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Re: Phylogeny of Maniraptora

Tim Williams wrote:
> The phugoid gliding phase posited by
> Chatterjee and Templin's (2003) for _Archaeopteryx_ illustrates one
> gradational pathway for the evolution of active flight.  Phugoid gliding 
> occupies the grey zone between passive gliding and active flight.

Doesn't this make a perhaps unwarranted presumption that passive gliding
precedes active flight?

> The fact that the forelimbs of
> _M. gui_ are considerably shorter than the hindlimbs (see Fig 1c in Xu et
> al. [2003]) gives pause to the interpretation that it was an active flier.

Why? Perhaps it used the forewings as stationary canards while flapping
with the hindwings?  Note that I don't necessarily buy into this
scenario myself.  I only point out that it can't be discounted without
investigation, and insofar as I know -- it hasn't been investigated
yet.  As always, there's more than one way to skin a cat.

> Also, the hypothesis that _M. gui_ was an active flier must account for how
> two pairs of wings were deployed in this behavior.  This burden does not
> fall on _Archaeopteryx_, since its two-winged configuration is the same as
> in all volant birds.

Perhaps large (non-vertebrate) dragonflies might be able to partially
address this issue.

>  As far as we know, there are no four-winged vertebrate
> fliers.  Bats (and presumably pterosaurs) may use both the fore- and
> hindlimbs to support the flight surface, but in this case all four limbs are
> integrated into the same flight surface - i.e., two wings, not four.

Doesn't this presume that all pterosaurs incorporate the hindlimb into
the flight membrane?  And mightn't that be totally inconsistent with the
known wing TE position in the vicinity of the elbow for all known
fossilized pterosaur wing membranes? Those known to me anyway.....
Perhaps in some pterosaurs the hindlimb/tail/uropatagium complex was
totally independent of the wing -- perhaps with about 1/3 the surface
area of the wing and half the aspect ratio, flying at about half the
lift coefficient.  It works, it solves some yaw command authority
issues, and it is consistent with the fossil record.  Note that I
personally think there were enough pterosaur niches to be filled and
enough pterosaur species filling them that both conditions might well
arise, with the independent condition more likely where yaw authority
issues provided a need.
> >Could it not have done both?
> At some stage, this is quite possible, if flapping was superposed upon
> previous passive gliding.

At some stage, this is quite possible, if passive gliding was superposed
upon previous active flapping.

And your scenario is quite possible too.  It doesn't address which came
first.  And perhaps neither avenue was exclusive to the other.

> >Did the "four-winged" stage precede two winged animals, or are the
> >"four-winged" theropods descended from theropods that sported only pectoral
> >wings?  There is probably no way of telling at this time.

Why do we always use binary logic?  What's wrong with trinary?

> Currently, there is no reason to assume that birds
> are descended from Beebe's "tetrapteryx" glider.  _M. gui_ and its fellow
> four-winged aerobats might have represented an aerodynamic dead-end.

I agree.