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

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 (September-October 2001)

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.

And us.

 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,
 though.

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
 temporal bar.

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
 dorsal surface.

This sentence suffers from runaway vocabulary :o)