Re: "Flight theory has legs"

["Jaime A. Headden" <qilongia@yahoo.com>]

Graydon (oak@uniserve.com) wrote:
 
<But you're -- if you're arguing for a scansorial orgin for birds -- not
just arguing for climbing, you're arguing for _leaping_.>

  I am not arguing for a scansorial origin of birds. I am arguing for a
scansorial adaptions in birds. The origin of flight seems to draw on some
preexisting adaptations, some of which may have drawn out as a result of
climbing or predation in the branches, rather than on the ground. The two
are not neccessarily the one and the same (origin of flight and climbing
in near-avian dinosaurs). Leaping is a method used by SOME, but not all,
gliders -- falling is another; this is, to wit, the main method for bats
to get into the air, as not all bast have a terrestrial habit.
 
<Chameleons have lots of specialized climbing adaptations for precise
movment on narrow branches, and they move slowly.>

  Chameleons are also prime targets for food and their locomotory speed
seems related to their method of evading detection, rather than any
constraint on speed for vertical-legged branch walkers.

<If you're looking at a scansorial origin for birds, they have to be
leapers or parachuters or something of that character, no?  That requires
the boing-boing-jump sort of tree dwelling, rather than step ... step ...
step ... sorts of tree dwelling that you get with chameleons or sloths or
whatnot.>

  I have no particular theory or favoritism to any particular one on the
origin of avian flight. My issue is on the adaptations that led to the
avian flying apparatus. So far, few methods have satisfactorily explained
the evolution of the shoulder. Many of the discoveries in the last 15
years have permitted a grand perspective on this phase in evolution, even
if it is about 20my too late. That climbing and predation seem to apply
various qualities that are evidenced, as shown since Ostrom, it is
becoming of some particular interest to evaluate these properly. 

<Sloths move around in trees just fine, yes.  What they *don't* do is jump
or parachute.  They're essentially cryptic animals -- lots of camoflage,
very slow locomotion, low metabolism.  That's not a particularly plausible
niche to get a flapping flyer out of, since flapping necessarily involves
a high energy output.>

  This is largely irrelevant, they were used to counter a p[oint you;d
made. It's already been stated that the paradigm being suggested is unique
and that it cannot be strictly constrained by what animals TODAY do.
However, this is a two-edged sword, and while using current behavior to
determine what can be known in dead animals is a BAD idea, so too is
pretty much any attempt without direct evidence of it, to infer any sort
of behavior in a dead animal. No one was really trying to say sloths were
predicatory to a flier, now were they? None of the points being made are
cases where a flier is likely to derive, or even a glider. Certainly the
flying snakes of Madagascar are limbless and don't leap, but they do well
on that front, themselves. Especially since they are particularly slow
when climbing.

<Dromeosaurs have knees about at the same level as their pubic boots; this
essentially takes the femur completely out of utility in that climbing
posture, because it will always be maximally or near maximally forward to
get the legs folded up enough to allow the manual claws to grasp the
tree.>

  Except when in the same animals the pubis is retroverted and the knee
would be, as in all other limbed climbing animals, be flexed and well
forwards of the hips. By the same token, squirrels do just well dragging
their bellies on the trunks of trees their climbing, and can lift it up at
need depending on speed. I doubt anyone has proposed that the animals
(dromaeosaurs) walked straight legged up trees or along branches, and a
crouching gait is seen in birds where upon the belly also extends well
below the position of the knee, flexed or not. Similarly, the more basal
deinonychosaurs, including *Microraptor* and *Sinornithosaurus*, have
femora exceeding the depth of the belly, and have shorter pubes than do
their larger descendants, the "velociraptorines" and "dromaeosaurines"
proper. Similarly, the basal dromaeosaurs, especially *Microraptor*, have
an arm that is longer than the leg, and this reach increases when the leg
is flexed and the arm is not. An animal just needs to "Groucho walk" up
the trunk, using the arms pole-climber style to stabilize and grip when
the animal is in a static position.

<If the dromeosaur was fully scansorial, spending all its time in trees,
it would have some kind of limb adaptation to either swing is legs out at
the hips or to have much shorter legs that allowed it a fuller range of
motion with the primary muscle groups.  (This is one of the neat things
about the reported '4 winged' forms; those sprawling hip adaptations are
what one would expect in something scansorial.)>

  As already discussed, there are plenty of straight legged animals to
refute NEEDING a sprawling limb, or even short legs (both of which
trunk-climbing birds manage to contradict). As for Paul's purported
sprawling hip designs, unlike a bird, no dromaeosaur has a femoral neck
nor does it have an unequivocal femoral caput that is higher than the
trochanteric crest, that would permit femoral lateral excursions. Such a
sprawling stance, similarly, would prevent the pes from contaction the
substrate on the same place as the other pes, but would be
outwardly-angled to it.

<Cats are all four limbs with sharp claws, though; you're talking about
something that has terrestrial pedal unguals and scansorial manual
unguals.  The monkeys and lemurs have hands _and often feet_ that can
close around the branch; are there any theropod hand morphologies that
allow that? (I understand not, but could easily be mistaken.)>

  Dromaeosaurs do not have terrestrial unguals, and I'd hate to find out
where this was stated. Though the claws are not as recurved as in
woodpeckers, they are certainly more recurved than in troodontids or the
larger dromaeosaurids, and are suited with a strongly recurved, possibly
equippable "piton"-like claw and a sharpy recurved hallux. Similarly, cats
tend to wear their pedal claws down; they do not make an issue of
sharpenting them like the manual claws, and these tend to be similar in
recurvature as in the smaller (<2.5m) dromaeosaurids.

<Wouldn't this require you to find that the feet of the putatively
terrestrially-clawed fossil forms aren't flat?>

  Given the presence of keratinous sheaths in NGMC 91 ("Dave") in the
pedal claws having a ~15 degree difference from the manual claws, this is
really questionable to infer they were exclusively terrestrial.

<The point I was after is that a proto-bird that's scansorially
quadrepedal is going to be capable of being terrestrially quadrepedal, and
that there is no least hint of that anywhere that I'm aware of.>

  I never said terrestrially quadrupedal. Just "quadrupedal."

<Hoatzin chicks are very cool things, but they don't brachiate, and
they're relatively slow clamber-climbers, no?>

  Never said the climber would have to brachiate. Or even remotely should
be able to. Most clambering/climbing animals don't, and its not really
considerable that a climbing animal should. This is a post-arboreal
adaptation that seems to occur only in herbivores, anyway. But the nature
of quadrupedal climbing in an extant bird, slow or not (its a juvenile
that loses its claws and therefore advanced capability to climb through
ontogeny, so?) is as tellings as its implied absence prior to being
mentioned. As Tim said, some adult birds use the wings as well when
climbing.

  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)

__________________________________
Do you Yahoo!?
SBC Yahoo! DSL - Now only $29.95 per month!
http://sbc.yahoo.com