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re: mesosaurs - what are they?



  My source, by the way, for the cited information in my last post is
http://tolweb.org/accessory/Phylogeny_and_Classification_of_Amniotes?acc_id=462,
which was composed in 1996 by Laurin and Gauthier, and hasn't been revised
much since, but includes some diagrams.

David Peters (davidrpeters@earthlink.net) wrote:

<My analysis places turtles outside of the diapsida, between Procolophon
and Anthodon. So now we're stuck with a potentially problematic clade
list.>

  Unfortunately, according to the way it's been defined and used by
authors over the last couple of years, If turtles are reptiles, and all
other reptiles are diapsids, turtles have largely become diapsid by
default, with the so-called Anapsida a reversal within Diapsida OR with
Anapsida being paraphyletic. Some phylogenies toy with moving Pareiasauria
around, forming a Parareptilia to which occassionally
Procolophonoidea/Procolophonidae/Procolophonia (now including *Owenetta*)
belongs, but this also seems to be dependant on where turtles lie. So far,
there is little reason to beleive in any _one_ phylogeny, until more and
more recent phylogenies agree. Dave's data for this contentious
arrangement would be nice to see.

<I don't think the tabular, big or small, or lack thereof, as been seen in
Mesosaurus. If wrong, please ref. Absent or fused in Sauropterygia.>

  Indeed. Sauropterygians are considered reptiles.

<If we allow that the ectopterygoid fused to the pterygoid in turtles, as
it appears, then this character is homologous and present whether you call
it a palatine fenestra or a suborbital fenestra. Either way, present in
basal sauropterygia and mesosaurs (the palate is on the Internet, but I
can send you the evidence).>

  If the ectopterygoid fuses to the ptewrygoid, then the condition becomes
a unique state, and the character defined by a foramen between these bones
more a matter of how it forms, not whether it be present. Or it's a
separate set of characters, and some that may only, when some states
agree, have something to do with one another in one taxon, but not in
others. Fusion of ectopterygoid to pterygoid is a derived state and would
acount for any possible loss of a suborbital foramen, even if one was
present in its ancestry. This is not explicit in the fossils, to date.
*Proganochelys* is still a derived animal compared to its
contemporaries.... Turtle-like procolophonoids and pareiasaurs have been
linked one way or the other to turtle ancestry, but not, to my knowledge,
mesosaurs being closer to archosaurs than have turtles been.

<As in Claudiosaurus, the last in the lineage to retain an identifiable
PP,  so this character is present in Sauropterygia.>

  By whose ideas does it include a post-parietal?

  Here are some other data from Laurin and Gauthier:
  http://tolweb.org/accessory/Autapomorphies_of_Diapsid_Clades?acc_id=465

<And here is the fault with all prior cladistic analyses! All of the above
are obscure cranial characters. Cladistic analysis, in its glory, espouses
a holistic approach, employing all the characters from every part of the
body.>

  Yes, it is true, the skull in all its complexity (being more bones in
one small area than most other regions of the body save wrist and ankle)
tends to be either a) the most popular region to observe change between
species, showing more plastic effects during evolution or b) more likely
to be preserved than, say, a foot (or at least a part thereof, including
teeth). In other regions, relative allometric conditions, locomotions, and
so forth, tend to exert a stronger effect on constraint or plasticity than
they do on the skull, leading to slightly less availability or likelihood
of intraspecific variation. There is a reason, it seems, why most
ceratopsid workers say that centrosaurine skeletons vary virtually not at
all, their variation ebing negligible or absent, apart from the skulls.
This is also why most cladistic analyses tend to emphasize 50% or more of
the matrices to the skull.

<In my analysis, Mesosaurs share with either Pachypleurosaurus or
Lariosaurus  or both, 128 out of 139 characters except characters relating
to:>

<Skull elongatation, rostrum length, orbit in anterior half of skull
 Skull width vs. height>

  I see ... are champsosaurs included in this matrix? As a group, or with
*Monjurosuchus* (basal choristoderan), *Lazarusuchus,* and *Champsosaurus*
separated? Skull length variation (note, no constraint on where it's
elongated relative to), width:height, tending to be related to "low, long,
flat skull" a feature present in pachypleurosaurs, secondarily in several
plesiosaur taxa, choristoderes, snakes, mesosaurs, thalattosaurs, and
mosasaur subgroups.

<Metacarpal length and alignment (here it is midway between the two)
 The number of phalanges in mdigit IV (both have four, Mesosaurus has
taken it one step further and has three)>

  Most reptiles in Diapsida, unless given reversion, should have FIVE
phalanges in manual and pedal digits IV: mdIV-1--5, pdIV-1--5.

<A fenestratated tarsus (but shares that with Claudiosaurus)>

  And with ... *Petrolacosaurus,* *Younginia* (other basal diapsids, so
should be present in the outgroup), *Prolacerta,* *Macrocnemus,*
*Trilophosaurus,* *Howesia* and *Mesosuchus,* *Proterosuchus,*
*Champsosaurus,* *Protorosaurus* and *Langobardisaurus,* and
Drepanosauridae, among Diapsida.  Taxa without (in many cases reversals),
include *Gephyrosaurus* (pachypleurosaur), lacertilians, *Rhynchosaurus,*
*Scaphonyx,* *Stenaulorhynchus* and *Hyperodapedon* (all rhynchosaurs,
reversed from the basal rhynchosaur condition), and *Euparkeria.*  Data
from Dilkes (1998: _Phil. Trans. (Bio. Sci.)_ 353 [1368]:501-541).

  Reisz & Scott (2002: _JVP_ 22(2):244-256) add further data built on
deBraga & Reisz (1996: _JVP_ 16(3):384-395) that implies a more
comprehensive basal diapsid phylogeny that supports parareptiles,
anapsids, or otherwise, including the separation of *Mesosaurus* from
diapsids in general. *Claudiosaurus* is excluded from the Diapsida in
Rieppel & Reisz (1999: _Annu. Rev. Ecol. Syst._ 30:1-22), further
suggestive that a foramen between the proximal tarsals is the norm, not
the exception, so it makes sense that *Mesosaurus* would have it and of no
particular value otherwise. Otherwise, the foramen is plesiomorphic for
Diapsida (Rieppel, 1993. Studies on skeleton formation
in reptiles. IV. The homology of the reptilian (amniote) astragalus
revisited. _JVP_ 13(1):31-47). It's even present in amphibians; the
punctuating artery translates proximally between the tibia, astragalus,
and calcaneum in some diapsids, especially in lepidosauromorphans and
archosauromorphans, then even further proximal in the leg.

<The longest metatarsal is V, and Lariosaurus has an elongated mt V just
shorter than IV.
 Likewise, pedal digit V is the longest.>

  In most amniotes, digit four is nearly the longest. Note that in
nothosaurs and *Mesosaurus* (and choristoderes if you're keeping record),
all have elongate fifth digits, but not the same length of metatarsal V,
and they even loose the hook-shaped mtV condition, as in seaturtles
(likely due to an aquatic habitus... function of the hook seems to be
related to means of walking [Robinson, 1975. The functions of the
hooked fifth metatarsal in lepidosaurian reptiles. _Coll. Int. CNRS_
218:461-83]), and the condition appears to be the same for both loss of
the hook AND longest fifth pedal digit in *Hovasaurus,* as well -- another
aquatic basal diapsid. This sounds like trying out the same thing in
multiple lineages, since none of these animals appears to possess other
relating features. Trilophosaurs and lepidosauromorphans, as well as basal
diapsids, all share a conservative body design that, if put into the water
and adapt to it, would imply they would do so in similar ways. If the stem
lineages of Sauropsida were essentially all very similar
(*Petrolacosaurus,* *Younginia,* *Claudiosaurus,* *Kuehneosaurus,*
*Trilophosaurus,* *Mesosuchus* [do not confuse with *Mesosaurus*],
*Prolacerta,* *Macrocnemus,* etc.) how hard, really, would it be to
confuse them as a lumped group? Now, this all seems awfully harsh on Dave,
I understand he is testing things, in this case, so I am just being a
nasty critic.

<And I'm not even counting pachystotic ribs.>

  Good, because they would be convergent in similar yet disparate animals.
Just like other locomotory or aquatic induced adaptations. Like those
listed above. Faculative, functional morphology is a kicker, ain't it?
Makes you sit and think about how much he characters really -- I mean
REALLY -- mean.

<Hardly superficial.>

  As above, very superficial. The animals (mesosaurs, geosaurs,
champsosaurs, nothosaurs, pachypleurosaurs, thalattosaurs, hovasaurs) all
seem to share a host of similar anatomical conditions including long
necks, long and low heads, short limbs dominated by feet and hands, with
longer hind than fore, long sinuous bodies with a tendency for thick ribs
and decoupled pelvic girdles and smaller shoulder girdles, mediolaterally
compressed tails, loss of a hooked fifth metatarsal and other apparently
terrestrial adaptations in sprawling quadrupedal animals, etc.... This is
why I urge caution.

  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)


                
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