Temporal range: Late Jurassic,
|Skeletal restoration showing known remains|
Coelurus ( // si-LURE-əs) is a genus of coelurosaurian dinosaur from the Late Jurassic period (mid-late Kimmeridgian faunal stage, 155–152 million years ago). The name means "hollow tail", referring to its hollow tail vertebrae ( Greek κοῖλος, koilos = hollow + οὐρά, 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.
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.   Additionally, two arm bones possibly belonging to this genus are known from the Cleveland-Lloyd Dinosaur Quarry in Utah.  It was not a large dinosaur. Its weight has been estimated at around 13 to 20 kilograms (29 to 44 lb),   with a length of about 2.4 meters (7.9 ft) and a hip height of 0.7 meters (2.3 ft).  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. 
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 does not 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. 
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]). 
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.  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. 
Coelurus was described in 1879 by Othniel Charles Marsh,  an American paleontologist and naturalist known for his " Bone Wars" with Edward Drinker Cope.  At the time, he only described what he interpreted as vertebrae from the back and tail, found at the same location as the type specimen of his new genus and species Camptonotus dispar (later renamed Camptosaurus because Camptonotus was already in use for a cricket). Marsh was impressed with the hollow interiors of the thin-walled vertebrae, a characteristic that gave the type species its name: Coelurus fragilis. He thought of his new genus as an "animal about as large as a wolf, and probably carnivorous".  Coelurus would prove to be the first named small theropod from the Morrison Formation,  although at the time Marsh was not certain that it was a dinosaur. He returned to it in 1881 and provided illustrations of some bones, along with putting it in a new order (Coeluria) and family ( Coeluridae). 
From there, the story becomes more complex. Apparently, the skeleton was scattered throughout the quarry,  with the remains being recovered from September 1879 to September 1880.  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. fragilis.  He returned to the genus in 1888 to add 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.  This species is not currently accepted as representing Coelurus in reviews of the genus, but has not been given its own genus.
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. longicollis.  He later gave them their own genus, Coelophysis. 
In 1903, Henry Fairfield Osborn named a second genus of small theropod from the Morrison Formation, Ornitholestes. This genus was based on a partial skeleton from Bone Cabin Quarry, north of Como Bluff.  Ornitholestes became intertwined with Coelurus in 1920, when Charles Gilmore, in his influential study of theropod dinosaurs, concluded that the two were synonyms.  This was followed in the literature for decades.    The two genera were not formally compared, however, nor was there a full accounting of what actually belonged to Coelurus, until John Ostrom's study in 1980. 
Gilmore had suspected that C. fragilis and C. agilis were the same, but Ostrom was able to demonstrate this synonymy. This greatly expanded the known material pertaining to C. fragilis, and Ostrom was able to demonstrate that Ornitholestes was quite different from Coelurus.  At the time, Dale Russell had proposed that C. agilis was a species of Elaphrosaurus based on the incomplete information then published;  Ostrom was also able to demonstrate that this was not the case. Additionally, he showed that one of the three vertebrae Marsh had illustrated for C. fragilis was actually a composite of two vertebrae,  one of which was later shown to come from another quarry and belonged not to Coelurus but to another, unnamed small theropod.   This unnamed genus would not be the last small theropod from the Morrison Formation to be confused with Coelurus; a later discovery (1995) of a partial skeleton in Wyoming was first thought to be a new larger specimen of Coelurus,  but further study showed it belonged to a different but related genus, Tanycolagreus. 
Only one species of Coelurus, the type species C. fragilis, is still recognized as valid today,  although six other species have been named over the years. C. agilis, as discussed, was named by Marsh in 1884 for what turned out to be additional parts of the skeleton of C. fragilis.   Cope's C. bauri and C. longicollis, named in 1887 from Late Triassic fossils from New Mexico,  were transferred by Cope in 1889 to his new genus Coelophysis.  C. daviesi was named by Richard Lydekker in 1888 for Harry Seeley's Thecospondylus daviesi, a neck vertebra from the Early Cretaceous of England,  but this species was later transferred to its own genus, Thecocoelurus.  C. gracilis, another Early Cretaceous species, was also named in 1888. It was coined by Marsh for what seems to be an assortment of limb remains,  but Gilmore could only find a single claw when he reviewed the species in 1920.  This species has been proposed as outside Coelurus since the 1920s (when Gilmore assigned it to Chirostenotes),  and has been regarded as a dubious species outside of Coelurus in recent reviews.    Finally, during the period when Ornitholestes was thought to be the same as Coelurus, its type species was recognized as distinct by Steel, as C. hermanni. 
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. Along with several other generalized coelurosaurians such as the compsognathids, Ornitholestes, and Proceratosaurus, it has had multiple placements around the base of Coelurosauria.      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.   Oliver Rauhut (2003) proposed that Coeluridae was composed of Coelurus plus the compsognathids,  but he and others have not since found the compsognathids to group with Coelurus.    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.  Senter proposed that Coelurus and Tanycolagreus were the only coelurids and were actually tyrannosauroids,  but the phylogenetic analysis of Turner et al. (2007b) found that Coelurus was a basal coelurosaur, although more derived than the tyrannosaurids.  Zanno in 2010 recovered Coelurus as a basal maniraptoran.  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 did not belong to other groups; thus, they accumulated many dubious genera.   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.  A wastebasket Coeluridae lingered into the early 1990s in some sources  (and appears in at least one 2006 source)  but since then it has only been recognized in a much reduced form.   Coeluridae received a formal phylogenetic definition in 2015, when it was defined as all species more closely related to Coelurus fragilis than to Proceratosaurus bradleyi, Tyrannosaurus rex, Allosaurus fragilis, Compsognathus longipes, Ornithomimus edmontonicus, or Deinonychus antirrhopus by Hendrickx, Hartman and Mateus.  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. 
Below is a cladogram placing Coelurus in Coelurosauria by Cau et al. in 2015. 
The type specimen of Coelurus agilis, YPM 2010 (now considered a synonym of Coelurus fragilis) was recovered in the Brushy Basin member of the Morrison Formation, in Albany County, Wyoming. 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.  This specimen is housed in the collection of the Yale Peabody Museum in New Haven, Connecticut.
The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons, and flat floodplains.  Vegetation varied from river-lining forests of conifers, tree ferns, and ferns, to fern savannas with rare trees.  It has been a rich fossil hunting ground, holding fossils of green algae, fungi, mosses, horsetails, ferns, cycads, ginkgoes, and several families of conifers. Other fossils discovered include bivalves, snails, ray-finned fishes, frogs, salamanders, turtles such as Uluops, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans like Fruitachampsa, several species of pterosaur like Kepodactylus, numerous dinosaur species, and early mammals such as docodonts, multituberculates, symmetrodonts, and triconodonts. 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.  Coelurus is regarded as a small terrestrial carnivore,  feeding on small prey items like insects, mammals, and lizards.  It is thought to have been a fast animal, certainly faster than the similar but shorter-footed Ornitholestes.  Coelurus is present in stratigraphic zones 2 and 5 of the Morrison Formation. 
- Ostrom, John H. (1980). "Coelurus and Ornitholestes: Are they the same?". In Jacobs, Louis L. (ed.). Aspects of Vertebrate History: Essays in Honor of Edwin Harris Colbert. Flagstaff: Museum of Northern Arizona Press. pp. 245–256. ISBN 978-0-89734-052-6.
- Carpenter, Kenneth; Miles, Clifford; Ostrom, John H.; Cloward, Karen (2005). "Redescription of the small maniraptoran theropods Ornitholestes and Coelurus from the Upper Jurassic Morrison Formation of Wyoming". In Carpenter, Kenneth (ed.). The Carnivorous Dinosaurs. Bloomington: Indiana University Press. pp. 49–71. ISBN 978-0-253-34539-4.
- Russell, Dale A. (1989). An Odyssey in Time: Dinosaurs of North America. Minocqua, Wisconsin: NorthWord Press. p. 80. ISBN 978-1-55971-038-1.
- Paul, Gregory S. (1988). "Early Avetheropods". Predatory Dinosaurs of the World. New York: Simon & Schuster. pp. 297–300. ISBN 978-0-671-61946-6.
- Carpenter, Kenneth; Miles, Clifford; Cloward, Karen (2005). "New small theropod from the Upper Jurassic Morrison Formation of Wyoming". In Carpenter, Kenneth (ed.). The Carnivorous Dinosaurs. Bloomington: Indiana University Press. pp. 23–48. ISBN 978-0-253-34539-4.
- Marsh, Othniel Charles (1879). "Notice of new Jurassic reptiles" (PDF). American Journal of Science. Series 3. 18 (108): 501–505. Bibcode: 1879AmJS...18..501M. doi: 10.2475/ajs.s3-18.108.501. S2CID 131001110.
- Jaffe, Mark (2000). The Gilded Dinosaur: The Fossil War Between E. D. Cope and O. C. Marsh and the Rise of American Science. New York, New York: Crown Publishing Group. ISBN 978-0-517-70760-9.
- Marsh, Othniel Charles (1881). "A new order of extinct Jurassic reptiles". American Journal of Science. 21 (124): 339–341. doi: 10.2475/ajs.s3-21.124.339. S2CID 130429452.
- Marsh, Othniel Charles (1884). "Principal characters of American Jurassic dinosaurs, Part 8: The Order Theropoda" (PDF). American Journal of Science. 27 (160): 29–40. doi: 10.2475/ajs.s3-27.160.329. S2CID 131076004. Retrieved 2011-11-06.
- Marsh, Othniel Charles (1888). "Notice of a new genus of Sauropoda and other dinosaurs from the Potomac Formation" (PDF). American Journal of Science. 35 (205): 89–94. Bibcode: 1888AmJS...35...89M. doi: 10.2475/ajs.s3-35.205.89. S2CID 130879860.
- Cope, Edward Drinker (1887). "The dinosaurian genus Coelurus". American Naturalist. 21 (4): 367–369. doi: 10.1086/274464.
- Cope, Edward Drinker (1889). "On a new genus of Triassic Dinosauria". American Naturalist. 23 (271): 626. doi: 10.1086/274979.
- Osborn, Henry Fairfield (1903). "Ornitholestes hermanni, a new compsognathoid dinosaur from the Upper Jurassic" (PDF). Bulletin of the American Museum of Natural History. 19 (12): 459–464. Archived from the original (PDF) on 2009-03-25. Retrieved 2008-01-20.
- Gilmore, Charles W. (1920). "Osteology of the carnivorous dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus" (PDF). Bulletin of the United States National Museum. 110 (110): 1–159. doi: 10.5479/si.03629236.110.i. hdl: 2027/uiug.30112032536010.
- Romer, Alfred S. (1956). Osteology of the Reptiles. Chicago: University of Chicago Press. p. 772pp. ISBN 978-0-89464-985-1.
- Romer, Alfred S. (1966). Vertebrate Paleontology (Third ed.). Chicago: University of Chicago Press. p. 468 pp. ISBN 978-0-7167-1822-2.
- Steel, R. (1970). "Part 14. Saurischia. Handbuch der Paläoherpetologie/Encyclopedia of Paleoherpetology". Gustav Fischer Verlag, Stuttgart: 1–87.
- Russell, Dale A. (1972). "Ostrich dinosaurs from the late Cretaceous of western Canada". Canadian Journal of Earth Sciences. 9 (4): 375–402. Bibcode: 1972CaJES...9..375R. doi: 10.1139/e72-031.
- Makovicky, Peter J. (1997). "A new small theropod from the Morrison Formation of Como Bluff, Wyoming". Journal of Vertebrate Paleontology. 17 (4): 755–757. doi: 10.1080/02724634.1997.10011024.
- Miles, Clifford A.; Carpenter, Kenneth; Cloward, Karen (1998). "A new skeleton of Coelurus fragilis from the Morrison Formation of Wyoming". Journal of Vertebrate Paleontology. 18 (3, Suppl): 64A. doi: 10.1080/02724634.1998.10011116.
- Holtz, Thomas R., Jr.; Molnar, Ralph E.; Currie, Philip J. (2004). Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 71–110. ISBN 978-0-520-24209-8.
- Lydekker, Richard (1888). Catalogue of the Fossil Reptilia and Amphibia in the British Museum (Natural History). Part I. Containing the Orders Ornithosauria, Crocodilia, Dinosauria, Squamata, Rhynchocephalia, and Proterosauria. London: British Museum (Natural History). pp. 1–309. ISBN 978-0-543-95654-5.
- von Huene, Friedrich (1923). "Carnivorous Saurischia in Europe since the Triassic". Bulletin of the Geological Society of America. 34 (3): 449–458. Bibcode: 1923GSAB...34..449V. doi: 10.1130/GSAB-34-449.
- Gilmore, Charles W. (1924). "A new coelurid dinosaur from the Belly River Cretaceous of Alberta". Canada Department of Mines Geological Survey Bulletin (Geological Series). 38 (43): 1–12.
- Norman, David B. (1990). "Problematic theropoda: "coelurosaurs"". In Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.). The Dinosauria. Berkeley: University of California Press. pp. 280–305. ISBN 978-0-520-06727-1.
- Gauthier, Jacques A. (1986). "Saurischian monophyly and the origin of birds". In Padian, Kevin (ed.). The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences. Vol. 8. pp. 1–55.
- Holtz, Thomas R., Jr. (1998). "A new phylogeny of the carnivorous dinosaurs" (PDF). Gaia. 15: 5–61. ISSN 0871-5424.
- Rauhut, Oliver W.M. (2003). "The interrelationships and evolution of basal theropod dinosaurs". Special Papers in Palaeontology. 69: 1–213.
- Rauhut, Oliver W.M.; Xu, Xing (2005). "The small theropod dinosaurs Tugulusaurus and Phaedrolosaurus from the Early Cretaceous of Xinjiang, China". Journal of Vertebrate Paleontology. 25 (1): 107–118. doi: 10.1671/0272-4634(2005)025[0107:TSTDTA]2.0.CO;2. ISSN 0272-4634.
- Smith, N.D.; Makovicky, P.J.; Hammer, W.R.; Currie, P.J. (2007). "Osteology of Cryolophosaurus ellioti (Dinosauria: Theropoda) from the Early Jurassic of Antarctica and implications for early theropod evolution". Zoological Journal of the Linnean Society. 151 (2): 377–421. doi: 10.1111/j.1096-3642.2007.00325.x.
- Senter, Phil (2007). "A new look at the phylogeny of Coelurosauria (Dinosauria, Theropoda)". Journal of Systematic Palaeontology. 5 (4): 429–463. doi: 10.1017/S1477201907002143. S2CID 83726237.
- Turner, A.H.; Pol, D.; Clarke, J.A.; Erickson, G.M.; Norell, M.A. (2007b). "A basal dromaeosaurid and size evolution preceding avian flight". Science. 317 (5843): 1378–1381. Bibcode: 2007Sci...317.1378T. doi: 10.1126/science.1144066. PMID 17823350.
- Zanno, L.E. (2010a). "A taxonomic and phylogenetic review of Therizinosauria". Journal of Systematic Palaeontology. 8 (4): 503–543. doi: 10.1080/14772019.2010.488045. S2CID 53405097.
- Lambert, David; the Diagram Group (1983). "Coelurids". A Field Guide to Dinosaurs. New York: Avon Books. pp. 44–47. ISBN 978-0-380-83519-5.
- Lessem, Don; Glut, Donald F. (1993). The Dinosaur Society Dinosaur Encyclopedia. Random House, Inc. ISBN 978-0-679-41770-5.
- Palmer, Douglas (2006). "Ruling Reptiles: Dinosaurs and Their Kin". The Illustrated Encyclopedia of the Prehistoric World. New Jersey: Chartwell Books, Inc. pp. 232–313. ISBN 978-0-7858-2086-4.
- Hendrickx, C.; Hartman, S.A.; Mateus, O. (2015). "An Overview of Non- Avian Theropod Discoveries and Classification". PalArch's Journal of Vertebrate Palaeontology. 12 (1): 1–73.
- Brusatte, S. L.; Carr, T. D. (2016). "The phylogeny and evolutionary history of tyrannosauroid dinosaurs". Scientific Reports. 6: 20252. Bibcode: 2016NatSR...620252B. doi: 10.1038/srep20252. PMC 4735739. PMID 26830019.
- Cau, Andrea; Brougham, Tom; Naish, Darren (2015). "The phylogenetic affinities of the bizarre Late Cretaceous Romanian theropod Balaur bondoc (Dinosauria, Maniraptora): Dromaeosaurid or flightless bird?". PeerJ. 3: e1032. doi: 10.7717/peerj.1032. PMC 4476167. PMID 26157616.
- Russell, Dale A. (1989). An Odyssey in Time: Dinosaurs of North America. Minocqua, Wisconsin: NorthWord Press. pp. 64–70. ISBN 978-1-55971-038-1.
- Carpenter, Kenneth (2006). "Biggest of the big: a critical re-evaluation of the mega-sauropod Amphicoelias fragillimus". In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 131–138.
- Chure, Daniel J.; Litwin, Ron; Hasiotis, Stephen T.; Evanoff, Emmett; Carpenter, Kenneth (2006). "The fauna and flora of the Morrison Formation: 2006". In Foster, John R.; Lucas, Spencer G. (eds.). Paleontology and Geology of the Upper Jurassic Morrison Formation. New Mexico Museum of Natural History and Science Bulletin, 36. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. pp. 233–248.
- Foster, John R. (2003). Paleoecological Analysis of the Vertebrate Fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A. New Mexico Museum of Natural History and Science Bulletin, 23. Albuquerque, New Mexico: New Mexico Museum of Natural History and Science. p. 29.
- Fastovsky, David E.; Smith, Joshua B. (2004). "Dinosaur paleoecology". In Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.). The Dinosauria (2nd ed.). Berkeley: University of California Press. pp. 614–626. ISBN 978-0-520-24209-8.
- Foster, J. (2007). "Appendix." Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. pp. 327-329.
- Coelurus from Palaeos.com.