Deinonychus antirrhopus was a wolf- or leopard-sized predator, reaching lengths of 3.5 meters and perhaps 60 to 80 kg. It lived in the early-middle Cretaceous, about 115 to 108 million years ago in the inland western region of the United states, ranging from what is today Montana to Utah and possibly east to the Atlantic coast. Its fossils are found in floodplain, tropical forest, and swampy environments. It lived alongside the giant predator Acrocanthosaurus, the immense sauropod Sauroposeidon, the smaller (but still large) sauropod Astrodon, nodosaur dinosaurs Sauropelta, and ornithopod dinosaurs Zephyrosaurus and Tenontosaurus, not to mention a variety of crocodiles, mammals, turtles, and lungfish.
There are two fossil sites where remains of several individual Deinonychus skeletons have been found around a Tenontosaurus fossil. This has led to speculation that Deinonychus hunted in packs, using their numbers to bring down much larger prey such as Tenontosaurus. However, there are plenty of other explanations for how many individual Deinonychus might come together. It is possible that they were simply an unorganized mob clustered around a carcass, in much the way that modern eagles, vultures, crocodiles, and Komodo dragons gather around large carcasses even though they did not cooperate to bring it down.
Regardless of whether Deinonychus was solitary or cooperative, it did seem to frequently feed upon Tenontosaurus. In addition to full Deinonychus fossils in close proximity to Tenontosaurus, many other Tenontosaurus specimens are associated with Deinonychus teeth (which were probably shed during feeding) or gouge marks on the bone from Deinonychus bites. It is not clear if Deinonychus actually killed these animals, or if it was just scavenging them. Certainly, Tenontosaurus is a much larger animal than Deinonychus and would have been a challenge for this smaller predator to bring down.