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

re: mesosaurus



David Peters (davidrpeters@earthlink.net) wrote:

<Not in Pachypleurosaurus and Mesosaurus>

[...]
 
<Not in Pachypleurosaurus and Mesosaurus>

[...]

<not in Pachy or Meso>

  This is all kind of the point.

I had written:

<<*Mesosaurus* (1) lacks the upper temporal fenestra;>>

To which Dave replied:

<Maybe not. That's what we need to determine with good evidence. And even
if so, the utf of Pachy. is on the verge of closing up.>

...and...

<<"euryapsid" (lower temporal arch lost);>>

<I know this is traditional, but new unpublished findings upset the
tradition (the missing puzzle piece). So, not lost in Pachy and Meso, but
transformed beyond recognition in derived taxa.>

  The euryapsid condition seems to involve a secondary closure. However,
given it's proposed position, as in other basal sauropsidans, *Mesosaurus*
may never have had the condition to begin with, and the diapsid condition
occured later in Sauropsida than the base lineages, and after the origin
of the Parareptilia (it's an idea, anyway).

<<(2) has a very short postorbital cranial region,>>
 
<true, but so does Pachy>

  Well, indeed, so does *Keichousaurus* and *Neusticosaurus,* all of which
are pachypleurosaurs. However, basal sauropterygians appears to include
the placodonts as well according to some (numerous works by Rieppel), and
these are short-skulled animals with elongated caudal regions of the
skull, as in the majority of sauropterygians, and would imply the
condition in pachypleurosaurs are a reversal, and similarities are
convergent. I am not a critic of this idea, but I don't precisely support
it.

<that's okay. We're not too far from Claudiosaurus and don't forget
pliosaurs.>

  Given their nesting within Plesiosauria, and their development from
long-necked forms, I will take these as convergent. Similarly, many
pliosaurs have over ten vertebrae in the neck, and these are generally
small.

<<has a humerus with straight cranial margin and curved caudal margin;>>

<as in Pachy>

  Actually ... yes ... http://www.geocities.com/sea_saur/limbs.jpg is an
example of some comparisons. *Mesosaurus*' humerus is a dead-ringer for a
longer plesiosaur humerus, and is not at all as rotund or "robust" as that
in *Pachypleurosaurus.*

<<Rieppel and Wild (1996) diagnosed some basal sauropterygians
(nothosaurs, including *Lariosaurus*) by: (1) long snout, small, narrow
head [_]; (2) pterygoid vacuities closed by extension of pterygoid [/];
(3) extreme dorsoventral flattening of temporal region with the
development of occipital flange on the squamosal and parietal, allowing
posterodorsal expansion of the jaw adductor musculature [_]; (4) long neck
[_]; (5) unique pectoral girdle: essentially circular in horizontal plane
with large medial opening [_]; (6) scapular blades greatly reduced [*] and
extend below clavicle [_]; (7) distal limb elements reduced [*], humerus
stout [_]; (8) large coracoid with posterior extension (back stroke of
anterior limbs?) [*]; (9) pelvic girdle somewhat similar, but more
primitive [_].>>

<and those few are???? Pachy and Lario!>

  Yes. One or two among the some ten plus we've discussed already.
Parsimony? I think not. As also discussed, the limb features are
locomotory, and using the similarity of mosasaur and seat-turtle limbs, or
plesiosaur and ichthyosaur limbs, would be in the same mean as implying
that the same limb condition among thalattosaurs, nothosaurs,
choristoderans, pachypleurosaurs, and so forth, were all homologous and
derived from a single ancestor, from which point all oher differences are
nullified because it's _obvious_ two different animals cannot develope
similar limb types to locomote in similar ways ... at all. Let us not also
forget marine animals like *Teleosaurus,* a crocodyliform.

<the rostrum is the only autapomorphy. And it's not so long in
Stereosternum, its morphological predecessor>

  It is extremely slender and narrow in *Stereosternum,* based on photos
of this long-necked, large-headed animal that I have.

<and that's okay.>

  Yes, Because convergence happens. Different animals can even develop
opposable digits, or stereoscopic vision, or long necks, or lateral
undulation of the spine, or longer digits than others....

<Stereosternum is a mesosaur. Saurosternon is a post-cranial basal
terrestrial diapsid, usually matched with Paliguana, a skull, I think and
according to Carroll 1988.>

  Yes, I realized this a while back, and use this reply to acknowledge the
mistake of similar names. This means Müller did not include mesosaurs, as
if that's a problem. However, right now, I will take Müller over Carroll
for position of *Saurosternon.*

<Lots of problems with this cladogram, which Müller is aware of.>

  I was more intserested in the data at the moment. The discussion stems
from that. Similarly, however, it does illustrate the disparity of the
same body type among diapsids, which was one of the main points of
illustrating it.

<Clarazia has placodont-like pavement teeth. "superficial attachment"???>

  Pavement teeth? Placodonts are thecodont, they have clear and deep roots
for the broad crowns. The teeth in *Clarazia* appear to not indicate the
presence of sockets, and look to be acrodont, as in some lepidosaurs,
which is why I think Müller used "superficial attachment."

<It might be useful to see the situation in Stereosternum, the antecedent
without the straining teeth.>

  Skulls I have seen of *Mesosaurus* imply it lacks straining teeth as
well, just much more numerous, smaller recurved conical crowns. Piscivory
adapatations as in some elasmosaurs. Teeth become larger and less numerous
per measured section in the rostral region of the skull.

<Take a look at Pachypleurosaurus and perhaps Claudiosaurus its
antecedent, I think you'll see the similarity in Mesosaurus.>

  I have taken a lot of what has been offered by Dave, and am not
convinced. This is why I refute the superifical features Dave has raised
in regards to what are essentially locomotory adaptations and the two or
three cranial features as leading to a relationship, whereas refutating
features outnumber these. Dave has also not responded to the arguments
I've made about locomotory convergence, but instead seems to offer any
character that can be found as being an absolute feature reflecting
phylogeny (which is true, actually, in some cases). David Marjanovic
suggested weighting these differentially, to determine their actual use in
the cladogram given the hypothesis I offer, but I have a few qualms about
differential weighting which I have discussed on the list before. I will
try to recover the topic in a different email.

<The above should also cover some issues of the temporal region.>

  What I have seen is "this could mean this," and "this could mean that,"
but offers no testable hypothesis of temporal evolution with a
parsimonious phylogeny. At least a phylogeny that seems to agree with
other people's ability to confirm.

  Cheers,

=====
Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
__________________________________ 
Do you Yahoo!? 
Read only the mail you want - Yahoo! Mail SpamGuard. 
http://promotions.yahoo.com/new_mail