Utahraptor

Utahraptor ostrommaysorum is the largest known dromaeosaurid. It measured roughly 5.5–6 m in length and weighed between 280 and 500 kg. These upper estimates are based on findings from the "Stikes Quarry" block, which contains material currently being studied. Jim Kirkland and his team named the species in 1993 to honour the work of John Ostrom and the Mays family, who provided funding for the research.

Its anatomy follows the general dromaeosaurid pattern but on a much larger scale. It featured a deep, sturdy skull, blade-like teeth, and three-fingered hands equipped with large recurved claws. The sickle claw on its second toe reached lengths of up to 24 cm along its outer curve. While Deinonychus was built for speed, Utahraptor was a heavier, more robust animal. This sturdy build was likely a result of allometric scaling, as a larger predator required stronger bones to support its increased mass.

Although no fossils preserve direct evidence of feathers, its position within the feathered clade Eumaniraptora and its relationship to Sinornithosaurus make their presence certain. Adult Utahraptor were too large for flight, but they would have possessed pennaceous feathers on their arms and a fan of feathers on their tails.

Utahraptor shared its environment with the iguanodontid Iguanocolossus, the armoured Gastonia, and several sauropod species. The "Stikes Quarry" find consists of a single sandstone block containing the remains of at least nine individuals across various age groups. While this assemblage could indicate social behaviour, researchers also consider the possibility that it was a "predator trap" where multiple animals were buried in quicksand.

Estimates suggest its bite force was modest for its size. Like other members of its family, Utahraptor likely used its sickle claws and forelimbs to pin down prey before delivering a fatal bite. Its strength and weaponry likely allowed it to target animals significantly larger than itself.

• CEU 184v.86 — original holotype material, Prehistoric Museum of Utah.
• The Utahraptor Megablock ("Stikes Quarry") — multi-individual block, in long-running preparation; held at Utah Geological Survey / NHMU.
• Additional Cedar Mountain Formation material from multiple Utah quarries.

Maximum size — published estimates have ranged widely (~300 kg to ~500 kg), with the upper end possibly larger pending Megablock analysis. Social behaviour — the Megablock is suggestive but not conclusive; obligate group behaviour is difficult to demonstrate. Species validity — single species U. ostrommaysorum; some workers consider that the Megablock material may eventually require additional taxonomic attention.

Utahraptor appears in Walking with Dinosaurs: Land of Giants (2002), Prehistoric Planet, and many recent dinosaur publications, often as a stand-in for the cinematic "raptor" but at a more accurate scale.

• dromaeosauridae
• deinonychus
• velociraptor
• microraptor

• Kirkland, J. I., Burge, D., & Gaston, R. (1993). A large dromaeosaurid (Theropoda) from the Lower Cretaceous of eastern Utah. Hunteria, 2, 1–16.
• Britt, B. B., et al. (2001). New osteological data and the affinities of Utahraptor from the Cedar Mountain Fm., Utah. Journal of Vertebrate Paleontology, 21 (suppl. 3), 36A.
• Kirkland, J. I., et al. (1997). Lower to Middle Cretaceous Dinosaur Faunas of the Central Colorado Plateau. Brigham Young University Geology Studies, 42, 69–103.
• Senter, P. (2007). A new look at the phylogeny of Coelurosauria. Journal of Systematic Palaeontology, 5, 429–463.