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Hip Structure in Dromaeosaurids (2004 SVP Abstracts)

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, 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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