Utahraptor was the largest raptor dinosaur that ever lived, making close relatives like Deinonychus seem positively shrimpy by comparison. Here are 10 facts you may (or may not) have known about this Cretaceous predator. (See also a gallery of Utahraptor pictures.)
1. Utahraptor was discovered in...you guessed it...Utah!
Dozens of dinosaurs have been discovered in the state of Utah, but very few names directly reference this fact. The "type fossil" of Utahraptor was unearthed in Utah's Cedar Mountain 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 Utahceratops.
2. Utahraptor is the largest raptor yet identified...
Utahraptor's claim to fame is that it was the largest 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 or so for the much later Deinonychus. (In case you were wondering, the two-ton Gigantoraptor from central Asia technically wasn't a raptor, but a large theropod dinosaur.)
3. ...and the claws on its hind feet were correspondingly huge.
Raptors were distinguished by the large, curving, single claws on each of their hind feet, which they used to disembowel their prey. Befitting its large size, Utahraptor possessed especially dangerous nine-inch-long claws (which sort of made it the dinosaur equivalent of a saber-tooth tiger!) Utahraptor probably dug its claws into plant-eating dinosaurs like Iguanodon; see Utahraptor vs. Iguanodon - Who Wins? for a blow-by-blow description of this battle.
4. 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 well-known raptors (like the North American Deinonychus and the Asian Velociraptor) flourished toward the end of the Cretaceous period, 50 million years after Utahraptor's day had come and gone.
5. ...a neat reversal of the usual evolutionary pattern.
In the history of life on earth, it's usually the case that large animals evolve from pint-sized predecessors (witness the tiny mammals, like Pakicetus, that gave eventually rise to the first whales). Utahraptor is the exception that proves the rule: this dinosaur lay near the root of the theropod family tree that spawned the much smaller raptors of the later Cretaceous period.
6. Utahraptor was a close relative of Achillobator.
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 20 feet from head to tail) but had some odd anatomical quirks of its own, notably the extra-thick Achilles tendons in its heels.
7. Utahraptor was probably covered with feathers...
Befitting their kinship with the first prehistoric birds, most, if not all, raptors were covered with feathers, at least during certain stages of their life cycles. Although no direct evidence has been adduced for Utahraptor having feathers, they were almost certainly present, if only in hatchlings or juveniles (and the odds are that full-grown adults were feathered as well).
8. ...and it probably had a warm-blooded metabolism, too.
Today, most paleontologists agree that theropod dinosaurs possessed some kind of warm-blooded metabolism--perhaps not the robust physiology of modern mammals, but something intermediate between reptiles and mammalians. As a large, feathered, actively predatory theropod, Utahraptor was almost certainly warm-blooded, which was bad news for its cold-blooded, plant-munching prey.
9. The species name of Utahraptor 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, Ostrom speculated that Deinonychus was a distant ancestor of modern birds, a theory that has since been accepted by the vast majority of paleontologists.
10. Utahraptor may have hunted in packs.
Since only isolated fossils of Utahraptor have been discovered, positing any kind of pack behavior is a delicate matter. However, there's strong evidence that the closely related North American raptor Deinonychus hunted in packs, and it may well be the case that pack hunting defined raptors every bit as much as those curved claws on their hind feet!