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Tan theropod with small horns above the eyes and a striped tail, walking in side view
Reconstruction of Allosaurus fragilis with proportions matching Scott Hartman's skeletal of UUVP 6000.Image credit: Fred Wierum, CC BY-SA 4.0, via Wikimedia Commons (unmodified except resizing)CC BY-SA 4.0 (unmodified)Source (opens in a new tab)

Allosaurus fragilis

Pronounced AL-oh-SORE-us FRAJ-ih-lis

Tetanurae ยท Allosauridae

The commonest large predator of the Late Jurassic Morrison Formation of the western United States, an allosauroid theropod known from hundreds of individuals, many from the Cleveland-Lloyd Quarry in Utah.

  • Bipedal
  • Apex predator
  • Movie icon
  • Possible pack behavior
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Period
Jurassicยท Kimmeridgian
Diet
Carnivore
Length
8.5 m
Mass
1,500โ€“1,700 kg
Location
Colorado, Utah, United States

Quick answers

How big was Allosaurus fragilis?
Allosaurus fragilis was about 8.5 m long. It stood about 1.9โ€“2.4 m tall at the hips. It weighed about 1.5โ€“1.7 t. (Paul 2010; Paul 2016)
When did Allosaurus fragilis live?
Allosaurus fragilis lived during the Late Jurassic, about 154.8โ€“145 million years ago. (Cohen et al. 2023; Chure & Loewen 2020)
Where have Allosaurus fragilis fossils been found?
Fossils of Allosaurus fragilis have been found in Morrison Formation (Colorado) and United States. (Marsh 1877; Madsen 1976)
What did Allosaurus fragilis eat?
Allosaurus fragilis was a carnivore. (Marsh 1877; Madsen 1976)
Who named Allosaurus fragilis, and when?
Allosaurus fragilis was described by Othniel Charles Marsh in 1877. (Marsh 1877)
Was Allosaurus fragilis in Jurassic Park?
Yes. Allosaurus fragilis appears in Jurassic World: Fallen Kingdom (2018) and Jurassic World Dominion (2022). A large predator sharing the screen with Cretaceous animals in Fallen Kingdom and Dominion. Allosaurus lived in the Late Jurassic, tens of millions of years before T. rex; the films mix animals from different eras. (Wikipedia: Dinosaurs in Jurassic Park 2026; Madsen 1976)

On this page

AL-oh-SORE-us FRAJ-ih-lis

Period
Jurassic154.8โ€“145 Ma
Diet
Carnivore
Length
8.5 m
Mass
1.5โ€“1.7 t
Location
United States
Classification
Tetanurae ยท Allosauridae
Described
1877, Othniel Charles Marsh

Overview

Allosaurus fragilis lived in western North America in the Late Jurassic, alongside Stegosaurus and giant sauropods such as Brachiosaurus. O. C. Marsh named it in 1877 from partial remains (YPM 1930) from Garden Park, Colorado, during the "Bone Wars".

It was a large, bipedal predator with a long, narrow skull, a pair of small horns in front of the eyes, three-fingered arms with large claws and serrated, blade-like teeth. The Cleveland-Lloyd Quarry in Utah has yielded bones of dozens of individuals, which James Madsen used for his 1976 monograph, still the standard description of the species.

Not every Morrison Allosaurus belongs to A. fragilis. Chure and Loewen (2020) named A. jimmadseni from the lower part of the formation, including the well-known specimen "Big Al" (MOR 693), which older size estimates used as a stand-in for A. fragilis.

Name meaning

From Greek allos ("different, other") and sauros ("lizard"), for its unusual vertebrae; Latin fragilis ("fragile") refers to the lightly built vertebrae (Marsh 1877).

Biology & evidence

What scientists think about how this animal lived, and how strong the evidence for each idea is.

Feeding

  • Allosaurus attacked with an open-mouthed strike, using its upper teeth like a hatchet.

    Evidence level: Hypothesis

    Its skull could withstand strong impacts though its bite was relatively weak, which led to the idea that it slammed its upper jaw into prey. This is an interpretation of a model, not an observed behaviour.

    Evidence for / against ยท 1
    Evidence for

    • Biomechanical model ยท Allosaurus skull finite-element model

      A finite-element model of the Allosaurus skull finds it strong under impact but with a modest bite muscle force.

Injuries & disease

  • Allosaurus often survived serious injuries and infections.

    Evidence level: Strong evidence

    Healed fractures and infected bones are common in Allosaurus skeletons, including a young animal with injuries to many bones.

    Evidence for / against ยท 1
    Evidence for

    • Specimen ยท Injuries and infections in Allosaurus skeletons

      Many Allosaurus bones, including a sub-adult with 19 injured or infected bones, show a hard, injury-prone life.

Behavior

  • Allosaurus preyed on or fought Stegosaurus.

    Evidence level: Strong evidence

    An Allosaurus bone was wounded by a Stegosaurus spike and healed, direct proof of a violent encounter between the two.

    Evidence for / against ยท 1
    Evidence for

    • Specimen ยท Allosaurus vertebra pierced by a Stegosaurus spike

      An Allosaurus tail vertebra with a healed puncture matching the size and shape of a Stegosaurus tail spike.

Social life

  • Allosaurus lived and hunted in groups.

    Evidence level: Disputed

    Many Allosaurus were found together at Cleveland-Lloyd, but the site is read as a trap or a drought gathering. Mass death does not show group living.

    Evidence for / against ยท 1
    Evidence against

    • Other ยท Cleveland-Lloyd Quarry taphonomy

      Tens of Allosaurus individuals mixed in one quarry, interpreted variously as a predator trap, a drought assemblage or a slow build-up.

Fossilised injury on a toe bone of the Allosaurus 'Big Al' (MOR 693), one of many pathologies on this individual.Image credit: Laura Vietti, CC BY 4.0, via Wikimedia CommonsCC BY 4.0Source (opens in a new tab)

Size

Length
8.5 m

(27.9 ft)

Height
2.1 mpublished range 1.9โ€“2.4 m

(6.9 ft)

Based on AMNH 5753 / USNM 4734 (femur c. 0.85 m)

Mass
1,500โ€“1,700 kg

Based on MOR 693 "Big Al" (volumetric model); general adult (Paul 2010)

Allosaurus fragilis was about 8.5 m (27.9 ft) long, roughly 4.7 times the height of a 1.8 m adult human.

Timeline

Lived from about 154.8 to 145 million years ago (Ma), within the Mesozoic era (252โ€“66 Ma).

Periods: Kimmeridgian (154.8โ€“149.2 Ma), Tithonian (149.2โ€“145 Ma), Late Jurassic (161.5โ€“145 Ma)

Ages are the ICS 2023/09 bounds of the Kimmeridgian and Tithonian, the stage(s) the fossils are assigned to; they bracket the record and are not dated first/last appearances. Chure & Loewen (2020) restrict A. fragilis to the upper part of the Morrison Formation, so the real range is narrower.

Location

Formations

Map data: Natural Earth (public domain). Modern positions, not where the continents were then.

These are the places where fossils have been found, not the full area where the animal lived.

Ecosystem

Lived in: Morrison

156โ€“147 million years ago ยท floodplain, river system, open woodland, lake

A vast, seasonally dry floodplain crossed by rivers in the western interior of North America in the Late Jurassic, with a wet and a dry season, conifer and fern vegetation along the water and giant sauropods in abundance.

Same ecosystem

Stegosaurus stenops

Jurassicยท KimmeridgianHerbivore

Same ecosystem

Brachiosaurus altithorax

Jurassicยท KimmeridgianHerbivore

Same ecosystem

Brontosaurus excelsus

Jurassicยท KimmeridgianHerbivore

Same ecosystem

Diplodocus carnegii

Jurassicยท KimmeridgianHerbivore

Same ecosystem

Apatosaurus ajax

Jurassicยท KimmeridgianHerbivore

Same ecosystem

Ceratosaurus nasicornis

Jurassicยท KimmeridgianCarnivore

Classification

  1. cladeDinosauria
  2. cladeSaurischia
  3. cladeTheropoda
  4. cladeTetanurae
  5. superfamilyAllosauroidea
  6. familyAllosauridae
  7. genusAllosaurus
  8. speciesAllosaurus fragilis

Taxonomic status: Valid

Fossil record

Cast of the Allosaurus specimen 'Big Al' (MOR 693) at the University of Wyoming Geological Museum.Image credit: Karl T. Bates, Peter L. Falkingham, Brent H. Breithaupt, David Hodgetts, William I. Sellers, and Phillip L. Manning, CC BY 4.0, via Wikimedia Commons, University of Wyoming Geological MuseumCC BY 4.0Source (opens in a new tab)
Specimens
Catalog numberStatusInstitutionFound
YPM 1930HolotypeYale Peabody Museum (YPM)1877
USNM 4734Referred specimenNational Museum of Natural History, Smithsonian Institution (USNM)1883

What has been found

Holotype: YPM 1930

  • Preserved
  • Partial
  • Reconstructed
  • Unknown / not recorded

Holotype: YPM 1930

Region by region (table)
Skeleton regions of Holotype: YPM 1930
RegionState
SkullUnknown / not recorded
MandibleUnknown / not recorded
TeethPartial
Cervical vertebraeUnknown / not recorded
Dorsal vertebraePartial
SacrumUnknown / not recorded
Caudal vertebraeUnknown / not recorded
RibsUnknown / not recorded
GastraliaUnknown / not recorded
Scapula / shoulder girdleUnknown / not recorded
HumerusPartial
Forearm (radius / ulna)Unknown / not recorded
Hand (manus)Unknown / not recorded
PelvisUnknown / not recorded
FemurUnknown / not recorded
Tibia / fibulaUnknown / not recorded
Foot (pes)Unknown / not recorded

Referred specimen: USNM 4734

Region by region (table)
Skeleton regions of Referred specimen: USNM 4734
RegionState
SkullPreserved
MandiblePreserved
TeethUnknown / not recorded
Cervical vertebraePreserved
Dorsal vertebraePreserved
SacrumUnknown / not recorded
Caudal vertebraePartial
RibsUnknown / not recorded
GastraliaUnknown / not recorded
Scapula / shoulder girdleUnknown / not recorded
HumerusPreserved
Forearm (radius / ulna)Unknown / not recorded
Hand (manus)Preserved
PelvisPreserved
FemurPreserved
Tibia / fibulaUnknown / not recorded
Foot (pes)Preserved
Skeletal restorations of Allosaurus specimens MOR 693 ('Big Al') and DINO 11531; grey elements are missing (Chure & Loewen 2020, after Scott Hartman).Image credit: Scott Hartman; modified by Daniel J. Chure and Mark A. Loewen, CC BY 4.0, via Wikimedia CommonsCC BY 4.0Source (opens in a new tab)

Gallery

Historical reconstruction: Charles R. Knight's early 20th-century restoration of Allosaurus feeding on a sauropod carcass, based on AMNH 5753.Image credit: Charles R. Knight, Public domain, via Wikimedia CommonsPublic domain

All specimens

Discovery

Year described
1877
Described by
Othniel Charles Marsh
Type specimen
YPM 1930

Discovery timeline

  1. 1877 ยท Formally described

    Marsh names Allosaurus fragilis

    O. C. Marsh names the genus from partial remains from Garden Park, Colorado.

    People: Othniel Charles Marsh

  2. 1883 ยท Specimen recovered

    Felch collects a complete Allosaurus at Garden Park

    Marshall Felch collects the skeleton later described by Gilmore (1920).

    People: Marshall P. Felch

  3. 1976 ยท Major study

    Madsen publishes the Allosaurus osteology

    James Madsen describes Allosaurus from the thousands of bones of the Cleveland-Lloyd Quarry.

    People: James H. Madsen Jr.

  4. 2020 ยท Revision

    Allosaurus jimmadseni split from A. fragilis

    Chure and Loewen name a second species from the lower Morrison Formation.

    People: Daniel J. Chure

In Jurassic Park & Jurassic World

Films

  • Jurassic World: Fallen Kingdom (2018)
  • Jurassic World Dominion (2022)

On screen

A large predator sharing the screen with Cretaceous animals in Fallen Kingdom and Dominion.

Film vs fossils

  • Allosaurus lived in the Late Jurassic, tens of millions of years before T. rex; the films mix animals from different eras.
  • It really did clash with Stegosaurus: a spike wound in an Allosaurus tail bone shows it.

Film titles are used for identification only. DinoDex is not affiliated with or endorsed by the Jurassic Park / Jurassic World franchise. All dinosaurs seen in the films

Sources

  1. (2026). Wikipedia: Dinosaurs in Jurassic Park. Wikipedia. https://en.wikipedia.org/wiki/Dinosaurs_in_Jurassic_Park (opens in a new tab)
  2. Bates, K. T., Manning, P. L., Hodgetts, D. & Sellers, W. I. (2009). Estimating mass properties of dinosaurs using laser imaging and 3D computer modelling. PLoS ONE 4(2): e4532. doi:10.1371/journal.pone.0004532 (opens in a new tab)
  3. Carpenter, K., Sanders, F., McWhinney, L. A. & Wood, L. (2005). Evidence for predator-prey relationships: examples for Allosaurus and Stegosaurus. In Carpenter, K. (ed.), The Carnivorous Dinosaurs, Indiana University Press, pp. 325-350.
  4. 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 (opens in a new tab)
  5. Cohen, K. M., Harper, D. A. T., Gibbard, P. L. & Car, N. (2023). International Chronostratigraphic Chart (v2023/09). International Commission on Stratigraphy. https://stratigraphy.org/ICSchart/ChronostratChart2023-09.pdf (opens in a new tab)
  6. Madsen, J. H., Jr. (1976). Allosaurus fragilis: a revised osteology. Utah Geological and Mineral Survey Bulletin 109: 1-163.
  7. Marsh, O. C. (1877). Notice of new dinosaurian reptiles from the Jurassic formation. American Journal of Science (series 3) 14: 514-516.
  8. Paul, G. S. (2010). The Princeton Field Guide to Dinosaurs. Princeton University Press.
  9. Paul, G. S. (2016). The Princeton Field Guide to Dinosaurs (2nd edition). Princeton University Press.