His 1964 discovery of "Deinonychus" was one of the most important fossil finds in history. "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. The conclusion that at least some dinosaurs had a high metabolism, and were therefore warm-blooded, was popularized by his student Robert Bakker. This helped to change the impression of dinosaurs as the sluggish, slow, cold-blooded lizards which had prevailed since the turn of the century.