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Re: To Sprawl or Not to Sprawl?...
I just wanted to clarify how my opinions have changed since my 2003 post that
Jaime referred to. At the time I had not seen good photos or specimens of
microraptor femora, so I was simply skeptical of the idea pending more
information. I was skeptical because no other dinosaur had a limb structure
like what was being described for a spread-eagle Microraptor (i.e. I was being
conservative). Greg Paul pointed out (correctly) that evolutionary novelties
can and do show up, so clearly Microraptor _could_ have evolved a spherical
femoral head and rearranged the insertion points of its proximal limb
musculature. Having seen specimens (including the AMNH Novitate that is
available to everyone) that show otherwise, I think it is appropriate to take a
more pro-active stance. The morphology of the femora and pelvic girdle of the
CAGS specimens of M. zhaoianus clearly show that they were of the typical
theropod (non-sprawling) variety, so speculations about the hind "wings" flappi!
or being employed in a sub-horizontal manner need to be amended.
This being said, Chris' question about the taphonomy needs to be addressed. As
Jaime notes, not all specimens of Microraptor are preserved on their bellies.
Most, but not all, Confusciousornis specimens are. Some are spread-eagle.
Many fairly modern perching birds are found this way in the Green River
Formation of Wyoming (Eocene). I think GP's explanation that Confusciousornis
bodies were wider than deep does explain part of this.
What are the differences between the two? From what I've read, Solenhofen
is a fine-grained, low sedimentation rate depositional environment. With
increasing time before entrainment, organisms become more likely to come to
rest in their most stable configuration (on their sides, for Archaeopteryx).
Liaoning (again, from what I've read), between volcanism and what not, appears
to be a quicker sedimentation environment. More rapid (but still low energy)
deposition is more likely to catch organisms in less stable positions. So I
think we should expect the vertebrates in Liaoning to be found in a variety of
positions. Compactment of from overlying sediments and motion during burial
would cause some of them to be partially disarticulated, or in other unnatural
positions, just as we find them.
Assuming the published accounts of the sedimentology of the two sites are
correct, there doesn't seem to be anything anomalous about the positions we
find the fossils in.
Zoology & Physiology
University of Wyoming
Laramie, WY 82070