On the second floor of Toronto's Royal Ontario Museum, the skeleton of an older, ganglier cousin of Tyrannosaurus rex stands tall. But if the creature were alive today, it might be limping. More than 70 million years ago, this Gorgosaurus would have been an apex predator in what are now the badlands of Montana and western Canada. Apex doesn't mean invincible, though. The animal's right shin is a mess of broken bone that healed over in life.

What broke the poor tyrannosaur's leg? Short of hopping in a time machine, researchers can't be sure. But elsewhere in the same museum, visitors can get a glimpse of one of the best—and most exquisite—suspects in this Cretaceous cold case.

Meet Zuul crurivastator, a