Re: Fwd: Gliders to Fliers? (Was Re: Ruben Strikes Back)

[dbensen <dbensen@gotnet.net>]

>>What would be the case for Theropods (if one assumes they
evolved at first on the ground)? Most of them have relatively short arms.
Were the hands once so  usefull as to cause the evolution of the bipedal
stance?...and then degenerated later??? Or rather were they just vestiges of
once useful wings?<<

Okay, here's my testament to dinosaur evolution.

    Dinosaurs evolved from quadripedal, cursorial archosaurs (like Lagosuchus)
in the early Triassic.  These archosaurs
were small predators of the scrub-and-desert that covered most of the world at
that time, and had several adaptations
for moving around in the sand.  One of these was hopping.
    Although bipedal movement is usually not any more efficient than quadripedal
(and sometimes more dangerous, as
mbonnan stated).  However, bipedal hopping is much better than quadripedal
running when the animal is on sand.  Many
desert-living rodents and marsupials have evolved bipedalism independently and
all live in such flat environs.  One
cannot say that bipedalism is a result of aborialism because most arboreal
mammals are quadripedal  Monkeys most
certainly run on four legs and tree kangaroos are bipedal because they evolved
from bipedal, hopping, ancestors.  So,
these lagosuchians made many specializations toward a hopping gait, and when the
global climate changed in the
Middle and Late Triassic, it was simpler, evolutionarily, to become bipedal
walkers rather than devolve all their new
traits.  These archosaurs were the dinosaurs, which only descended back onto
four legs when they had to (for reasons
of balance).  Dinosaurs are bipedal by default because they evolved from bipedal
ancestors, just as mammals are
quadripedal by default because we evolved from weasel or rat-like scurriers who
needed a limber, sprawling posture.
    As to arborality, some of these cursorial dinosaurs climbed trees.  They did
not do it the way mammals did it (as
stated by mbonnan, their limbs were far to stiff to allow tree-hugging) they
scaled trees in the manner of their
descendants, birds.  Nuthatches, small insectivorous birds, climb trees by
gripping the bark with their sharp toe claws
and then jumping strait up.  As they lose their upward momentum, they reach out
and grasp the bark with their toes, get a
purchase, then jump again.  Nuthatches _skitter_ up trees very quickly, and they
do it with the stiff limbs of their
ancestors.
    There is no reason why small, tree-climbing dinosaurs, who had the advantage
of claws on both their hind and
fore-limbs could not have scaled trees in the same manner as nuthatches.  In
fact, this behavior has left traces in the
body structure of some later dinosaurs.  The hyper-extendable claws of those
flightless birds, the dromaeosaurs, were
used as pitons to climb up and wound large prey.  These adaptations are almost
certainly derived from similar claws
which were used as pitons for scaling trees.
    I do not say that the nuthatches' behavior is descended from their dinosaur
ancestors, they probably revolved the
method because it works for animals with such a body type.  I am also not saying
that I am right and Dinogeorge is
wrong, it makes just as much sense as mine does.  Only new fossils, or, perhaps
a better look at old fossils, can resolve
this argument.


Dan