Science, Tech, Math › Animals & Nature 10 Facts About Utahraptor, the World's Biggest Raptor Share Flipboard Email Print Stocktrek Images/Getty Images Animals & Nature Dinosaurs Carnivores Basics Paleontologists Dinosaurs & Birds Herbivores Marine Reptiles Prehistoric Mammals Amphibians Birds Habitat Profiles Mammals Reptiles Wildlife Conservation Insects Marine Life Forestry Evolution View More By Bob Strauss Science Writer B.S., Cornell University Bob Strauss is a science writer and the author of several books, including "The Big Book of What, How and Why" and "A Field Guide to the Dinosaurs of North America." our editorial process Bob Strauss Updated July 07, 2019 Weighing nearly a full ton, Utahraptor was the largest, most dangerous raptor that ever lived, making close relatives like Deinonychus and Velociraptor seem positively shrimpy by comparison. Utahraptor Is the Largest Raptor Yet Discovered Utahraptor's claim to fame is that it was by far the biggest raptor ever to walk the earth; adults measured about 25 feet from head to tail and weighed in the neighborhood of 1,000 to 2,000 pounds, compared to 200 pounds for a more typical raptor, the much later Deinonychus, not to mention the 25- or 30-pound Velociraptor. In case you were wondering, the two-ton Gigantoraptor from central Asia technically wasn't a raptor, but a large, and confusingly named theropod dinosaur. The Claws on Utahraptor's Hind Feet Were Almost a Foot Long Among other things, raptors are distinguished by the large, curving, single claws on each of their hind feet, which they used to slash at and disembowel their prey. Befitting its large size, Utahraptor possessed especially dangerous-looking nine-inch-long claws (which sort of made it the dinosaur equivalent of the Saber-Toothed Tiger, which lived millions of years later). Utahraptor probably dug its claws on a regular basis into plant-eating dinosaurs like Iguanodon. Utahraptor Lived During the Early Cretaceous Period Perhaps the most unusual thing about Utahraptor, aside from its size, is when this dinosaur lived: about 125 million years ago, during the early Cretaceous period. Most of the world's well-known raptors (like Deinonychus and Velociraptor) flourished toward the middle and end of the Cretaceous period, about 25 to 50 million years after Utahraptor's day had come and gone—a reversal of the usual pattern in which small progenitors tend to give rise to plus-size descendants. Utahraptor Was Discovered in Utah Dozens of dinosaurs have been discovered in the state of Utah, but very few of their names directly reference this fact. The "type fossil" of Utahraptor was unearthed from Utah's Cedar Mountain Formation (part of the larger Morrison Formation) in 1991 and named by a team including paleontologist James Kirkland; however, this raptor lived tens of millions of years before its fellow Utah namesake, the recently described (and much bigger) horned, frilled dinosaur Utahceratops. Utahraptor's Species Name Honors Paleontologist John Ostrom The single-named species of Utahraptor, Utahraptor ostrommaysorum, honors the famous American paleontologist John Ostrom (as well as the dinosaur robotics pioneer Chris Mays). Way back before it was fashionable, in the 1970s, Ostrom speculated that raptors like Deinonychus were the distant ancestors of modern birds, a theory that has since been accepted by the vast majority of paleontologists (though it's not clear whether raptors, or some other family of feathered dinosaur, lay at the root of the bird evolutionary tree). Utahraptor Was (Almost Certainly) Covered in Feathers Befitting their kinship with the first prehistoric birds, most, if not all, raptors of the late Cretaceous period, like Deinonychus and Velociraptor, were covered with feathers, at least during certain stages of their life cycles. Although no direct evidence has been adduced for Utahraptor possessing feathers, they were almost certainly present, if only in hatchlings or juveniles—and the odds are that full-grown adults were plushly feathered as well, making them look a bit like giant turkeys. Utahraptor Is the Star of the Novel "Raptor Red" Though the honor of its discovery went to James Kirkland (see above), Utahraptor was actually named by another eminent paleontologist, Robert Bakker—who then went on to make a female Utahraptor the main protagonist of his adventure novel Raptor Red. Correcting the historical record (and the errors perpetrated by movies like Jurassic Park), Bakker's Utahraptor is a fully fleshed-out individual, not evil or malicious by nature but simply trying to survive in its harsh environment. Utahraptor Was a Close Relative of Achillobator Thanks to the vagaries of continental drift, most of the North American dinosaurs of the Cretaceous period had similar-looking counterparts in Europe and Asia. In the case of Utahraptor, the ringer was the much later Achillobator of central Asia, which was slightly smaller (only about 15 feet from head to tail) but possessed some odd anatomical quirks of its own, notably the extra-thick Achilles tendons in its heels (which doubtless came in handy when it was gutting prey like Protoceratops) from which it derives its name. Utahraptor Probably Had a Warm-Blooded Metabolism Today, most paleontologists agree that the meat-eating dinosaurs of the Mesozoic Era possessed some kind of warm-blooded metabolism—perhaps not the robust physiology of modern cats, dogs, and humans, but something intermediate between reptiles and mammals. As a large, feathered, actively predatory theropod, Utahraptor was almost certainly warm-blooded, which would have been bad news for its presumably cold-blooded, plant-munching prey. No One Knows If Utahraptor Hunted in Packs Since only isolated individuals of Utahraptor have been discovered, positing any kind of pack behavior is a delicate matter, as it is for any theropod dinosaur of the Mesozoic Era. However, there's strong evidence that the closely related North American raptor Deinonychus hunted in packs to bring down larger prey (like Tenontosaurus), and it may well be the case that pack hunting (and primitive social behavior) defined raptors every bit as much as their feathers and the curved claws on their hind feet!