Sinosauropteryx
"Sinosauropteryx" was a small theropod with an unusually long tail and short arms. The longest known specimen reaches up to in length, with an estimated weight of It was a close relative of the similar but older genus "Compsognathus", both genera belonging to the family Compsognathidae. Only one species of "Sinosauropteryx" has been named: "S. prima", meaning "first" in reference to its status as the first feathered non-avialian dinosaur species discovered. Three specimens have been described. The third specimen previously assigned to this genus represents either a second, as-yet unnamed species or a distinct, related genus.
Sinosauropteryx
"Sinosauropteryx" was anatomically similar to "Compsognathus", differing from its European relatives in its proportions. The skull of "Sinosauropteryx" was 15% longer than its thigh bones, unlike in "Compsognathus", where the skull and thigh bones are approximately equivalent in length. The arms of "Sinosauropteryx" (humerus and radius) were only 30% the length of its legs (thigh bone and shin), compared to 40% in "Compsognathus". Additionally, "Sinosauropteryx" had several features unique among all other theropods. It had 64 vertebrae in its tail. This high number made its tail the longest relative to body length of any theropod. Its hands were long compared to its arms, about 84% to 91% of the length of the rest of the arm (humerus and radius), and half the length of the foot. The first and second digits were about the same length, with a large claw on the first digit. The first fingers were large, being both longer and thicker than either of the bones of the forearm. The teeth differed slightly (they were heterodont) based on position: those near the tips of the upper jaws (on the premaxillae) were slender and lacked serrations, while those behind them (on the maxillae) were serrated and laterally compressed. The teeth of the lower jaws were similarly differentiated.