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Re: Phylogeny of Maniraptora
Ralph W. Miller III <email@example.com> wrote:
Needless to say, Philip Currie's analysis did not take account of
_Microraptor gui_, the "four-winged dinosaur" described in _Nature_ in
January 23, 2003. By the informal reasoning stated in the first line
above, if this basal dromaeosaur could fly, then it may be considered (as)
bird, even though its appearance in the Early Cretaceous precludes it from
qualifying as a theropod ancestor of Jurassic birds.
This highlights the major problem in defining clades on the basis of
presumed behavior. I'm not claiming Phil Currie did this, but Gauthier and
deQueiroz (2001) endeavored to use active flight as the defining feature of
Avialae (birds). Using character-based definitions for clades is dangerous
enough; using behavior-based definitions is playing with dynamite.
As you point out, the debate over whether _Microraptor_ and even
_Archaeopteryx_ were capable of "true" (i.e. active or sustained) flight
makes this a contentious issue. 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.
As Gregory S. Paul has noted, _Microraptor gui_ shares several features
derived birds that are not evident on _Archaeopteryx_ specimens. The
_Nature_ article by Xu et al. puts forward the hypothesis that this animal
represents a gliding tetrapteryx stage from the dromaeosaur lineage that
ultimately gave rise to flying birds. Did it indeed glide, but not fly?
Alas, we're probably never going to know. The fact that the forelimbs of
_M. gui_ are considerably shorter than the hindlimbs (see Fig 1c in Xu et
al. ) gives pause to the interpretation that it was an active flier.
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. 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.
Could it not have done both?
At some stage, this is quite possible, if flapping was superposed upon
previous passive gliding.
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.
You're right; we'll need more fossils to tell us whether the "two-winged"
configuration of birds or the "four-winged" configuration of _M. gui_ (and
_Cryptovolans pauli, if the two are not the same) is primitive for the
Paraves/Eumaniraptora. 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.
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