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Re: The actual running Archie paper...



First let me defend the ground-up theory:

One of the three issues the authors claim marr the success of the ground-up
theory is the gap between the estimated running speed of _Ax_ (2m/sec) and
its estimated minimum flying speed (6m/sec)".

Unless it's a mis-sprint, that top running speed is about 4mph.  Surely mice
can do 2m/sec? Some snakes approach or exceed it, and I suspect some
arthropods can too.  It must be about one sixth the absolute human maximum.
The reference for this figure is Hecht's paper in "The Beginnings of Birds"
p289-292, though it's possible this reference is for the minimum flying
speed only.

It is so difficult to defend any estimate of this kind but something nearer
2/3 the standard fit human figure would be much easier for me to believe.
_Ax_'s legs are really quite long and very thin - and although they may be
long in order to allow the wings to operate, and thin to save weight, this
configuration is also a well-known recipe for speed.  _Ax_ also had narrow
hips.  Er, I would guess at least 3/4 of chicken-running speed.  Maybe
Beatrice Cunningham will bring us up to speed on what this might be.  And if
chickens turn out to run at only 3m/sec, then I will go for at least 5m/sec
for _Ax_ anyway.

"Proponents of the arboreal model consider _Ax_ to have been a tree-climber
but evidence in support of this is weak at best."  Who do they quote as
giving evidence for _Ax_'s tree-climbing ability?  Padian and Chiappe (and
also Ostrom '79, probably not a strong pro-trees-downer at that time).  Few
who have read the P&C '98 Biol. Review paper: "The Early Evo. of Birds" (or
the Sci Am Feb 98) will be impressed by that "argument".

"Despite lacking the pulley-like action of the supracoracoid muscle of
modern birds..."   Look at page 217 of "Pred. Dinos.World" (if you haven't
got one, get one.)  The picture clearly illustrates a *very important
feature* (interestingly contradicted by the text):  that the
supracoracoideus muscle pulls *forward and upwards* not just in the modern
bird shown but also in  _Ax_ and Coelophysis too!  The pulley is *not* there
entirely to change the direction the muscle pulls in (though the final angle
wrt horizontal of the tendon where it attaches to the humerus varies:
_Coelo_:30 degs, _Ax_: 45, and "moderns": 60 degs) but it is there because
the S.C. gets bigger with more evolved flying.  The picture quite clearly
shows the tendon passing over a promontory on the _Ax_ coracoid that changes
the direction of the muscle by something like 135 degs.  Is that not a loop?
In order to constitute a pulley, does the line have to go through a closed
hole?  Is your outside elbow tendon not a pulley just because it doesn't go
through a tunnel?  Whatever word we want to call it, the triosseal canal of
birds is a red herring.  A neat trick, but it's the other end of the muscle
that is special and has reversed (and enlarged).

So _Ax_ does have a pulley-like system very much like modern birds after all
(it's just that the S.C. muscle was rather smaller - though of course bats
manage on a very different system, so we ought to be careful about claiming
"Bumblebees can't fly") but even if it didn't, the S.C. does much the same
job as it did in _Coelo_ though with different power.

The paper's message is that the running speed of _Ax_ may have been
considerably increased by the wings' contribution long before take-off, and
who can doubt this (though as I've suggested above *this* probably wouldn't
be a problem for ground-up).  However they're rather contradictory about the
importance of lift and thrust:
"As the lift due to flapping relieves the hindlimbs of bodyweight support,
its running speed is further increased..." suggests lift helps increase
running speed.  But...
"...thrust is the only force that exerts work on _Ax_ along its entire
take-off run..." ...well, all right, but... "Thus we regard thrust and not
lift as the primordial force ultimately responsible for sustained flight"
... doesn't build well on the earlier claim, and what with one thing and
another, the impression that last sentence delivers to me is not one of
clarity of thought and mastery of the subject.

I think they're saying lift helps a bit but thrust helps more.  An
illustration of this is the loaded Harrier jump jet which cannot take off
vertically because its engines can't budge it in a vertical direction at
that weight.  However, if energy is built up in an easy direction
(horizontally), it may be gradually accumulated, and later transformed into
a vertical direction by the ski jump at the end of the flight deck.
"Because the direction of thrust is perpendicular to to that of gravity, not
against it, objections to the cursorial theory on the basis of strenuous
energetic demads may not be relevant."  Unfortunately, objections to the
cursorial theory are by no means mainly on the grounds that _Ax_ could not
take off from the ground even with a run; they have more to do with the
impossibility of a purely running hairy-armed ancestor progressively
evolving increased thrust and lift.  Attempts to use _Ax_ to illustrate
issues in the early evolution of flight are a waste of time.

So the basis of the paper is an unnecessary attempt to make a not very
surprising claim (thrust and lift increase running speed) and not done in a
very convincing way.

There are a few special errors: "This study indicates that _Ax_'s wings may
have been an efficient aerodynamic thrust generator".  It doesn't at all.
It merely states that *if they did* it would be useful to the ground-up
theory but it doesn't say anything about how the wings generate lift.

Though the authors are familiar with alternatives to their ground-up theory,
they still appear to be entirely unaware of an alternative to the "Most
maniraptorans had no flying ancestors" theory.  Any members of this list who
refereed the paper will no doubt be very resentful that the comments they
must have made on this were ignored by the editors.  Still, we can't all be
scientists.  "Some of [these non-avian maniraptoriforms] have even been
found to possess fully fledged wings.  Thus the structures and functions
necessary for wing-generated thrust were already present in the flightless
ancestors of birds."  That's what they think.

The text to figure 1 included: "During the downstroke the airfoil aligns
itself to the upcoming airflow by rotating aeroelastically, trailing edge
up,..."  ...but get this... "...owing to the pressure build up between the
wing and the ground."  Do they mean to suggest that this "aeroelastic"
passive rotation effect disappears in high-flying birds?  No, please don't
tell me "ground effect"; their "owing to" means they think the ground is
essential for the whole phenomenon.  If they didn't, they'd have said "below
the wing".  "This aeroelastic response is possible because the feathered
wing of _Ax_ is attached to the body only at the shoulder."  Mmm.  It's
funny how an unhappy construction can make one smile.

In Figure 2, although it is of course difficult to show exactly where the
reactive force of the ground on the foot is applied when the animal and its
legs are moving, in pictlet "a" where the animal is standing still, it would
have been nice for this force and the weight to have gone the same point,
and also not to have gone through the heel.

Also in Figure 2, the velocity of the _Ax_ is shown in various stages V',
V'' etc, but described as *incremental* velocities.  They are not.  If they
were, the increasing length of V', V'' etc would mean increasing
acceleration.

Finally in Figure 2, they say lift from the wings gradually replaces the
reaction from the ground, which is fine (though I would have preferred "We
term this: 'vertical force migration' " to "This is called '....' ").
However they describe exactly the same thing happening in the horizontal
direction, indicating wing thrust as progressively growing as the bird
gathers speed.  I don't think so!  Does your bicycle gain more torque as you
move up through the gears and approach maximum speed?  Mine doesn't.  If
you've got to push against something that's actually going backwards
relative to you, you can't push so hard as you accelerate.  That's why
rockets go faster than jets.  However, anyone reading the "Methods" section
at the end of the paper would get the impression the authors had a deeply
detailed understanding of the process.  Odd that it shouldn't show in the
diagrams (or the main text).

I liked the pictures of _Ax_ itself, though a purist might say the pubis
pointed too far backwards.

The final sentence has little justification (IMO):
"The origin of bird flight from cursorial theropods is, however, not only
the least conflicted [?] hypothesis given the available phylogentic and
functional data but, as illustrated here, is also aerodynamically
achievable."

The last four words would be right if they referred to their basic argument,
well expressed, applied to _Ax_ itself, though not to the process of
evolution leading up to it.  Everything else in the sentence is wrong.



JJ


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