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Re: The Continuing Story of Gliders to Dinosaurs



Grant Harding wrote (and I'm replaying this backwards
for form):

<P.S. Where is _Marasuchus_ generally classified on
the ornithosuchian family tree?>

  Let's stick with Archosauria, please? It's a little
less jumbled and more -- secure -- phylogenetically
speaking. Therefore, Mara is an archosaur, as opposed
to *Pachyrhachis*, a lepidosaur, and *Ornithosuchus*
is neither.

<Now, could a jerboa-analogous, cursorial, leaping
_Marasuchus_ have had descendants adapted for
arboreality, which then *retained* the leaping ability
and evolved into indri-analogous vertical
tree-leapers?  This would make sense.  Any problems
with the idea?  (Like, for example, in becoming
adapted for climbing, would they have automatically
*lost* the leaping ability, thus making this phylogeny
less credible?)>

  *Marasuchus* and *Scleromochlus* are certainly good
canidates for secondarily arboreal animals, but we
need their precursors, those MT and possibly ET taxa
that we need to find that will help sort this all out
[the base of the ornithodiran, dinosaurian, and
pterosaurian trees, the first being plausible only in
light of Peters' work -- and I'm not dismissing it,
Dave].

  Essentially, however, both have very short arms
compared to their legs, and so are already showing
strong cursorial adaptations. Since I think only
sprawl-legged lizards will actually _run_ in the
trees, this leaves these animals as more likely
post-arboreal/scansorial or simply just cursorial
animals. Dinosaurs became hyperterrestrial, with
maniraptoriforms as hypercursors, and sauropods and
some ornithschians as hypocursors. This is based on
observable data on femur/tibia/mt3 ratios (any two as
well as all three) as well as in large part from
Bakker, 1986, 1989, and Holtz, 1994. Alexander has
also done some limb morphometrics, which I have seen
some of, but must catch the papers themselves to
reference them.

  *Marasuchus* makes a good subscansor (only partially
scansorial, meaning with strong cursorial or just
terrestrial bent) with some apparent reduced forelimb
capability to climb, so I wouldn't dismiss the animal
as a possible post-arboreator. Likewise, while the
idea of the animal as a secondarily flightless animal
is plausible, given the forelimb (see previous post on
ceratosaur forelimbs, true for Mara as well) this
animal had to have gone through _many_ changes in a
_very_ short time to become effectively flightless and
re-adapt it's limbs for grasping as seen in primitive
dinosaurs (ornithschian, saurpodomorphan, or
theropodan).

  So, Mara neither especially benefits nor hampers
BCF, which was why the animal was pulled up to begin
with. it _is_ good evidence for ornithodiran
monophyly, though, but this remains for a complete,
published, quantifiable paper on Peters'
pterosaur/lepidosauriform paradigm. And I would
certainly read it. The same goes for Olshevsky's BCF,
once he gets the latest version into print. Tell us,
George, so I can reserve a copy!


=====
Jaime "James" A. Headden

"Come the path that leads us to our fortune."

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