Sinosauropteryx (meaning "Chinese lizard-wing") is the first and most primitive genus of dinosaur found with the fossilized impressions of feathers. It lived in China during the early Cretaceous period and may have been a close relative of Compsognathus. It was the first dinosaur genus discovered in the famous Liaoning Province. The largest known specimens are 1-1.20 meters (3 ft) in length, most of which was taken up by its extremely long tail. The remarkably well-preserved fossils show that Sinosauropteryx was covered with a furry down of very simple feathers - though some contention arose with an alternative interpretation of the filamentous impression as collagen fiber remains. These filaments consisted of a simple two-branched structure, roughly similar to the secondarily primitive feathers of the modern kiwi.
Three or four specimens are known: the holotype GMV 2123 (NIGP 127586), NIGP 127587, D 2141, and GMV 2124. The assignment of the latter to S. prima is disputed.
Most paleontologists do not consider Sinosauropteryx to be a bird, because phylogenetically, it lies far from the clade Aves, usually defined as Archaeopteryx + modern birds. The scientists who discovered and described Sinosauropteryx, however, used a character-based (apomorphy) definition of the Class Aves, that is, any animal with feathers is a bird. They argued that the filamentous plumes of Sinosauropteryx represent true feathers with a rachis and barbs, and therefore that Sinosauropteryx should be considered a true bird. They classified it as belonging to a new biological order, Sinosauropterygiformes, family Sinosauropterygidae, within the subclass Sauriurae.
The specimen GMV 2124 was found with three mammal jaws in its stomach region. Hurum, Luo & Kielan-Jaworowska (2006) identified these jaws as belonging to Zhangheotherium (two jaws) and Sinobaatar (the third jaw), showing that these two mammals were part of the Sinosauropteryx diet. Interestingly, Zhangheotherium is known to have had a poisonous spur, like the modern platypus, showing that Sinosauropteryx fed on possibly poisonous mammals.
More recently, it was proposed that GMV 2124 does not belong to Sinosauropteryx but to a distinct (possibly closely related) species with proportionally longer tibiae and a shorter tail.
See also: Feathered dinosaurs
Some researchers interpret the filamentous impression around Sinosauropteryx fossil as remains of collagen fiber, forming a frill on the back of the animal and underside of its tail.
This hypothesis denies the proposal that Sinosauropteryx is the most basal known theropod genus with feathers and also questions the current theory of feather origins itself. It calls into question the theory that the first feathers evolved not for flight but for insulation, and that they made their first appearance in relatively basal dinosaur lineages that later evolved into modern birds. 
However, even if all of the known specimens of Sinosauropteryx that preserve feathery structures are invalidated, Dilong, which is another basal, non - maniraptoran coelurosaur, is known to have simple filamentous protofeathers preserved with its fossils.
1. ^ a b Ji, Q., and Ji, S. (1996). "On discovery of the earliest bird fossil in China and the origin of birds." Chinese Geology 10 (233): 30-33.