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Re: Differences between *Vancleavea* and thalattosaurs
Thank you, Dave [Peters]. 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?
There's nothing wrong with using proportional characters in principle,
and I don't see why you call them "horribly prone to convergence", or
what you mean by "tend to group into suites of characters". What I said
is that it's not easy to deal with them, and I alluded to this paper
which explains the best method so far, stepmatrix gap-weighting:
John J. Wiens: Character analysis in morphological phylogenetics:
problems and solutions, Systematic Biology 50(5), 689 -- 699
It's a bit time-consuming, but feasible; I used it in the 2008b paper
with Michel Laurin.
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.
That is true. The correlation should be quantified, and the character
dropped if the correlation is too high.
The theoretically best method (which also measures phylogenetic signal
at the same time) is described in this paper:
F. Robin O'Keefe & Peter J. Wagner: Inferring and testing hypotheses of
cladistic character dependence by using character compatibility,
Systematic Biology 50(5), 657 -- 675 (September-October 2001)
Unfortunately you need to be an expert programmer to use it.
<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.
It's not. There's a cladogram in the more recent of the two papers on it.
Interpolating a length of parietal contact that must be scored as
unknown for any fenestra-bearing taxon must be pretty difficult,
Just to repeat it: I agree that this must be scored as unknown for any
taxon with an upper temporal fenestra, because they _can't_ have a long
contact -- the fenestra prevents it. The obvious is always inapplicable.
<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
Oh yeah, that, too.
<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.
Are the cervicals smaller than the dorsals? Perhaps that was meant. (But
in that case that's what the character ought to be called.)
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
This sentence suffers from runaway vocabulary :o)