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



The CHOP! demon has struck again (see, told ya Don), so here is the part of my 
message that was chopped, with the chopped bit put back in (the bit between 
'had a reversed' and "orientation', which changed the entire meaning of the 
sentence):

------------
This reminds me of the argument that fossil birds such as _Archaeopteryx_ had a 
reversed hallux, simply because this toe is preserved in a reversed 
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.
------------


--- On Tue, 1/26/10, Tim Williams <tijawi@yahoo.com> wrote:

> From: Tim Williams <tijawi@yahoo.com>
> Subject: Re: FYI -- Microraptor gui, Burnham and Alexander, PNAS...
> To: dinosaur@usc.edu
> Cc: tijawi@yahoo.com
> Date: Tuesday, January 26, 2010, 8:32 PM
> 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-dimens
formed 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 3 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.Â
> 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 
ui_ (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.Â
> 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 
> 
> 
> 
> 
> 
> 
> 
> Â Â Â 
>