Sinosauropteryx - Classification

Classification

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.

There is only one named species of Sinosauropteryx, S. prima. A possible second species is represented by the specimen GMV 2124 (aka NGMC 2124), which was described as another, larger specimen of S. prima by Ji and Ji in 1997. However, in a 2002 presentation and abstract for the Society of Vertebrate Paleontology, Nick Longrich showed that this specimen differs in several anatomical aspects from the others, including its relatively large size, proportionally longer shins, and shorter tail. Longrich suggested that GMV 2124 was a compsognathid coelurosaurian, while Sinosauropteryx proper was a more primitive kind of coelurosaurian or even a basal carnosaurian. In 2007, Gishlick and Gauthier concurred that this specimen was probably a new taxon, and tentatively re-classified it as Sinosauropteryx? sp., though they suggested it may belong in a new genus. Also in 2007, Ji, Ji and colleagues wrote that GMV 2124 is probably a new genus, noting the differences in tail length and hindlimb proportions.