Allosaurus fragilis

Allosaurus fragilis was one of the principal large predators of Late Jurassic western North America. It had a relatively light, open skull, small crests above the eyes, recurved cutting teeth, muscular hind limbs and grasping hands with three clawed fingers. Numerous specimens make Allosaurus anatomically well known, but the genus has a long and complicated taxonomic history. Material once grouped under A. fragilis has been divided among other species, and the International Commission on Zoological Nomenclature designated the nearly complete Smithsonian skeleton USNM V 4734 as the species' neotype in 2023.

Its fossils occur between approximately 154 and 148 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The skeleton is known from many individuals, but museum mounts may combine bones from several animals or use restored and cast elements. USNM V 4734 is especially important because it is a largely associated individual and now defines the name A. fragilis. Its skull was deep but full of openings that reduced weight, while the neck and forelimbs were built to contribute to prey handling.

• Paired bony crests above and in front of the eyes
• Large, lightly constructed skull with extensive openings
• Recurved, laterally compressed teeth with serrated edges
• Strong, mobile neck capable of rapid head movements
• Three-fingered grasping hands with large curved claws
• Powerful hind limbs balanced by a long muscular tail
• Relatively flexible lower jaw and cranial joints

Othniel Charles Marsh named Allosaurus fragilis in 1877 from fragmentary Morrison Formation fossils catalogued as YPM 1930. The name Allosaurus means 'different lizard', referring to vertebrae that appeared unusual at the time, while fragilis referred to their lightly constructed form. Marshall P. Felch collected the much more complete USNM V 4734 at Felch Quarry, Colorado, in 1883–1884. Charles W. Gilmore described that skeleton in detail in 1920. Because Marsh's original holotype was too incomplete to diagnose reliably, the ICZN formally replaced it with USNM V 4734 as the neotype on 29 December 2023.

Naming authors: Othniel Charles Marsh.

A. fragilis inhabited the seasonal river floodplains and open woodlands represented by the upper Morrison Formation. The environment alternated between wetter and drier conditions and supported stegosaurs, ornithopods and several kinds of enormous sauropod, as well as competing theropods such as Ceratosaurus and Torvosaurus.

Allosaurus was an active carnivore and scavenger, but its exact feeding technique remains debated. Biomechanical models agree that the skull could withstand substantial stress, while differing over how bites, neck-driven pulling and head movements were combined. Injuries and healed pathologies show that individuals survived serious trauma. Claims of coordinated pack hunting are not supported by direct evidence; bonebeds can form through many processes.

• USNM V 4734 became the official name-bearing specimen 146 years after the species was named.
• The Smithsonian mount contains real fossil bones and poses the animal crouched over a nest rather than running after prey.
• Allosaurus replaced and absorbed several historical genus names as its anatomy became better understood.
• Many fossils traditionally called A. fragilis require renewed comparison with the 2023 neotype and other named Allosaurus species.

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1. 01
   The Paleobiology Database

   Open source
2. 02
   Notice of new dinosaurian reptiles from the Jurassic formation

   Othniel Charles Marsh · American Journal of Science · 1877

   Open source
3. 03
   Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus

   Charles W. Gilmore · Bulletin of the United States National Museum · 1920

   Open source
4. 04
   Allosaurus fragilis: A Revised Osteology

   James H. Madsen Jr. · Utah Geological Survey Bulletin 109 · 1976

   Open source
5. 05
   Cranial design and function in a large theropod dinosaur

   Emily J. Rayfield, David B. Norman, Celeste C. Horner and 4 coauthors · Nature · 2001

   Open source
6. 06
   Multibody dynamics model of head and neck function in Allosaurus (Dinosauria, Theropoda)

   Eric Snively, John R. Cotton, Ryan Ridgely, Lawrence M. Witmer · Palaeontologia Electronica · 2013

   Open source
7. 07
   Opinion 2486 (Case 3506): Allosaurus Marsh, 1877—usage conserved by designation of a neotype for its type species Allosaurus fragilis

   International Commission on Zoological Nomenclature · Bulletin of Zoological Nomenclature · 2023

   Open source
8. 08
   Just Our Type: Smithsonian's Dinosaur Skeleton Becomes the Scientific Standard for Prehistoric Predator

   Smithsonian Institution · 2024

   Open source