Tyrannosaurs weren’t always tyrants. For millions of years, the ancestors of the regal T. rex were relegated to second-class predator status while a different dinosaur dynasty ruled over what is now North America: towering allosaurs.

But the allosaurs went extinct during the late Cretaceous, allowing tyrannosaurs to seize the throne and then evolve into large killing machines like T. rex and Tarbosaurus.

To better understand how and when tyrannosaurs became giants, paleontologists have sought examples of their lineage from when they were small. Their latest discovery is a tiny tyrannosaur that lived in the shadow of larger predators some 96 million years ago.

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Called “Moros intrepidus,” the new species is the oldest Cretaceous-period tyrannosaur ever found in North America and among the smallest in the world, measuring only about as big as a deer. Because scientists have previously found large tyrannosaurs in North America that date to 81 million years ago, the newly discovered species helps narrow the window of when tyrannosaurs became huge.