Sinornithosaurus millenii ("Chinese lizard-bird of the new millennium") is a feathered dromaeosaurid dinosaur genus from the Lower Cretaceous (Middle Barremian) of the Yixian Formation in China. It is the fifth feathered dinosaur exposed, and is the neighboring of them all to the birds. It provides additional proof supporting the "ground up" theory of flight, which proposes that feathers first developed in terrestrial dinosaurs, instead of in climbers. It also suggests that additional dromaeosaurids, like Velociraptor, may have had feathers.
The feeling of proto-feathers was found in the rock surrounding Sinornithosaurus. They were calm of filaments, and showed two features that indicate they are early feathers. First, several filaments were connected together into "tufts", similar to the way down is structured. Second, a row of filaments (barbs) were joined as one 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 current feathers have, which allow the feathers of modern birds to form a discrete vane.
This chains the "ground up" theory of avian flight. The "tree down" theory postulates that birds evolved from tree-climbing (arboreal) dinosaurs, which glided from tree to tree. The "ground up" theory, on the other hand, suggests that birds descended from running dinosaurs, which used their feathers for lagging or as part of mating displays, before they started using them to fly.
In addition to the feathers, Sinornithosaurus might flap its arms — it is the first dinosaur exposed with a bird-like shoulder girdle. It also has a bird-like pelvic girdle and hind limbs, and very long arms.
The dromaeosaurids are a collection of agile, meat-eating dinosaurs with large claws and big brains, which comprise the Deinonychus and the Utahraptor. As a group, they have been badly represented in the fossil record, and are known only from scattered bones and partial skeletons.
Sinornithosaurus lived about 125 million years before in the Barremian age of the Lower Cretaceous time, which makes it the earliest and almost certainly the most primitive dromaeosaurid yet discovered. Sinornithosaurus is the fifth known feathered dinosaur genera, but all the others are higher. 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 higher species may have lost them.
Analysis of known features also indicates that the dromaeosaurid family is more intimately related to birds than the troodontids (see also: cladistics). Most paleontologists now consider that birds branched off from the dromaeosaurids, and much earlier than was previously believed, maybe 150 million years past, during the late Jurassic.
Sinornithosaurus was exposed 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 configuration; the same incredibly rich location where four dinosaurs with feathers were exposed previously, Protarchaeopteryx, Sinosauropteryx, Caudipteryx, and Beipiaosaurus.