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Phylogeny of Maniraptora



In his concluding remarks to Chapter 11 -- "Feathers and Flight: Current
Ideas" from the 2002 book, _Dinosaurs: the Science Behind the Stories_,
Philip Currie writes:

"Active flight is what separates birds from their non-avian theropod
ancestors.  However, because there is a continuous series of evolutionary
steps between birds and their ancestors, it is difficult to know when
exactly active flight began.  Paleontologists therefore often consider
_Archaeopteryx_ as the division between theropods and birds, because this
animal has been proclaimed the "first bird" since 1861.  Using this scheme,
any animal that shares a common ancestor with _Archaeopteryx_ and modern
birds would be considered as a bird.  Theropods more primitive than this
common ancestor would not be considered as birds, even if they had feathers
and rudimentary wings.

"Under a modern and biological classification, birds are a subset of the
Dinosauria.  Thus, dinosaurs are not extinct, and are still one of the most
successful groups of vertebrates.  In spite of the fact that birds are
dinosaurs, they are also a very specialized group that we continue to call
birds."

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 quoted
above, if this basal dromaeosaur could fly, then it may be considered (as) a
bird, even though its appearance in the Early Cretaceous precludes it from
qualifying as a theropod ancestor of Jurassic birds.  On the other hand, its
precise phylogenetic relationship with _Archaeopteryx_ is a matter of
considerable debate.  It seems to me that the theropod family tree gets very
bushy near the base of Avialae, and there must be a tremendous diversity of
missing taxa, obscuring ancestor/descendant relationships.  Liaoning gives
us a glimpse of this.

If _Microraptor gui_ had been the oldest fossil discovered with asymmetrical
feathers, would it not have been given priority as the "first bird," and
therefore framed our identification of synapomorphies diagnosing the node
Avialae?

As Gregory S. Paul has noted, _Microraptor gui_ shares several features with
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?
Could it not have done both?  What is the evidence that _Archaeopteryx_ was
better equipped for flight than _Microraptor gui_?  What is the evidence for
the contrary view?

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.

This paper does not name the synapomorphies that exclude _M. gui_ from avian
status, but it is referred to as a "non-avian dromaeosaurid."  To what
extent is this taxonomic status -- and that of _Archaeopteryx_ -- arbitrary?
I recognize that the _Nature_ paper is only a preliminary description, and
that alternative views may result as more scientists study the specimens and
as new fossils are discovered and described.

-------Ralph W. Miller III
       ralph.miller@alumni.usc.edu