Re: To Sprawl or Not to Sprawl?...

[DinoBoyGraphics@aol.com]

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!
 ng!
 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. 


Scott Hartman
Zoology & Physiology
University of Wyoming
Laramie, WY 82070

(307) 742-3799