RE: Differences between *Vancleavea* and thalattosaurs

[Jaime Headden <qi_leong@hotmail.com>]

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 
it.

<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 
thalattosaurians.

<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 
temporal adductor).

<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 
*Herrerasaurus*.

<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 
archosauromorphs.

<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 
cladistically.
 
<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 
several questions.>

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. 

Cheers,

Jaime A. Headden

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