Fossil foot of a Deinonychus on display in the Museum of the Rockies in Bozeman, Montana. This specimen was collected in Carbon County, Montana.

Deinonychus antirrhopus was first discovered in 1931 near Billings, Montana. Its name means "terible claw", because it could lift the middle toe of its heavily clawed foot and slice with it like a razor.

In the late 1960s, paleontologist John Ostrom published a series of papers about Deinonychus that completely revolutionized the way we think about dinosaurs. Previously, scientists thought of them as plodding, slow, cold-blooded creatures. But Ostrom proved that Deinonychus couldn't be cold-blooded, and was undoubtedly lightning-fast and very agile. Within just a few years, everyone accepted that dinosaurs had to be warm-blooded, and most were extremely active and mobile. This was the "Dinosaur Revolution" or "Dinosaur Renaissance", and it governs the way we think about dinos today.

Deinonychus lived 115 to 108 million years ago in western North America. It was about 11 feet in length, and tended to run with its head lowered and tail stuck straight out as a counterbalance. In this position, it was only about three feet high at the hip. It weighed no more than 160 pounds. The 16-inch-long skull had 70 curved, blade-like teeth in a very narrow snout and it had stereoscopic vision. Deinonychus had strong forelimbs, powerful and very large hands, and three claws on each hand. The middle of the three toes on each hind foot had a sickle-shaped claw. The snap-tendons on that toe permitted it to be used like a scythe!