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Microraptor: Sprawling Revisted
Viewers of the NOVA program "The Four-Winged Dinosaur" will recall a
similarity in this argument, one approached previously with several other
papers but wwithout the aggressive testing, of flight plan models in a
sprawl-legged microraptorian.
In Alexander et al. (2010), the authors approach this topic by using a
transfer-prepared specimen (previously undescribed) called LHV 0026. Now, a
short note on transfer preparation: The process involved does little to
actually preserved the fossil itself, save to separate it from the unstable
rock matrix it is preserved in/on. In this casde, the material is removed
completely from the underlying matrix, and replaced onto a new one. Anyone
interested in Jiufotang preservation will note that all specimens are virtually
flat (they are not intact in all three dimensions, although they produce a
light relief from the surface of matrix that is left after the layers are
removed which overlie the bedding plane where the integument is splayed out
upon. Transfer preparing the specimen removes the final section of matrix
UNDER the fossil (all integument is lost) and remounts it. This is often done
to care for larger fossils (in order to make all sides of a fossil available,
but retain the "bonebed" look) but also smaller fossils in extremely fragile
rock (such as the oil shales of the Messel). So far, the bedding rock of the
Jiufotang does not require transfer to preserve the fossils, but the idea here
is that the material can become visible on all sides. This does nothing to
reveal the shape of the bones, which are still flattened and in many cases
crushed. There are virtually no Jiufotang fossils which are not like this.
As such, the process of transfer casting to preserve the model of an
articulatable specimen is flawed on the outset, without the assumption that no
distortion has occured, but aside from the discussion in the supplemental
information, no further details of the preparation and casting of this iconic
specimen are made, and no photos of the material are made (just the modeled
skeleton).
Further arguments on the model as a glider are based on the articulation of
the splay of the femur, but fortunately the authors refrained from arguing that
the specimen itself shows this splay. What they do argue, however, is a form
of sleight of hand:
"Primitively, early archosaurs are sprawling, with the legs set laterally and
elevated at around 75° (6), a preadapted posture for gliding. Modern birds
normally have the thigh elevated and sprawled to the side in different degrees;
for example, it is nearly perpendicular to the midline in loons and grebes (7).
This variation shows that the degree of splaying needed to use the hindlegs in
gliding is not unusual when compared with that in modern birds. The absence of
an antitrochanter and a supraacetabular shelf (SAC) in the eumaniraptorans,
including dromaeosaurids, would make elevation and splaying of the legs even
easier (8)."
[6. Bakker, R. T. 1971. Dinosaur physiology and the origin of mammals.
_Evolution_ 25: 636–658.
7. Hertel, F. & Campbell, K. E, Jr. 2007. The antitrochanter of birds: Form and
function in
balance. _Auk_ 124:789–805.
8. Longrich, N. 2006. Structure and function of hindlimb feathers in
*Archaeopteryx lithographica.* _Paleobiology_ 32:417–431.]
One will note first that the word dinosaur does not appear anywhere in this
sentence, which is in keeping with one of these authors' previously reported
statements arguing that birds derived from nondinosaurian 9even
nonarchosaurian) terrestrial vertebrates (just no idea what it is, but
apparently it cannot be a dinosaur). Secondly, a large leap is made, seemingly
with a continuous morphology presented: The sprawling femur of early
archosaurs followed by the elevated and rotated femur in birds. It is almost
as if the femur was continuously splayed in evolution from this basal to this
derived from, and thus there is no way the femur could in fact be parasagittal
in orientation. Thirdly, it excludes the option that the femur could evolve
from a splayed into an erect posture, despite this being the case with
dinosaurs, or that if it did, it would be 1) unlikely to shift into a sprawling
posture, or 2) that the femur was not in fact sprawled. The argument, then, is
that the femur is sprawled in this specimen (an eversion of 25 degrees in the
paper and supplement) because basal archosaurs and birds have an everted femur.
Problems abound with this despite the apparent models:
1) The authors argue that the ilium lacks a supra-acetabular shelf. -- Hwang
et al (2002, _A M Novitates_ 3381) shows that this is in fact incorrect in
specific when applied to two well-preserved specimens referred to *Microraptor
zhaoianus*, and Paul (2002, _Dinosaurs of the Air_) does so in general,
examining specimens of *Archaeopteryx* and *Velociraptor*; the shelf exists,
even though it is short.
2) Eversion of the femur in an archosaurian-like sprawl requires an elevated
caput -- The authors do not demonstrate the morphology of the femur (or, in
fact, the morphology of anything in the paper; they go so far as to hypothesize
a V-shaped tail fan for their models despite one of the specimens they
photograph, *Microraptor gui* holotype IVPP V13352, as having a diamond-shaped
tail fan -- the image in the supplement is cut at this point), which would be
required to determine the actual splay of the femur they argue is apparent
based solely on an attempt at phylogenetic bracketing (trumped by actual
morphology). This is further contradicted by Hwang et al. (2002, _A M
Novitates_ 3381) where the caput is essentially horizontally oriented, as it is
in *Velociraptor mongoliensis*.
At this point, the argument in the paper tests a model being thrown in
various methods, and the flight path analyzed. I am tempted to side with Jim
Cunningham as the relevant voice of skepticism here, as arguing for a stable
glide model is in fact the wrong idea when attempting to derived an evolving
flyer. A dynamic, modular flight system (as in birds) required a chaotic,
unstable system to operate in, and modulating the body to compensate.
Nonexistent or gently controlled air flow is not a good way to determine
performance of a glide path, especially since NO living glider today maintains
an absolutely fixed physical posture during its trajectory, and this goes
triple for parachuting animals.
In their conclusion, Alexander et al. state: "Obviously, the living animal
was capable of active control, but we suggest that the tandem wing
configuration may have been advantageous because it requires less active
stabilizing ability." I am not quite sure how one can develop a system of
unstable flight control from a passively stable system. Abilities to develop
control over the wing and perform a dynamic powered operation seem to require a
powered operation to precede it, although perhaps I am simply being too
unimaginative.
Cheers,
Jaime A. Headden
"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
"Human beings, who are almost unique in having the ability to learn
from the experience of others, are also remarkable for their apparent
disinclination to do so." --- Douglas Adams (Last Chance to See)
"Ever since man first left his cave and met a stranger with a
different language and a new way of looking at things, the human race
has had a dream: to kill him, so we don't have to learn his language or
his new way of looking at things." --- Zapp Brannigan (Beast With a Billion
Backs)
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