Hip Structure in Dromaeosaurids (2004 SVP Abstracts)

[Dino Guy Ralph <ralph.miller@alumni.usc.edu>]

On pp. 251-252 of the 304 page Abstracts for the upcoming SVP meeting 
(November 3-6,
2004 in Denver, Colorado) currently available on line at
http://www.vertpaleo.org/meetings/2004_SVP_abstracts.pdf, you will note the 
following abstract, which provides a hypothesis for the function of the 
"hind-wings" of _Microraptor gui_, the baffling "four-winged dinosaur," and 
the implications for the origin of flight in the theropod lineage:

Xu, X., Z., Zhang, F., Wang, X. and Kuang, X.: FUNCTIONAL HIND-WINGS CONFORM 
TO THE HIP STRUCTURE IN DROMAEOSAURIDS
XU, Xing, ZHOU, Zhonghe, Institute of Vertebrate Paleontology & 
Paleoanthropology, Beijing, China;  ZHANG, Fucheng, Institute of 
Paleontology & Paleoanthropology, Beijing, China; WANG, Xiaolin, Institute 
of Vertebrate Paleontology & Paleoanthropology, Beijing, China; KUANG, 
Xuewen, Tianjin Museum of Natural History, Tianjin, China
 An arboreal-gliding hypothesis of flight origin has long been thought to 
be implausible with the theropod ancestry of birds supported by all modern 
cladistic analyses.  Recent discoveries of basal dromaeosaurids with pedal 
features comparable to arboreal birds indicate that some non-avian theropods 
might have been arboreal.  Furthermore, the long and asymmetric metatarsal 
feathers of these dromaeosaurids are inconsistent with any cursorial habit. 
The above evidence was presented to support an arboreal-gliding hypothesis, 
but whether the functional hind-wings are present in basal dromaeosaurids 
remains an unresolved issue.  Here we propose that while taking off, the 
hind-limbs of basal dromaeosaurids were capable of stretching posteriorly 
and also deflecting laterally in a position that the long pennaceous 
feathers of the hind-limbs were placed in a subparallel position with 
respect to the tail.  In this posture, the legs and tail surface made a 
lifting-surface altogether.  Such a posture is slightly different from the 
parasaggital posture of dinosaurs and inconsistent with the osteological 
features of the pelvis and hind-limbs of eumaniraptorans.  A significant 
modification seen in eumaniraptorans is the posterolateral divergence of the 
iliac blades, a design that allows the posterolateral orientation of the 
hind-wings.  This modification, together with the opisthopubic condition, 
makes a flat caudal portion of the pelvis for eumaniraptorans.  Recent 
reexaminations of _Archaeopteryx_ specimens revealed the proportionately 
long pennaceous feathers along the tibia, which suggests the presence of a 
diminutive hind-wing.  Based on this evidence, we propose that primitive 
eumaniraptorans developed two lift-generating airfoils: the front-wings 
(also the thrust-generator) and the hind-wings (formed by both hind-limbs 
and tail); during early avian evolution the front-wings became the main 
airfoil while the hind-wings lost their role in producing lift.  _M. gui_ 
represents an early stage in the evolution of flight with two 
lift-generating surfaces; _Archaeopteryx_ reduced its leg feathers but 
compensated with a large feathered tail.

--------
"Dino Guy" Ralph W. Miller III
Docent at the California Academy of Sciences
proud member of the Society of Vertebrate Paleontology
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