Sinornithosaurus (derived from a combination of Latin and Greek, meaning 'Chinese bird-lizard') is a feathered dromaeosaurid dinosaur genus from the Lower Cretaceous Period (Middle Barremian) of the Yixian Formation in what is now China. It was the fifth known feathered dinosaur genus discovered and at that time the closest of them all to the birds. It also suggests that other dromaeosaurids, like Velociraptor, may have had feathers.

Fuzzy-feathered runner

The impression of feathers was found in the rock surrounding Sinornithosaurus. They were composed of filaments, and showed two features that indicate they are early feathers. First, several filaments were joined together into "tufts", similar to the way down is structured. Second, a row of filaments (barbs) were joined together to a main shaft (rachis), making them similar in structure to normal bird feathers. However, they do not have the secondary branching and tiny little hooks (barbules) that modern feathers have, which allow the feathers of modern birds to form a discrete vane.

This was taken to support the "ground up" theory of avian flight. The "tree down" theory postulates that birds evolved from tree-climbing (arboreal) dinosaurs, who glided from tree to tree. The "ground up" theory, on the other hand, suggests that birds descended from running dinosaurs, who used their feathers for insulation or as part of mating displays, before they started using them to fly. However, the discovery of Microraptor's four wings and the increasing knowledge of early avian evolution indicates that this is likely a gross oversimplification, with both modes of evolving flight occurring in various "paravian" (close to the ancestors of birds) dinosaur species according to their respective mode of life.

In addition to the feathers, Sinornithosaurus could flap its arms — it is one of the first dinosaurs discovered with a bird-like shoulder girdle. It also has a bird-like pelvic girdle and hindlimbs, and very long arms.

Classification

NGMC 91, or "Dave", is a probable specimen of Sinornithosaurus.

Sinornithosaurus was a member of the family Dromaeosauridae, a group of agile, meat-eating dinosaurs with a distinctive sickle-shaped toe claw, which also includes Deinonychus and Utahraptor. It lived about 125 million years ago in the Barremian age of the Lower Cretaceous period, which makes it one of the earliest and most primitive dromaeosaurids yet discovered. The presence of feathers on such an early dromaeosaurid indicates that later dromaeosaurids may also have had feathers (instead of scales or simpler "proto-feathers").

Sinornithosaurus is known from at least two species. S. millenii ("millennium Chinese bird-lizard") is the type species, described in 1999. A second species, S. haoiana ("Hao's Chinese bird-lizard") was described by Liu et al. in 2004 based on a new specimen which differed from S. millenii in features of the skull and hips. An incredibly well-preserved microraptorian nicknamed "Dave" (specimen NGMC 91) may represent a third species of Sinornithosaurus, or a juvenile.

Discovery

The holotype fossil of Sinornithosaurus millenii.

Sinornithosaurus was discovered by Xing Xu, Xiao-Lin Wang and Xiao-Chun Wu of the Institute of Vertebrate Paleontology and Paleoanthropology of Beijing. An almost-complete fossil with feather impressions, was recovered from Liaoning Province, China, in the Yixian Formation; the same incredibly rich location where four dinosaurs with feathers were discovered previously, Protarchaeopteryx, Sinosauropteryx, Caudipteryx, and Beipiaosaurus.

References

• Liu, J., Ji, S., Tang, F. & Gao, C. (2004). "A new species of dromaeosaurids from the Yixian Formation of western Liaoning.". Geological Bulletin of China 23 (8): 778-783.  (abstract)
• Xu, X., Wang, X.L., and Wu, X.C. (16 September 1999). "A dromaeosaurid dinosaur with a filamentous integument from the Yixian Formation of China". Nature 401: 262–266.  (abstract, doi:10.1038/45769)

External links

• Wang, L.. "Dinosaur fossil yields feathery structures", Science News, March 10, 2001. 
• Sinornithosaurus at DinoData

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