When I looked at my PAUP results, that's the
first place I looked. It wasn't there. When I found it in the thalattosaurs I
took a deep breath and realized, well, that explains a lot-- but it also raises
I am coming to a realization, going over the papers, that some features of
*Vancleavea* are weirder than you, Dave ;) And that is meant in all good humor.
Jaime A. Headden
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