Coelurus

Coelurus (/sɨˈljʊərəs/ si-LEWR-əs) is a genus of coelurosaur dinosaur from the Late Jurassic period (mid-late Kimmeridgian faunal stage, 153–150 million years ago). The name means "hollow tail", referring to its hollow tail vertebrae (Greek κοιλος, koilos = hollow + ουρα, oura = tail). 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. Only one species is currently recognized as valid: the type species, C. fragilis, described by Othniel Charles Marsh in 1879. It is known from one partial skeleton found in the Morrison Formation of Wyoming, United States. It was a small bipedal carnivore with elongate legs.

Description
Coelurus is known from most of the skeleton of a single individual, including numerous vertebrae, partial pelvic and shoulder girdles, and much of the arms and legs, stored at the Peabody Museum of Natural History; however, the relative completeness of the skeleton was not known until 1980. The fossils were recovered from Reed's Quarry 13 at Como Bluff, Wyoming.[1][2] Additionally, two arm bones possibly belonging to this genus are known from the Cleveland Lloyd Dinosaur Quarry in Utah.[2] It was not a large dinosaur. Its weight has been estimated at around 13 to 20 kilograms (29 to 44 lb),[3][4] with a length of about 2.4 meters (7.9 ft) and a hip height of 0.7 meters (2.3 ft).[3] From reconstructions of the skeleton, Coelurus had a relatively long neck and torso due to its long vertebrae, a long slender hindlimb due to its long metatarsus, and potentially a small slender skull.[2]

The skull is unknown except for possibly a portion of lower jaw found at the same site as the rest of the known Coelurus material. Although it has the same preservation and coloring as the fossils known to belong to the Coelurus skeleton, it is very slender, which may mean it doesn't belong to the skeleton; this bone is 7.9 centimeters long (3.1 in) but only 1.1 centimeters tall (0.43 in). In general, its vertebrae were long and low, with low neural spines and thin walls to the bodies of the vertebrae. Its neck vertebrae were very pneumatic, with numerous hollow spaces on their surfaces (pleurocoels); these hollows were not evenly distributed among the vertebrae and varied in size. The neck vertebrae were very elongate, with bodies four times longer than wide, and they articulated with concave faces on both ends (amphicoely). The back vertebrae were not as elongate, lacked surface hollows, and had less developed concave faces and bodies that were hourglass-shaped. The tail vertebrae also lacked surface hollows.[2]

The only bone known from the shoulder girdle is a fragment of scapula. The upper arm had a distinct S-shaped curve in side view and was slightly longer than the forearm (11.9 centimeters [4.7 in] versus 9.6 centimeters [3.8 in]). The wrist had a semilunate carpal† similar to that of Deinonychus, and the fingers were long and slender. The only bone known from the pelvic girdle is paired and fused pubis bones, which had a prominent, long "foot" at the end. The thigh bones had an S-shape when viewed from the front. The metatarsals were unusually long and slender, nearly the length of the thigh bones (the best preserved thigh bone is about 21 centimeters long [8.3 in]).[2]

Coelurus, Ornitholestes, and Tanycolagreus
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. Now that Coelurus and Ornitholestes have been more fully described, it is possible to distinguish them by various characteristics of their anatomy. For example, they had visibly different proportions: Coelurus had a longer back and neck than Ornitholestes, and longer, more slender legs and feet.[2] Coelurus and Tanycolagreus are more similar, but differ in a variety of details. Such details include the shape of the upper arm, forearm, and thigh bones; the location of muscle attachments on the thigh bone, proportionally longer back vertebrae; and, again, the very long metatarsus of Coelurus.

Classification
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. It, along with several other generalized coelurosaurians such as the compsognathids, Ornitholestes, and Proceratosaurus, has had multiple placements around the base of Coelurosauria.[6][7][8][9][10] 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.[8][11] Oliver Rauhut (2003) proposed that Coeluridae was composed of Coelurus plus the compsognathids,[8] but he and others have not since found the compsognathids to group with Coelurus.[9][10][12] 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.[12] Senter proposed that Coelurus and Tanycolagreus were the only coelurids and were actually tyrannosauroids,[12] but the phylogenetic analysis of Turner et al. (2007b) found that Coelurus was a basal coelurosaur but was more derived than the tyrannosaurids.[13] Zanno in 2010 recovered Coelurus as a basal maniraptoran.[14] Coelurus is sometimes put into its own family, Coeluridae, although the membership of the family has not been stable.

Before the use of phylogenetic analyses, Coeluridae and Coelurosauria were taxonomic wastebaskets used for small theropods that didn't belong to other groups; thus, they accumulated many dubious genera.[4][15] 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.[16] A wastebasket Coeluridae lingered into the early 1990s in some sources[17] (and appears in at least one 2006 source)[18] but since then it has only been recognized in a much reduced form.