John Ostrom's 1964 discovery of "Deinonychus" is one of the most important fossil finds. "Deinonychus" was an active predator that clearly killed its prey by leaping and slashing or stabbing with its "terrible claw". Evidence of a truly active lifestyle included long strings of tendons running along the tail, making it a stiff counterbalance for jumping and running. One conclusion was that at least some dinosaurs had a high metabolism, and so in some cases were warm-blooded. This was popularized by Ostrom's student Robert Bakker. The impression of dinosaurs as being slow, cold-blooded, small-brained reptiles needed to be rethought, at least for some of the carnivores.