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Re: Gliders to Fliers?



I wrote:

<<...Now, while the reduced arms in ceratosaurs are
not precisely an impairment to climbing, and the
flexible hand (I do agree on this point) makes a good
grasping tool, the consistent shortness of the arm in
archosaurs until maniraptorans does not appear to
allow for a avian-style wing until the mid- to late
Jurassic...>>

and Dan Bensen wrote:

<Why not have the grasping arm having evolved for some
reason other than climbing and then primitive, small
ceratosaurs using that adaptations to get into the
trees. These squirly ceratosaurs might have evolved
flight the same way that flying squirrels are.>

  You mean, "Seem to be evolving flight". Don't assume
an end product; this is _a posteriori_ reasoning, and
in this case, is not useful. However, the form of
flying squirrel gliding is so different structurally
from that of frogs, snakes, birds, and lizards, that
there should be no attempt to group them in the same
form. The only animals that perform any aerial action
similar to birds are flying fish, who use their
pectoral fins (our forelimbs) to "fly." The flap of
skin between limbs, extended by ribs, or between toes,
use different parts of the body (not non-digital
extensions from the forelimb) to hold a flying or
gliding membrane to hold themselves in the air for
whatever period of time.

  Now, known ceratosaurs do not present any apparent
trend towards longer, more climbing-able equipment in
the arms, and possess relatively primitive hips (bowed
femoral shaft, very broad supraacetabular crest, etc.)
that would not have reduced stress on the hip enough
to allow the animal to climb by means that used these
regions primarily. Forelimb wise, they _do_ show a
trend towards shrinkage of the fore and elongation of
the hind, as in abelisaurs from dilophosaurs from
coelophysids. Shoulders are primitively independantly
mobile, as suggested from narrow ribcages, broad
scapulae, lack of apparent furcular fusion (I may have
missed something here, will have to check my furcula
refs), and unossified or un-coossified sternae (not
that I know of any). There are numerous refs to this
post and the one previous on pterosaurs, and I will
try to pull them up for the purpose of perusal.


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

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

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