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Re: Journal of Negative Results -- Ecology & Evolutionary Biology
On 1/28/07, evelyn sobielski <koreke77@yahoo.de> wrote:
The lack of anything avian from the
Morrison Fm.
First of all, which definition of "Aves"? I assume sensu Chiappe (=
clade(Archaeopteryx + Aves sensu stricto))? Or sensu Marjanovic (:=
clade(Vultur <-- Velociraptor, Troodon, Oviraptor, and a buttload of
other traditionally non-avian specifiers))? (Clearly not sensu
Gauthier, i.e., the crown group.)
Second of all, as I recall there's undiagnostic eumaniraptoran
material from the Brushy Basin Member that could be avian sensu
Chiappe or sensu Marjanovic (please correct me if I'm wrong).
Finally, as you note further below, Archie's position is uncertain,
and it's conceivable that many animals considered deinonychosaurian
are avian sensu Chiappe. (Although in this case Aves sensu Marjanovic
is a bit more stable and simply drops Archaeopteryx.)
lets me conclude that "ornithization" -
accumulation of sufficient "bird" features by one or
more theropod lineage(s) to allow powered flight -
took place in Eurasia (at least initially; Rahonavis I
*think* is too primitive in regards to its avian
parts),
Its position seems about as volatile as that of Archie, but anyway, it
occurs far too late to tell us anything geographic about the origins
of flight.
It's also not a given that Archie was capable of powered flight.
but given the gap above the Mirrison Fm.
(which IIRC still stands) and the uncertainty of
Archie's phylogenetic position I'm not 100% on this.
The Enantiornithes certainly were equipped for
transoceanic dispersal (as seen by Alexornis,
Yungavolucris, Lectavis, the avisaurids, probably also
Nanantius...) as were contemporary lineages that
diverged during c.130 mya Jurassic at latest and later
produced the Hesperornithes, Ichthyornithes etc. But
the exact dispersal pattern is still rather unclear;
the Late Cretaceous record of avians from NAm is what
one could possibly wish for (though most is in bits
and pieces) but before 100-90 mya it drops off rather
abruptly; it would be nice to be able to tell whether
this is due to sampling bias or to genuine absence
from a comprehensive sample.
True. It seems to me that a lot of dinosaurian taxa are scarce for
that time period, so I would think sampling might be a likelier
explanation, although that should be backed up with some numbers.
--
T. Michael Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com
ISPN Forum: http://www.phylonames.org/forum