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re: mesosaurus



David Peters (davidrpeters@earthlink.net) wrote:

<BTW, how does PAUP differentiate massive convergence versus homology? Or
does it take human intervention (weighin characters, etc>) to note the key
differences?>

  PAUP* does not differentiate convergence versus homology. This must
depend on the trees, their support, and in the end, a good dose of
supportive data. That is, consistent analytical support of a particular
arrangement, or the plastic ability of a particular feature to appear in
groups of animals for whatever reason. For instance, in animal
biomechanics textbooks, one can find sections on "digging animals,"
"diving animals," and so forth, and one sees a few features crop up
consistently in all of them that have produced such morphologies as
"eel-like body" in sharks, lampreys, hagfish, and the various groups of
disparate teleosteans which are currently named true and false eels, but
which when taken a close look at and through the power of genes, show
otherwise. Digging animals tend to be "mole-like" and show crooked arms,
short forelimbs with huge manus and claws, and giant olecranons. They've
appeared in marsupials, insectivorans, basal afrotheres, etc., and these
adaptations even occur in some edentates. Myrmecophagy in orycteropodids
and "xenarthrans" was more than two times the charm, it occured over three
times, including palaeanodonts and typical anteaters. This is why seeing a
large group of disparate anatomies (thalattosaurs and nothosaurs among
sauropterygians, mesosaurs, geosaurs and teleosuchids, pachypleurosaurs,
some sphenodontians, and choristoderes) occur makes one look a tad deeper.
Indeed, one of the best ways to discern convergence is to look at the
basal forms, which Dave and I have done to some degree, and these tend to
lead to convergent development of the long, skinny, short-limbed swimming
body form from more terrestrial, larger headed, longer legged, shorter
necked body forms, which remain distinct but similar, meaning primitive
forms were conservative, whereas similar lifestyles and selection would
promote convergent development otherwise.

<The answer will not only affect the mesosaur question, but another one
that recently came up about "worm lizaards" and "legless lizards", which
are all definitely lizards, but are they all related? Or does burrowing
impose convergence?>

  Burrowing lifestyles in some vertebrates, as in *Necrolestes*
("marsupial mole"), *Xenocranium* (palaeanodont), *Spathorhynchus*
(amphisbaenid), *Mesoscalops* (insectivoran), promote a heavy, reinforced
cranium with tiny eyes, developed auditory system, downturned rostrum with
upturned and reinforced snout. These suggest that these features are not
as phylogenetically informative among groups of taxa as they would seem to
be. However, genetic studies of living squamates points to the glass
snaked as being derived from lizards and separately so were amphisbaenids,
as well as true ophidian snakes, whereas leggedness may have been retained
into the booid lineages also perpetuating madstoiid and
"pachyrachine"-type snakes, meanwhile all other snakes lost theirs. Then
theres the separate lineages of sea snakes, appearing for all the world as
one lineage, yet genes point to multiple origins. A massive case of
variation and convergence can be found in the new study supporting clades
called Coronaves and Mesaves. Also genetic with some morphological support
found in the literature.

  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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