Utahraptor ostrommaysi

Utahraptor ostrommaysi greatly expanded the known size range of the sickle-clawed dromaeosaurids. At about 5.5 metres long, it was much stockier than Velociraptor or Deinonychus, with a deep torso, robust femora and an enlarged claw bone on the second toe of each foot. Its fame exceeds the completeness of its published anatomy: the named material is disarticulated, several bones used in early reconstructions have been reidentified, and much of the spectacular Stikes Quarry assemblage remains under preparation. That nine-ton block contains thousands of bones from young to adult Utahraptors alongside at least two plant-eating dinosaurs. It promises a much fuller picture of growth and anatomy, but the animals’ association in a natural trap cannot by itself demonstrate coordinated pack hunting.

Its fossils occur between approximately 136 and 132 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

The holotype, CEUM 184v.86, is a large second-toe ungual rather than a whole skeleton. Referred material includes additional foot bones, a premaxilla, teeth, vertebrae and substantial hind-limb elements from several individuals. The largest published femora are unusually stout for a dromaeosaur and combine mass-supporting features with the more birdward arrangement of theropod hip musculature. A 2024 comparative study used an estimated mass near 550 kilograms for one large individual, while other museum syntheses favour about 350 kilograms; both are model-dependent. The animal carried three-fingered hands, but several supposedly blade-like hand claws in early accounts were later reidentified as foot claws. As in other dromaeosaurids, the enlarged second toe was held clear of the ground and the long tail was stiffened by elongated bony processes. No Utahraptor fossil preserves feathers, although a feathered body is strongly inferred from its place among paravian theropods.

• Exceptionally large, powerfully built dromaeosaur body plan
• Enlarged, strongly recurved ungual on the second toe
• Robust femur with adaptations for supporting high body mass
• Three functional fingers with recurved claws
• Serrated, recurved predatory teeth
• Long tail stiffened by elongated vertebral processes
• Skull still reconstructed from incomplete and disarticulated material
• Feathered covering inferred phylogenetically but not directly preserved

Brigham Young University field work led by James A. Jensen recovered large dromaeosaur bones near Moab in 1975 after local collector Lin Ottinger reported the site, but the material was not named. In 1991, Robert Gaston found a giant sickle claw at the Gaston Quarry; James Kirkland, Donald Burge and Gaston described material from that quarry and a second locality in 1993 as Utahraptor ostrommaysi. The name honoured John Ostrom, whose Deinonychus research helped transform dinosaur science, and Chris Mays, who supported the project. Later authors often changed the ending to ostrommaysorum. A 2019 nomenclatural analysis concluded that the original ostrommaysi must be retained under the zoological code, so this profile uses that spelling while preserving the database’s older slug for stable links. The Stikes Quarry megablock was excavated beginning in the 2000s and transferred to the Natural History Museum of Utah; its laborious preparation continues to expose skull, juvenile and adult material.

Discovery credit: Lin Ottinger and the Brigham Young University field team, Robert Gaston, James I. Kirkland, Donald Burge.

Naming authors: James I. Kirkland, Donald Burge, Robert Gaston.

Utahraptor lived on the low-relief terrestrial landscapes recorded by the upper Yellow Cat Member of the Cedar Mountain Formation. Calcareous soils, spring-related deposits, ponds and episodic channels developed under a strongly seasonal, at least seasonally dry climate. A volcanic-ash-derived horizon at Utahraptor Ridge has been dated to 135.10 ± 0.34 million years, placing that part of the member in the Valanginian rather than the younger Barremian age used in many older books. Gastonia, Hippodraco, iguanodont-grade herbivores and sauropods occur in the wider member. Those animals were spread across different localities and horizons, so a formation-level list is not a single frozen community.

Teeth, jaws and the enlarged foot claw identify Utahraptor as a predator, but exactly how it subdued prey remains unresolved. Dromaeosaur foot-claw models favour grasping or pinning over a simple slashing kick, yet those tests are not direct observations of Utahraptor. The Stikes Quarry contains young and adult Utahraptors around larger herbivores in a spring-fed sand trap. The published sedimentology explains how repeated entrapment and rapid burial could concentrate the bodies; it does not show that the raptors arrived, hunted or lived as a coordinated pack. No nest, trackway or healed combat series currently demonstrates group hunting, parental care, top speed or a preferred prey species.

• The famous holotype is one giant toe-claw bone, not the near-complete skeleton often shown in museums.
• A nine-ton block from Stikes Quarry contains thousands of bones and individuals ranging from very young animals to adults.
• Radiometric dating published in 2023 made the Utahraptor Ridge fauna several million years older than many classic estimates.
• The spelling ostrommaysorum is widespread, but a 2019 code analysis concluded that the original ostrommaysi is the correct form.
• Utah designated Utahraptor its official state dinosaur in 2018 while retaining Allosaurus as the state fossil.

A research repository is not necessarily a public exhibit. Loan and display status can change, so check with the institution before visiting.

1. 01
   The Paleobiology Database

   Open source
2. 02
   A large dromaeosaur (Theropoda) from the Lower Cretaceous of eastern Utah

   James I. Kirkland, Donald Burge, Robert Gaston · Hunteria 2(10) · 1993

   Open source
3. 03
   New osteological data and the affinities of Utahraptor from the Cedar Mountain Formation (Early Cretaceous) of Utah

   Brooks B. Britt, Daniel J. Chure, Kenneth L. Stadtman and 3 coauthors · Journal of Vertebrate Paleontology 21(3 supplement), 36A · 2001

   Open source
4. 04
   Depositional constraints on the Lower Cretaceous Stikes Quarry dinosaur site: Upper Yellow Cat Member, Cedar Mountain Formation, Utah

   James I. Kirkland, Edward L. Simpson, Donald D. DeBlieux and 3 coauthors · PALAIOS 31(9) · 2016

   Open source
5. 05
   Berriasian–Valanginian geochronology and carbon-isotope stratigraphy of the Yellow Cat Member, Cedar Mountain Formation, eastern Utah, USA

   Robert M. Joeckel, Celina A. Suarez, Noah M. McLean and 7 coauthors · Geosciences 13(2), 32 · 2023

   Open source
6. 06
   Commentaries on different uses of the specific epithet of the large dromaeosaurid Utahraptor Kirkland et al., 1993

   Thiago Vernaschi V. Costa, Normand David · Bulletin of Zoological Nomenclature 76 · 2019

   Open source
7. 07
   The evolution of femoral morphology in giant non-avian theropod dinosaurs

   Romain Pintore, John R. Hutchinson, Peter J. Bishop and 2 coauthors · Paleobiology 50(2) · 2024

   Open source
8. 08
   Utahraptor ostrommaysorum

   Natural History Museum of Utah

   Open source
9. 09
   Utahraptor: Dino Directory

   Natural History Museum, London

   Open source
10. 10
    Utahraptor Megablock Fossil Project

    Utah Geological Survey

    Open source
11. 11
    Utahraptor exhibit handout and current museum information

    Utah State University Eastern Prehistoric Museum

    Open source
12. 12
    Utahraptor State Park—Preserving Dinosaur and Human History

    Utah Geological Survey · 2025

    Open source