Coelurus (/sɪˈljʊərəs/ si-LURE-əs) is a genus of coelurosaurian dinosaur from the Late Jurassic period (mid-late Kimmeridgian faunal stage, 155–152 million years ago).

Although its name is linked to one of the main divisions of theropods (Coelurosauria), it has historically been poorly understood, and sometimes confused with its better-known contemporary Ornitholestes.

Like many dinosaurs studied in the early years of paleontology, it has had a confusing taxonomic history, with several species being named and later transferred to other genera or abandoned.

Coelurus was described in 1879 by Othniel Charles Marsh,[1] an American paleontologist and naturalist known for his "Bone Wars" with Edward Drinker Cope.

Marsh was impressed with the hollow interiors of the thin-walled vertebrae, a characteristic that gave the type species its name: Coelurus fragilis.

[5] Marsh elected to place some of the material in a new species, C. agilis, on the strength of a pair of fused pubic bones he thought belonged to an animal three times the size of C.

gracilis, based on unknown remains only represented today by a single claw bone pertaining to a small theropod from the Early Cretaceous Arundel Formation of Maryland.

Despite their professional animosity, Cope also assigned species to Coelurus; in 1887, he named fossils from the Late Triassic of New Mexico as C. bauri and C.

[5] At the time, Dale Russell had proposed that C. agilis was a species of Elaphrosaurus based on the incomplete information then published;[15] Ostrom was also able to demonstrate that this was not the case.

[3] The three best-known small theropods of the Morrison Formation — Coelurus, Ornitholestes, and Tanycolagreus — were generalized coelurosaurs, and they have been mistaken for each other at various times.

[18] Since the growth of phylogenetic studies in the 1980s, Coelurus has usually been found to be a coelurosaurian of uncertain affinities, not fitting with the better-known clades of the Cretaceous.

[26][27][28][19][29] The phylogenetic analysis conducted by Rauhut (2003) and Smith et al. (2007) found that Coelurus was more closely related to compsognathids than to other coelurosaurs.

[19][29][31] However, a work published by Phil Senter in 2007 following the description of Tanycolagreus found it and Coelurus to be closely related at the base of Tyrannosauroidea.

Before the use of phylogenetic analyses, Coeluridae and Coelurosauria were taxonomic wastebaskets used for small theropods that did not belong to other groups; thus, they accumulated many dubious genera.

[25][23] As late as the 1980s, popular books recognized over a dozen "coelurids", including such disparate forms as the noasaurid Laevisuchus and the oviraptorosaurian Microvenator, and considered them descendants of the coelophysids.

[37] It remains unclear whether or not this group contains any species other than Coelurus itself, and while Tanycolagreus is often included, support for this relationship has been weak in most of the studies that recovered it.

The specimen was collected by Reed in gray sandstone and brown/green claystone that were deposited during the Kimmeridgian stage of the Jurassic period, approximately 157 to 152 million years ago.

Such dinosaurs as the theropods Ceratosaurus, Allosaurus, Ornitholestes, and Torvosaurus, the sauropods Apatosaurus, Brachiosaurus, Camarasaurus, and Diplodocus, and the ornithischians Camptosaurus, Hesperosaurus, Nanosaurus, Fruitadens, Dryosaurus, and Stegosaurus are known from the Morrison.