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Re: FYI -- Microraptor gui, Burnham and Alexander, PNAS...



Dan Chure <danchure@easilink.com> 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...)

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 & 
Bennett, 2007).

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
flight posture."

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 
life.  

This reminds me of the argument that fossil birds such as _Archaeopteryx_ had a 
reverse
 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 
prehension?  

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. 


Cheers

Tim