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SinornithosaurusSinornithosaurus millenii (meaning "Chinese bird lizard of the new millennium") is the fifth feathered dinosaur discovered, and is the closest of them all to the birds. It provides additional evidence supporting the "ground up" theory of flight, which proposes that feathers first developed in terrestrial dinosaurs, instead of in climbers. It also suggests that all dromaeosaurids, like the Velociraptor, may have had feathers. Fuzzy-feathered runner The impression of proto-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), similar to the way normal bird feathers are designed. 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 supports 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. In addition to the feathers, Sinornithosaurus could flap its arms — it is the first dinosaur discovered with a bird-like shoulder girdle. It also has a bird-like pelvic girdle and hindlimbs, and very long arms. Classification The dromaeosaurids are a group of agile, meat-eating dinosaurs with large claws and big brains, which include the Deinonychus and the Utahraptor. As a group, they have been poorly represented in the fossil record, and are known only from scattered bones and partial skeletons. Sinornithosaurus lived about 125 million years ago in Barremian age of the early Cretaceous period, which makes it the earliest and probably the most primitive dromaeosaurid yet discovered. Sinornithosaurus is the fifth known feathered dinosaur genera, but all the others are more advanced. The presence of proto-feathers on an early dromaeosaurid indicates that all dromaeosaurids may have had proto-feathers instead of scales, even if later and more advanced species may have lost them. Analysis of known features also indicates that the dromaeosaurid family is more closely related to birds than the troodontids. (See also: cladistics.) Most paleontologists now believe that birds branched off from the dromaeosaurids, and much earlier than was previously believed — maybe 150 million years ago, during the late Jurassic. Discovery 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 proto-feather impressions, was recovered from Liaoning Province, China, in the Yixian rock formation — the same incredibly rich location where four dinosaurs were previously discovered with feathers: Protarchaeopteryx, Sinosauropteryx, Caudipteryx, and Beipiaosaurus. Further reading - "A dromaeosaurid dinosaur with a filamentous integument from the Yixian Formation of China," by Xing Xu, Xiao-Lin Wang and Xiao-Chun Wu. September 16, 1999. Nature, issue 401, pages 262 to 266. (abstract)
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