Coelophysis is an extinct genus of coelophysid theropod dinosaurs that lived approximately 215 to 201.4 million years ago during the late Triassic period from the middle Norian to Rhaetian age in what is now the southwestern United States. Megapnosaurus was once considered to be a species within this genus, but this interpretation has been challenged and the genus Megapnosaurus is now considered valid.

Coelophysis was a small, slenderly built, ground-dwelling, bipedal carnivore that could grow up to 3 m (9.8 ft) long. It is one of the earliest known dinosaur genera. Scattered material representing similar animals has been found worldwide in some Late Triassic and Early Jurassic formations.

The type species C. bauri, originally placed in the genus Coelurus by Edward Drinker Cope in 1887, was described by the latter in 1889. The names Longosaurus and Rioarribasaurus are synonymous with Coelophysis. Coelophysis is one of the most specimen-rich dinosaur genera.

In April of 1887 American paleontologist Edward Drinker Cope first described vertebrate remains from the Triassic of New Mexico as new species of the genus Coelurus, including nearly all parts of the skeleton except the skull and teeth. While Coelurus had been considered by its describer Othniel Charles Marsh as an uncertain type of reptile, the new material described by Cope demonstrated that it was a dinosaur, and likely closely related to Megadactylus (now Anchisaurus) from the Triassic of Massachusetts. Cope chose to name two new species from his collection of New Mexican material, the first being Coelurus longicollis for multiple vertebrae and a femur, while the second was Coelurus bauri, named after German paleontologist Georg Baur, included the same regions of the skeleton. Following this description, Cope reclassified his New Mexican material in July 1887 from Coelurus to the German taxon Tanystrophaeus based on the similarities of the vertebrae, creating the new combinations Tanystrophaeus longicollis and Tanysteophaeus bauri. Cope described additional material of both T. longicollis and T. bauri to cover more of the anatomy of both species, and he also named the new species Tanystrophaeus willistoni for a partial pelvis even smaller than both other species, named after American paleontologist Samuel Wendell Williston. Following this, Cope came to the realization that the vertebrae of Tanystrophaeus he considered similar to the New Mexican species were not definitively caudal vertebrae, and as a result the species were in need of a new genus name. As a result, in 1889 Cope described the new genus Coelophysis, with three species C. longicollis, C. bauri, and C. willistoni, the name being a combination of the Ancient Greek words κοῖλος (koilos), "hollow", and φύσεις (fysis), "form".^{[citation needed]}

The original material of Coelophysis described by Cope was all collected by David Baldwin in 1881 from three localities of Rio Arriba County, New Mexico, with the only surviving collection information being labels written by Baldwin associated with the specimens, indicating a provenance of 120 m (400 ft) below a gypsum layer at "Arroyo Seco" and "Gallina Canyon". The layer of gypsum identified by Baldwin is now known as part of the Middle Jurassic Todilto Formation that overlies the Chinle Formation within the region of Ghost Ranch that "Arroyo Seco" (Arroyo Canjilon or Canjilon Creek) flows past, so while no other information on the area Baldwin collected can be known for certain, he must have collected from within a 4.8 km (3 mi) region around Ghost Ranch where the river cuts through the Chinle Formation below the Todilto gypsum layer. Williston and Ermine Cowles Case believed they located the area of Baldwin's collecting in a 1912 visit to the region where they located fragments they assigned to Coelophysis on the surface.

The German paleontologist Friedrich von Huene revisited the material and taxonomy of Coelophysis in a 1906 review of Triassic dinosaurs, following the species identifications of Cope and illustrating the material for the first time, with the additional description of elements of the hand of C. longicollis and vertebrae of C. bauri that Cope had not described. Huene did not see justification in considering Coelophysis a close relative of Anchisaurus (a replacement name for Megadactylus), instead classifying the genus within Coeluridae. In a further 1915 review limited to the New Mexican collection of Cope, Huene separated the specimens into the three species differently from previously, assigning bones based on size with C. longicollis as the largest species and C. willistoni as the smallest and suggesting that more than three species may be present. No type species for Coelophysis was designated by Huene, nor were specimen numbers identified, complicating the assignment of material to the species. In 1986 American paleontologist Kevin Padian revisited the type material of Coelophysis and identified which material was referred to which species by Cope and Huene in their studies, and identified some material in the Cope collection that had yet to be described.

During the summer of 1946, American paleontologist Edwin H. Colbert began searching for Triassic fossils in the Petrified Forest National Park of northern Arizona, under the employment of the American Museum of Natural History. This exploration continued the following year, where Colbert planned to stop at Ghost Ranch on the way to Petrified Forest, where many skulls of phytosaurs had been found. Colbert, along with George Simpson, Bill Fish, George Whitaker, and Tom Ierardi, were the field crew for the 1947 season, with Simpson and Fish focusing on Eocene mammals of northern New Mexico while Colbert, Whitaker, and Ierardi would continue to northern Arizona for Triassic reptiles. On 21 June, Colbert, Whitaker and Ierardi began exploring the sediments of the Chinle Formation in the area near Ghost Ranch, uncovering a complete phytosaur skull, and on 22 June Whitaker discovered some fragments including a claw that Colbert identified as Coelophysis, from a talus slope above an intermittent stream. Upon excavating bones on the surface and removing some of the talus, a fossil layer was found containing numerous Coelophysis bones. After several more days of excavations, it was understood that the fossil layer contained hundreds of skeletons of Coelophysis in a single concentration, and upon the return of Simpson and Fish to the locality the former stated that "this was the greatest find ever made in the Triassic of North America".

Excavations of the Ghost Ranch Coelophysis quarry were continued throughout 1947 and 1948 by the crew of the AMNH, which also included Carl Sorensen after arrival from New York City, assisted by the landowners Arthur and Phoebe Pack and farm hand Herman Hall. Seven large blocks were excavated, each containing numerous skeletons, numbered I to VI for identification of location when transported back to the AMNH. Around 105 smaller blocks were also collected, created by the channels excavated to separate the larger blocks. A further 6 blocks, VII to XIII, were excavated as the quarry expanded to several meters in diameter, with the blocks being distributed amongst the AMNH, Museum of Northern Arizona, New Mexico Museum of Natural History, and Yale Peabody Museum. All of the excavations at Ghost Ranch took place within the upper sandstone level of the Chinle Formation, believed to be Norian in age at the time and making the expanse of Coelophysis one of the longest deposits within the Chinle despite being one of the earliest known dinosaurs. Colbert believed that the original material of Coelophysis may have been collected from the surface in a similar method to Whitaker's discovery, as part of the same fossil layer that was excavated by the AMNH.