Sinosauropteryx - Description

Description

Sinosauropteryx prima were small bipedal theropods, noted for their short arms, large first fingers (thumbs), and long tails. The species includes some of the smallest known adult non-avian theropod specimens, with the holotype specimen measuring only 68 cm (27 in) in length, including the tail. This individual was relatively young. The longest known specimen reaches up to 1.07 m (3.5 ft) in length, with an estimated weight of 0.55 kg (1.2 lb).

Sinosauropteryx were anatomically similar to Compsognathus, differing from their European relatives in their proportions. The skulls of Sinosauropteryx were 15% longer than their thigh bones, unlike Compsognathus, in which the skulls and thigh bones are approximately equivalent in length. The arms of Sinosauropteryx (humerus and radius) were only 30% the length of their legs (thigh bone and shin), compared to 40% in Compsognathus. Additionally, Sinosauropteryx had several features unique among all other theropods. S. prima had 64 vertebrae in their tails. This high number helps give them the longest tails relative to body length of any theropod species. Their 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 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.

A pigmented area in the abdomen of the holotype has been suggested as possible traces of organs, and was interpreted as the liver by John Ruben and colleagues, which they described as part of a crocodilian-like "hepatic piston" respiratory system. A later study, while agreeing that the pigmented area represented something originally inside the body, found no defined structure and noted that any organs would have been distorted by the processes that flattened the skeleton into an essentially two-dimensional form. Dark pigment is also present in the eye region of the holotype and another specimen.

Three specimens have been assigned to Sinosauropteryx prima: the holotype GMV 2123 (and its counter slab, NIGP 127586), NIGP 127587, and D 2141. Another specimen, IVPP V14202, was assigned to the genus but not to a 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 late Barremian to early Aptian stages of the Early Cretaceous.