Deinonychus was a lightly built, fast-moving, agile, bipedal (walked on two legs), bird-like dinosaur. It was built to kill. This meat-eater had a curved, flexible neck and a big head with sharp, serrated teeth in very powerful jaws. Each of its three fingers on each hand had large, sharp, curved claws. It had four-toed feet; the second toe had a 5-inch (13-cm) sickle-like claw, and the other toes had smaller claws. Its long tail had bony rods running along the spine, giving it rigidity; the tail was used for balance and fast turning ability. Deinonychus had a relatively large brain and large, keen eyes. Deinonychus was about 10 feet long (3 m), 5 feet tall (1.5 m), and weighed up to 175 pounds (80 kg). This dinosaur was roughly 4 ft (1.2 m) tall at the shoulder. Its femur (thigh bone) was 31 cm long. Deinonychus lived during the Cretaceous period, about 110 to 100 million years ago. Among the contemporaries of Deinonychus were Spinosaurus, Archaeornithomimus (another swift, bird-like theropod), Sauroplites (an ankylosaurid, a plated herbivore), Sauropelta (a nodosaur, another armored herbivore), Probactrosaurus (an iguanodontid), Titanosaurs (a sauropod), and Tenontosaurus (a hypsilophodontid). Deinonychus may have hunted in packs, attacking even very large animals, perhaps even large sauropods and ankylosaurids. Tenontosaurus and Ankylosaurus may have been among its prey; it was an unarmored but fast hypsilophodontid whose fossils have been found near Deinonychus fossils. Deinonychus, along with the other dromaeosaurids, were among the smartest of the dinosaurs, as calculated from their brain:body weight ratio. This made them very deadly predators. Deinonychus was a dromaeosaurid, whose intelligence (as measured by its relative brain to body weight, or EQ) was the highest among the dinosaurs. Deinonychus was a carnivore, a meat eater. It probably ate just about anything it could slash and tear apart. When hunting in packs, Deinonychus could probably kill any prey it desired. Deinonychus walked on two slender, bird-like legs; it must have been a fast runner, considering its legs and light weight. When it ran, it rotated its huge foot-claw upwards and ran on the other toes.