Open Paleo

Coelurus

Marsh, 1879 seel-YOOR-us

From Greek 'koilos' (hollow) and 'oura' (tail); meaning 'hollow tail', referring to the extensively excavated, thin-walled vertebrae described in the holotype.

154.8–143.1 million years ago Kimmeridgian–Tithonian, Late Jurassic
Late Jurassic stages

Coelurus is a genus of small coelurosaurian theropod from the Late Jurassic Morrison Formation of Wyoming, United States. The single valid species, Coelurus fragilis, was described by Othniel Charles Marsh in 1879 and is known from a partial skeleton with characteristically hollow, thin-walled vertebrae. It was a lightly built bipedal carnivore with elongate hind limbs.

Under dispute

The placement of Coelurus is debated. Naish and Cau (2022) recover it in a Coeluridae with Tugulusaurus and Tanycolagreus, the sister of the other tyrannosauroids, and Hartman and colleagues (2019) find coelurids branching just after the tyrannosauroids; Rauhut (2003) instead recovered it with the compsognathids, and Delcourt and colleagues (2025) as the earliest-branching ornithomimosaur. It is placed conservatively at Coelurosauria incertae sedis.

Diagnosis

  1. Dentary very gracile
  2. All vertebral centra elongated and very thin-walled
  3. All neural spines long and low
  4. Paired pleurocoels on some cervicals; triangular diapophyseal ala angled sharply ventrolaterally; centroprezygapophyseal lamina forming a fossa lateral to the neural canal; centrodiapophyseal lamina present; caudal prezygapophyses very short
  5. Pubic foot very arcuate ventrally and projecting posterodorsally; pubic fenestra at the middle of the pubic apron
  6. Metatarsus very long and gracile, almost as long as the tibia or femur

Species

Coelurus fragilis Marsh, 1879

Type species

From Latin 'fragilis' (fragile, slender), referring to the delicate, thin-walled hollow vertebrae of the holotype.

Syntypes
YPM 1991, YPM 1992, YPM 1993 , Yale Peabody Museum of Natural History
Material
Proximal and mid-caudal vertebrae (centra and neural arches) and a cervical vertebra, the thin-walled vertebrae illustrated by Marsh (1881).
Notes
Coelurus fragilis was based on a series of vertebrae; Marsh marked YPM 1991 as the type, and Carpenter and colleagues (2005) treat YPM 1991-1993 as syntypes (no lectotype has been designated). The much more complete skeleton YPM 2010 is the type of Coelurus agilis Marsh (1884), now referred to C. fragilis.
Horizon
Morrison Formation, Kimmeridgian–Tithonian, Late Jurassic , 154.8–143.1 million years ago
Locality
Quarry 13 East, Wyoming, United States

Synonyms

  • Coelurus agilis junior subjective

    Erected by Marsh (1884) on the more complete skeleton YPM 2010; Carpenter and colleagues (2005) regarded it as a junior subjective synonym of Coelurus fragilis.

Notable specimens

  • YPM 2010

    Yale Peabody Museum of Natural History complementary

    More complete partial skeleton (cervical, dorsal, and caudal vertebrae plus pelvic and limb elements) from Quarry 13, Como Bluff. Originally the holotype of Coelurus agilis Marsh 1884, referred to C. fragilis by Carpenter and colleagues (2005); the principal source for the genus's modern anatomy.

    Carpenter et al. (2005)

References

  1. Marsh, O. C. (1879). Notice of new Jurassic reptiles. American Journal of Science s3-18(108): 501-505. American Journal of Science (AJS). doi:10.2475/ajs.s3-18.108.501 marsh1879a
  2. Ostrom, J. H. (1980). Coelurus and Ornitholestes: Are they the same?. Aspects of Vertebrate History: Essays in Honor of Edwin Harris Colbert (Jacobs, L. L., ed.) 245-256. Museum of Northern Arizona Press.

    Compares the type material of the two genera and concludes they are distinct rather than synonymous.

    ostrom1980a
  3. Carpenter, K.; Miles, C.; Ostrom, J. H.; Cloward, K. (2005). Redescription of the small maniraptoran theropods Ornitholestes and Coelurus from the Upper Jurassic Morrison Formation of Wyoming. The Carnivorous Dinosaurs (Carpenter, K., ed.) 49-71. Indiana University Press, Bloomington.

    Redescribes both genera as distinct taxa, provides revised diagnoses, and treats YPM 2010 (the Coelurus agilis type) as referred material of C. fragilis.

    carpenter2005b
  4. Delcourt, D.; Grillo, O. N.; Hendrickx, C.; Kellermann, M.; Langer, M. C. (2025). The coelurosaur theropods of the Romualdo formation, early Cretaceous (Aptian) of Brazil: Santanaraptor placidus meets Mirischia asymmetrica. The Anatomical Record ar.70085. Wiley. doi:10.1002/ar.70085

    Phylogenetic reanalysis recovering Coelurus fragilis as basal-most ornithomimosaur under equal-weight parsimony; authors note this does not settle the longstanding placement ambiguity.

    delcourt2025a
  5. Paul, G. S. (2024). The Princeton Field Guide to Dinosaurs (3rd ed.). Princeton University Press.

    Source of the mass estimate.

    paul2024a
  6. Jones, B. (2026). The Princeton Encyclopedia of Dinosaurs: Theropods. Princeton University Press.

    Source of the hip height estimate.

    jones2026c
  7. Hartman, S.; Mortimer, M.; Wahl, W. R.; Lomax, D. R.; Lippincott, J.; Lovelace, D. M. (2019). A new paravian dinosaur from the Late Jurassic of North America supports a late acquisition of avian flight. PeerJ 7: e7247. PeerJ. doi:10.7717/peerj.7247

    New paravian description's theropod phylogeny placed Coelurus near basal Tyrannosauroidea.

    hartman2019a
  8. Marsh, O. C. (1884). Principal characters of American Jurassic dinosaurs; Part VIII, The order Theropoda. American Journal of Science 27(160): 329-340. American Journal of Science. doi:10.2475/ajs.s3-27.160.329

    Named Coelurus agilis (YPM 2010), later synonymized with Coelurus fragilis.

    marsh1884a
  9. Marsh, O. C. (1881). A new order of extinct Jurassic reptiles (Coeluria). American Journal of Science s3-21(124): 339-341. doi:10.2475/ajs.s3-21.124.339 marsh1881c
  10. Naish, D.; Cau, A. (2022). The osteology and affinities of Eotyrannus lengi, a tyrannosauroid theropod from the Wealden Supergroup of southern England. PeerJ 10: e12727. PeerJ. doi:10.7717/peerj.12727

    Recovered Coelurus in a Coeluridae with Tugulusaurus and Tanycolagreus, the sister of the other tyrannosauroids.

    naish2022a
  11. Rauhut, O. W. M. (2003). The interrelationships and evolution of basal theropod dinosaurs. Special Papers in Palaeontology 69: 1-213. The Palaeontological Association.

    Recovered Coelurus with the compsognathids in a Coeluridae.

    rauhut2003b