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Re: FYI -- Microraptor gui, Burnham and Alexander, PNAS...
Dan Chure <firstname.lastname@example.org> wrote:
> Does anyone have the citation for
> this yet?
Alexander, D.E., Gong, E., Martin, L.D., Burnham, D.A., and Falk, A.R. (2010).
Model tests of gliding with different hindwing configurations in the
four-winged dromaeosaurid _Microraptor gui_. PNAS published online before
print January 25, 2010, doi:10.1073/pnas.0911852107
(I hope the little degrees signs come out OK in this copy-and-pasted
Abstract: "Fossils of the remarkable dromaeosaurid _Microraptor gui_ and
relatives clearly show well-developed flight feathers on the hind limbs as well
as the front limbs. No modern vertebrate has hind limbs functioning as
independent, fully developed wings; so, lacking a living example, little
agreement exists on the functional morphology or likely flight configuration of
the hindwing. Using a detailed reconstruction based on the actual skeleton of
one individual, cast in the round, we developed light-weight, three-dimensional
physical models and performed glide tests with anatomically reasonable hindwing
configurations. Models were tested with hindwings abducted and extended
laterally, as well as with a previously described biplane configuration.
Although the hip joint requires the hindwing to have at least 20Â of negative
dihedral (anhedral), all configurations were quite stable gliders. Glide
angles ranged from 3Â to 21Â with a mean
estimated equilibrium angle of 13.7Â, giving a lift to drag ratio of 4.1:1 and
a lift coefficient of 0.64. The abducted hindwing modelâs equilibrium glide
speed corresponds to a glide speed in the living animal of 10.6 msâ1. Although
the biplane model glided almost as well as the other models, it was
structurally deficient and required an unlikely weight distribution (very heavy
head) for stable gliding. Our model with laterally abducted hindwings
represents a biologically and aerodynamically reasonable configuration for this
four-winged gliding animal. _M. gui_'s feathered hindwings, although effective
for gliding, would have seriously hampered terrestrial locomotion."
The opening line of the main text put me in a bad humor, right off the bat:
"Evidence now exists that should settle the long-running debate
over a ground-up origin of avian flight vs. the evolution of
avian flight from a trees-down glider. This evidence shows that
the protoavian was arboreal (1) rather than a terrestrial cursor as
many have suggested (2â4)."
In other words, the artificial dichotomy between "trees-down" and "ground-up"
is revived. It ignores abundant evidence pointing to the possibility of a
'dual-mode' lifestyle, i.e. divided between both ground and trees (e.g., Glen &
Then there's this statement, further on:
"New anatomical information based on the discovery of several hundred specimens
similar to the four-winged glider _M. gui_ (and related taxa) has produced
converging lines of evidence demonstrating that the original
describers of _M. gui_ (5) were correct in their interpretation of the
I really hope the authors are not arguing that posture-in-death reflects
posture-in-life. Strange things happen to ligaments after a creature dies.
Besides, once a creature has shed its mortal coils it no longer feels
excruciating pain when muscles are stretched or twisted beyond their limits in
This reminds me of the argument that fossil birds such as _Archaeopteryx_ had a
orientation, with metatarsal I articulating directly on the posterior surface
of metatarsal II. This ignores the fact that this attachment would disrupt
passage of the flexor tendons. This situation is untenable in a living
theropod (who prefer to avoid snapping their own tendons), but not so much of a
problem for a dead one.
Finally, another red flag comes up in the Discussion:
"We suggest that _Microraptor_ was an adept glider and would have had little
difficulty gliding from tree trunk to tree trunk or climbing trees, but would
have been very awkward and vulnerable on the ground."
How adept can you be in the trees, when movement at the wrist and ankle joints
are each confined to a single plane (arboreal mammals tend to have highly
flexible wrists and ankles), and neither the hand nor pes was capable of
I think Alexander &c have it backwards: _Microraptor_ was comfortable on the
ground (it has the proportions of a cursor), but clumsy in trees. There's no
evidence that _Microraptor_'s osteology was substantially different to that of
any other maniraptoran.