# Allosaurus fragilis

> 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.

- Classification: Dinosauria (clade) > Saurischia (clade) > Theropoda (clade) > Tetanurae (clade) > Allosauroidea (superfamily) > Allosauridae (family) > Allosaurus (genus) > Allosaurus fragilis (species)
- Length: 8.5 m
- Hip height: 1.9–2.4 m
- Mass: 1.5–1.7 t
- Age: 154.8–145 million years ago (Late Jurassic)
- Fossils found in: Morrison Formation (Colorado); United States
- Diet: carnivore
- Described: Othniel Charles Marsh, 1877
- In films: Jurassic World: Fallen Kingdom (2018), Jurassic World Dominion (2022)

Canonical page: https://dinodex.club/dinosaurs/allosaurus-fragilis
Last updated: 2026-09-27

## 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).

## Sources

1. 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. https://doi.org/10.1371/journal.pone.0004532
2. 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. https://doi.org/10.7717/peerj.7803
3. 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
4. Madsen, J. H., Jr. (1976). Allosaurus fragilis: a revised osteology. Utah Geological and Mineral Survey Bulletin 109: 1-163.
5. Marsh, O. C. (1877). Notice of new dinosaurian reptiles from the Jurassic formation. American Journal of Science (series 3) 14: 514-516.
6. Paul, G. S. (2010). The Princeton Field Guide to Dinosaurs. Princeton University Press.
7. Paul, G. S. (2016). The Princeton Field Guide to Dinosaurs (2nd edition). Princeton University Press.

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Cite as: "Allosaurus fragilis". DinoDex. https://dinodex.club/dinosaurs/allosaurus-fragilis (updated 2026-09-27).