Re: fossil birds and Recent parakeets

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>Now to my question.  A few years ago, the dominant belief amoungst
>paleo-ornithologists was that modern birds are monophyletic above the
>level of <I think it is> the clan that Hesperornis belongs to.  OK, I can
>buy that reasoning.  The major proponents of this phylogeny (I believe
>it was Olson, and possibly Feduccia as well), also proposed that the
>split between "shore" birds and "land" birds happened in the
>Late Cretaceous.

The shore and land split in the Cretaceous is now no longer as strongly
supported, since no K-bird has been shown to be member of a modern bird
order.  The shore/land split within the modern bird clade must have occured
early in the Cenozoic.

>
>My questions are these (they all relate to semantics...sorry folks):
>
>1) My long-held understanding of monophylogeny is that it is defined
>   as the radiation of organisms from a "common" ancestor.  But
>   couldn't a "common ancestor" be defined as a genus, or even, in
>   some rare cases, a family?  As I recall the definition in it's
>   strictest sense, a "common ancestor" is a group of individuals of
>   a single species that radiate (through speciation), due to
>   geographic isolation or whatever.

Yes.

>But the problem with modern
>   birds is that, because of the constraints placed on their
>   skeletal structure to enable them to fly, taxonimists have a
>   hard time seeing enough differences in their skeletons to use
>   bone structure in their phylogeny.  Flight apparently causes
>   hyper-convergence. In fact, so does perching (the developement of the
>opposable claw is required for any bird who spends nearly all of it's time
>in the trees, regardless of it's ancestory).
>

First off, flight apparatus does NOT cause hyper-convergence, as the wings
of pterosaurs, bats, and birds are all easily distinguishable from one
another.  Features of the skeleton (including many in the skull, hips, and
limbs) are useful in determining bird relationships.  Also, DNA and other
biomolecules are helpful in discerning modern bird phylogenies.

>2) Could it therefore be possible that modern birds are actually
>   polyphyletic, and we just don't have any way to distiguish between
>   primitive and derived traits?

Possible, yes, but it isn't parsimonious.  Remember that the result of a
phylogenetic analysis is an hypothesis of phylogeny (the best, most
parsimonious result of the data used), but may not be the true phylogeny.
The way we can distinguish primitive and derived characters is outgroup
analysis.

>3) If birds are monophyletic from a single species, _before_ the
>   KT boundary, then that assumes that either birds suffered a
>   near-extinction sometime in the Cretaceous, _or_ that there was
>   only one clan of flying theropod in the first place.
>

Yes and Yes.  There (probably) was only one origin of birds, and most of
the lineages (Enantiornithes, Hesperornithiformes, Ichthyornithiformes,
etc.) went extinct at or before the K-T boundary.

>4) Does Sinornis from the lower Cretaceous have any pleisiomorphies
>   that Archeopteryx lacks?

No.  Sinornis is derived or the same in all characters relative to
Archaeopteryx, but primitive in many compared to more advanced birds.

>
>5) If all modern birds are monophyletic above the level of Hesperornis, what
>is the name of this clade?  I can find no name for it in the literature.

To pattern cladists, this group is Aves (Archaeopteryx and all its
descendants are Avialae, not Aves).  To most others, it's Neornithes.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.