Tyrannosaurus rex, one of the most feared predators in the Age of Dinosaurs, may have been built for endurance, not speed. A paper published Wednesday takes recent research on how mammals move and applies it to dinosaurs. Its conclusions support theories that the massive meat-eaters hunted in packs and opens a window into the ecology of the ancient forests they roamed. “We’re trying to figure out how much energy is going into and flowing through paleo ecosystems,” said Hans Larsson of Montreal’s McGill University, one of the paper’s co-authors. “If we can’t get an estimate for what it takes to feed the apex predators, then we have no chance of estimating anything else.” To figure out how much T. rex needed to eat, the scientists first had to figure out how it moved, including how fast it could run.