Sinosauropteryx
Three specimens have been assigned to "Sinosauropteryx prima": the holotype GMV 2123 (and its counter slab [opposite face], NIGP 127586), NIGP 127587, and D 2141. Another specimen, IVPP V14202, was assigned to the genus but not to the only species by Zhang and colleagues. The assignment of an additional larger specimen to "S. prima", GMV 2124, was later found to be in error. All of the fossils were found in the Jianshangou or Dawangzhangzi Beds of the Yixian Formation in the Beipiao and Lingyuan regions of Liaoning, China. These fossil beds have been dated to 124.6–122 million years ago, during the early Aptian stage of the Early Cretaceous.
Sinosauropteryx
Despite its feathers, most palaeontologists do not consider "Sinosauropteryx" to be a bird. Phylogenetically, the genus is only distantly related to the clade Aves, usually defined as "Archaeopteryx lithographica" plus modern birds. The scientists who described "Sinosauropteryx", however, used a character-based, or apomorphic, definition of the Class Aves, in which any animal with feathers is considered to be a bird. They argued that the filamentous plumes of "Sinosauropteryx" represent true feathers with a rachis and barbs, and thus that "Sinosauropteryx" should be considered a true bird. They classified the genus as belonging to a new biological order, Sinosauropterygiformes, family Sinosauropterygidae, within the subclass Sauriurae. These proposals have not been accepted, and "Sinosauropteryx" is generally classified in the family Compsognathidae, a group of small-bodied long-tailed coelurosaurian theropods known from the Late Jurassic and Early Cretaceous of Asia, Europe, and South America.