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Allosaurus

Marsh, 1877 AL-o-SAWR-us /ˌæləˈsɔːrəs/

From Greek 'allos' meaning 'different' or 'other' and 'sauros' meaning 'lizard'; meaning 'different lizard', alluding to its lightweight vertebrae

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

Allosaurus is a genus of theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic period (Kimmeridgian to late Tithonian ages). The first fossil remains that could definitively be ascribed to this genus were described in 1877 by Othniel C. Marsh. The name "Allosaurus" means "different lizard", alluding to its lightweight vertebrae, which Marsh believed were unique. The genus has a very complicated taxonomy and includes at least three valid species, the best known of which is A. fragilis. The bulk of Allosaurus remains come from North America's Morrison Formation, with material also known from the Alcobaça, Bombarral, and Lourinhã formations in Portugal. It was known for over half of the 20th century as Antrodemus, but a study of the abundant remains from the Cleveland-Lloyd Dinosaur Quarry returned the name "Allosaurus" to prominence. As one of the first well-known theropod dinosaurs, it has long attracted attention outside of paleontological circles.

Diagnosis

  1. Vertebral centra deeply excavated on the sides to reduce weight, producing an hourglass form in some elements
  2. Vertebrae biconcave with shallow cavities, centra not pneumatic but greatly lightened by lateral excavations
  3. Anterior caudal centrum so strongly constricted that its least transverse diameter (38 mm) is far less than its anterior face diameter (90 mm)
  4. Foot bones very slender

Species (2)

Allosaurus fragilis Marsh, 1877

Type species

From Latin 'fragilis' meaning 'fragile', referring to the lightweight construction features of the vertebrae that Othniel Charles Marsh noted when describing the species in 1877.

Neotype
USNM 4734 , National Museum of Natural History, Smithsonian Institution
Material
Nearly complete skeleton including the skull and lower jaws, presacral vertebrae, a complete sacrum, about thirty caudal vertebrae, the pelvis, ribs, and all four limbs and feet.
Horizon
Brushy Basin Member, Morrison Formation, Kimmeridgian–Tithonian, Late Jurassic , 154.8–143.1 million years ago
Locality
Felch Quarry 1, Garden Park, Colorado, United States

Synonyms

  • Epanterias amplexus junior subjective

    Based on AMNH 5767; generally treated as a junior synonym of Allosaurus fragilis, though debate persists. Epanterias is also widely considered nomen dubium due to lack of diagnostic features.

Allosaurus jimmadseni Chure & Loewen, 2020

Honors James H. Madsen Jr., for work excavating, preparing, and curating Cleveland-Lloyd Allosaurus material.

Holotype
DINO 11541 , Dinosaur National Monument
Material
Nearly complete articulated skeleton: left half of skull with occluded mandible, cervical-caudal vertebral column, ribs, gastral basket, pectoral girdle and forelimb, complete pelvis, hindlimbs, and partial pes.
Horizon
Salt Wash Member, Morrison Formation, Kimmeridgian, Late Jurassic , 154.8–149.2 million years ago
Locality
Dinosaur National Monument (locality DNM 116), Utah, United States
Diagnosis
  1. Ventral margin of the jugal is virtually straight, lacking the pronounced sigmoidal convexity present in A. fragilis
  2. Nasals bear paired, blade-like crests along their lateral margins forming a pronounced nasolacrimal crest, absent in A. fragilis
  3. A row of neurovascular foramina pierces the medioventral wall of the maxillary antorbital fossa
  4. Relatively taller lacrimal horns than in A. fragilis
  5. Straight posteroventral jugal ramus of the maxilla where it articulates with the jugal, rather than the pronounced ventral deflection seen in A. fragilis
  6. Axial intercentrum is rotated dorsally and has a flared rim in lateral view

Synonyms of the genus

Name Type Reason
Antrodemus junior subjective Although described before Allosaurus, the type material is too poor to be diagnostic; Allosaurus is given priority and Antrodemus treated as a junior synonym.
Creosaurus junior subjective Williston (1901) noted that Marsh had never adequately distinguished Creosaurus from Allosaurus; subsequent work has confirmed the synonymy.
Labrosaurus junior subjective Type material is now considered indistinguishable from Allosaurus.
Madsenius nomen nudum Informal name; never met ICZN requirements for formal publication and is treated as referable to Allosaurus. Per Olshevsky's Dinosaur Genera List.
Wyomingraptor nomen nudum Coined by an anonymous caller on Dr. Bob's Dinofacts radio show; never officially described in scientific literature.

Notable specimens

  • “Big Al”

    MOR 693 Museum of the Rockies Found 1991 by Kirby Siber iconic

    A 95 percent complete, partially articulated subadult, one of the most famous Allosaurus specimens. Renowned for its 19 broken or infected bones, including a long-lived foot infection that may have contributed to its death; later referred to A. jimmadseni.

    Chure & Loewen (2020)

  • “Big Al II”

    SMA 0005 Sauriermuseum Aathal exceptional preservation

    A second well-preserved Allosaurus from Howe Ranch. Preserves a 20 by 20 centimeter skin impression at the base of the tail showing large scales up to 2 centimeters across, though the scales resemble sauropod skin and associated non-theropod remains cast some doubt on the attribution.

    Chure & Loewen (2020)

  • AMNH 5753

    American Museum of Natural History Found 1879 by H. F. Hubbell historic

    Found in 1879 but left unpacked until 1903, when it proved to be one of the most complete theropod specimens then known. Put on view in 1908 as the first free-standing mount of a theropod dinosaur, famously posed as if scavenging an Apatosaurus, though never scientifically described.

    Breithaupt (1999)

  • AMNH 680

    American Museum of Natural History largest individual

    The largest definitive Allosaurus specimen, estimated at about 9.7 meters long and 2.3 to 2.7 tonnes, used to anchor maximum-size estimates for the genus.

    Chure (2000)

ICZN cases

References

  1. Marsh, O. C. (1877). Notice of new dinosaurian reptiles from the Jurassic formation. American Journal of Science s3-14(84): 514-516. American Journal of Science (AJS). doi:10.2475/ajs.s3-14.84.514 marsh1877c
  2. Gilmore, C. W. (1920). Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus. Bulletin of the United States National Museum 110: 1-154. Smithsonian Institution. doi:10.5479/si.03629236.110.i

    Redescribes the neotype USNM 4734 (as Antrodemus); source of the holotype material.

    gilmore1920a
  3. Breithaupt, B. H. (1999). AMNH 5753: The world's first free-standing theropod skeleton. Journal of Vertebrate Paleontology 19(3, Supplement): 33A. doi:10.1080/02724634.1999.10011202 breithaupt1999a
  4. Chure, D. J. (2000). A new species of Allosaurus from the Morrison Formation of Dinosaur National Monument (UT-CO) and a revision of the theropod family Allosauridae. Doctoral thesis, Columbia University.

    Ph.D. dissertation.

    chure2000a
  5. Paul, G. S.; Carpenter, K. (2010). Allosaurus Marsh, 1877 (Dinosauria, Theropoda): proposed conservation of usage by designation of a neotype for its type species Allosaurus fragilis Marsh, 1877. Bulletin of Zoological Nomenclature 67(1): 53-56. doi:10.21805/bzn.v67i1.a7 paul2010a
  6. Chure, D. J.; Loewen, M. A. (2020). Cranial anatomy of Allosaurus jimmadseni, a new species from the lower part of the Morrison Formation (Upper Jurassic) of Western North America. PeerJ 8: e7803. doi:10.7717/peerj.7803

    Formally erects Allosaurus jimmadseni on holotype DINO 11541 from the lower Morrison Formation; source of the species diagnosis.

    chure2020a
  7. International Commission on Zoological Nomenclature (2023). Opinion 2486 (Case 3506) - Allosaurus Marsh, 1877 (Dinosauria, Theropoda): usage conserved by designation of a neotype for its type species Allosaurus fragilis Marsh, 1877. Bulletin of Zoological Nomenclature 80(1): 65-68. doi:10.21805/bzn.v80.a015 iczn2023a
  8. Paul, G. S. (2024). The Princeton Field Guide to Dinosaurs (3rd ed.). Princeton University Press.

    Source of the appearance features.

    paul2024a
  9. Hendrickx, C.; Bell, P. R.; Pittman, M.; Milner, A. R. C.; Cuesta, E.; O'Connor, J.; Loewen, M.; Currie, P. J.; Mateus, O.; Kaye, T. G.; Delcourt, R. (2022). Morphology and distribution of scales, dermal ossifications, and other non-feather integumentary structures in non-avialan theropod dinosaurs. Biological Reviews 97(3): 960-1004. doi:10.1111/brv.12829

    Documents preserved pebbly scale impressions in several specimens.

    hendrickx2022a
  10. Williston, S. W. (1901). The dinosaurian genus Creosaurus, Marsh. American Journal of Science s4-11(62): 111-114. American Journal of Science. doi:10.2475/ajs.s4-11.62.111

    Noted that Creosaurus was not adequately distinguished from Allosaurus.

    williston1901a