Three specimens have been assigned to Sinosauropteryx prima: the holotype GMV 2123 (NIGP 127586), NIGP 127587, D 2141. The assignment of a fourth, larger specimen to S. prima was later found to be in error.
Sinosauropteryx prima is among the smallest known non-avian theropods, with the type specimen (a nearly mature individual) measuring only 68 centimeters (2.2 ft) in length, including a very long tail. The largest known specimens reach up to 98 cm (3 ft) in length. Sinosauropteryx is distinguished from other small dinosaurs by several features, including having a skull longer than its upper leg bone (femur) and very short, stout forelimbs, with the arms being only 30% the length of the legs. Overall, Sinosauropteryx had proportionately shorter limbs than its close relative Compsognathus.
In addition, Sinosauropteryx had several features unique among all other theropods (bipedal, mainly carnivorous dinosaurs). S. prima had 64 vertebrae in its tail, giving it the longest tail relative to body length of any theropod. It also had very large fingers for its small arms, with the second finger and claw being longer than the entire lower arm (radius).
Despite its feathers, most palaeontologists do not consider Sinosauropteryx to be birds. 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.
Below is a cladogram showing the placement of Sinosauropteryx within Coelurosauria by Senter et al. in 2012. Template:Clade