Segisaurus lived about 183 million years ago during the Jurassic period. Segisaurus was roughly the size of a goose and was a primitive bipedal theropod. Segisaurus was roughly 1 meter (3.3 feet) long, half a meter (1.65 feet) tall and weighed about 4-7 kilograms. It was nimble and insectivorous, although it may have scavenged meat also. It was bird-like in structure, with a flexible, elongated neck and stout body. Segisaurus was three-toed and had powerful legs that were long compared to its body length. Like its legs, Segisaurus had a long tail and long forearms. Its collar bone was not unlike a bird's, thus strengthening scientists' arguments that dinosaurs were related to avians. Segisaurus is described from the only specimen ever found, the holotype UCMP 32101, which was a sub-adult. The full size of Segisaurus as an adult may never be known. Strangely, clavicles were found on the Segisaurus specimen, which were unknown in other dinosaurs from that era. A speculative interpretation by Charles Lewis Camp was that the "splint-like" neck ribs supported a Draco-like patagium along the neck, to improve the animal's ability to move quickly. Segisaurus is significant because it demonstrates that the clavicle was primitively present in early theropods.
Segisaurus was described in 1936 by the paleontologist Charles Lewis Camp, based on specimen UCMP 32101, a fragmentary fossil skeleton which consisted of portions of the limbs, pelvis, and vertebrae. Cranial material was not recovered. Segisaurus went relatively ignored for the next half century. When the specimen was examined during this period, all who viewed it commented on the presence of clavicles and the apparently "solid" bones that the dinosaur had. Segisaurus appeared to be closely related to the better-known Coelophysis, but unlike the hollow bones of Coelophysis, Segisaurus had solid bones. This caused some scientists question whether Segisaurus was a theropod at all. In 2005, a re-examination of the Segisaurus holotype revealed that contrary to reports it did in fact have hollow bones.  In this study, Carano et al. found that although it was very unusual, Segisaurus was firmly a coelophysoid, and probably a close relative of Procompsognathus.
Distinguishing anatomical featuresEdit
A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.
According to Rauhut (2003), Segisaurus can be distinguished based on the following features:
- the dorsal centra are not very constricted ventrally
- the scapula is slender
- the humeral shaft has stronger torsion (~50 degrees) than does that of Coelophysis
- the presence of a large ischial fenestra (according to Carrano et al., 2005)
- the humeral deltopectoral crest is rectangular