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Sifaka and Chukar Behavior: Trees Down or Ground Up?
I've had time to digest the finds a little. Plus I've listened to the
discussions, and very cogent they have been, largely I have been
interested in the functional aspects of this new, peculiar morphology.
Whether the name be *Microraptor gui*, *Cryptovolans pauli*, or even
*Microraptor pauli* or *Cryptovolans gui*, the point is that the
collection of fossils represent an interesting and unparalleled adaptation
in the manner of getting from Point A to Point B. Where this fits in the
general scheme of flight origin may have everything to do with the feather
First, some comments.
Czerckas et al. actually refuted Norell et al.'s finding of "tibia
feathers" and stated they were continuous with the wing feathers, implying
then that there was a fully-formed, functional avian wing going on. These
were similarly argued onlist. Stephan Czerckas even created a model
showing exactly this. It is now clear that the addition of six new fossils
shows that the original hypothesis, long leg feathers, was right, and that
the arm did not have long wing feathers, at least not very much longer
than the bony arm itself. Even hummingbirds have longer primaries than the
Both in the arm and leg, the feathers taper as a set to a single, pointy
tip, rather than a broad fan. The restoration of Portia Sloan indicates a
narrow yet rounded wing on both sets of limbs, yet the fossils appear to
indicate that they tapered very strongly on the arm, but were broad on the
leg; thus, the Sloan restoration, as in Figure 1c of the paper, has a
broader wing and narrow leg fan than should be neccessary. The leg
feathers similarly appear to orient from the metatarsus at a narrow angle
of about 60 degrees or so, and would not likely have interferred in
bipedal walking, or trunk-climbing. The tail fan in the fossil is
indicated not as a broad, lozenge-shaped structure but rather as a
smaller, narrow rhomboid structure in TNP00996 (Figure 2e), rather similar
to that in "rhamphorhynchoid" pterosaurs, including the balance of a long,
slender tail, underexaggerated in the restorations.
Sifakas lack any leg-gliding equipment, leaping aided almost wholly by a
"halo" of body hair including some extra long underarm hair badly in need
of trimming, inducing greater drag than in sleeker animals or other
monkeys. Some monkeys such as *Saimiri* do not have such a halo and make a
great show of leaping about, so its not as if this underarm hair is
neccessary for the "gliding" leap of *Propithecus*. I wonder if Ken Dial
is interested in shaving sifakas with Feduccia to test their aerial
ability or the part that the halo really plays in their leaps....
Anyways, back to the show.
So we now have two possible hypotheses for avian flight orgin, ground up
supported by Ken Dial's experiments with galliforms, and trees down
supported by Xu et al. on new (and not so new) fossils from the middle
Cretaceous of Liaoning. But does the new material from Liaoning really
support trees down? One of the problems was raised by Tim Williams late on
the 22nd: ambush predators leap feet first, not using their wings, and in
dromaeosaurs, this is with the rear set of wings, with a broader, more
drag-inducing set of plummage involved. The second problem, raised in
discussion with Randy Irmis, is that given the feet-first ambush strategy,
how does gliding, maintaining a broad and stable wing shape, explain the
avian flight stroke? And how does a small set of fossils with no known
correlates or even close relatives possessing the unique plummage, show
how avian flight began?
Simply put, it does not. These most likely appear to be secondary
adaptations on the part of the *Microraptor*/*Cryptovolans* complex, in
that they occur no where else, and generally do not explain the origin of
flight; rather, a means of getting around within a canopy. An ambush
predator cannot afford to maximize predatory equipment into a locomotory
structure; loss in basal ungulates of the predatory feet to favor getting
around reduced their ability to hunt as well as their more predatory
sister-group, carnivorans and creodonts. Similarly, animals that glide
cannot simply switch to predatory mode while in flight, meaning that the
wing structures are independant of predation. Previous arguments to the
interrelationship of integument and predation involve the only
well-supported hypothesis for the origin of the flight stroke; the
snap-action of the shoulder and wrist do not work in a gliding mechanism,
in which they must be fixed. Flexing these joints would reduce the ability
to glide, and work only in one plane or two, with retraction or folding of
the flight surface. Though one might posit that the folding of the
membrane would serve in aerial maneuvering, and effectively utilize the
"flight" joints that were present in the fossils, this would still not
explain how a glider could develop a fully formed flapping mechanic,
increase the pectoralis or form a larger keel for leverage, develop the
humeral elevator and triosseal canal to enhance it, decrease the angle of
the furcula and increase the degree of craniocaudal curvature in the rami
of the furcula, etc.. The problem appears to be that the flapping mechanic
was not employed in the glider, or any such gliding mechanism, yet the
glider is there.
What this appears to be, rather, is that one group of small, predatory
theropods in the trees became adapted to gliding independantly of all
other avian-style theropods, and that in no way do these finds support
either the trees down or the ground up hypotheses. Merely an interesting
sideline. The tetrapteryx of Beebe and the feathered legs of Feduccia et
al. are interesting predictors of four-winged animals, but again do not
show how flapping flight would have evolved.
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)
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