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.

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.

“We now know it took less than 15 million years for them to go from these subsidiary secondary players in the environment to top of the food chain,” said Lindsay Zanno, a paleontologist at North Carolina State University. She and her colleagues published their discovery Thursday in the journal Communications Biology.

In 2012, while Zanno was fossil hunting in central Utah, she noticed a dinosaur leg bone sticking out from a rocky hillside. The next year, she and her team excavated the fossilized remains. Although the bones were poorly preserved, she could tell by how thin they were that they belonged to a theropod, which is the major group of carnivorous dinosaurs that include allosaurs and tyrannosaurs.

Her colleague Aaron Giterman at the North Carolina Museum of Natural Sciences painstakingly pieced the leg together over the course of a year. After examining the dinosaur’s tiny foot and upper leg, Zanno determined it belonged to a tyrannosaur. From hip to toe it measured about 4 feet long, and the dinosaur was estimated to weigh about 170 pounds. T. rex, for comparison, strutted around on legs that were about 12 feet long and weighed more than 10,000 pounds.

The next steps were to find whether the tiny tyrannosaur belonged to a new species or was a juvenile of a known species.

Aurore Canoville, a postdoctoral researcher in Zanno’s lab, collected small samples from the bones. Using a microscope, she identified growth marks in the bone tissue that provided clues to the dinosaur’s age, much like tree rings. Canoville saw evidence suggesting the tyrannosaur was at least 6 or 7 years old. The growth marks, she said, were also spaced closer and closer together, indicating the tyrannosaur was nearing full maturity when it died.

“When we found out it was actually almost an adult, we could tell that it was a new species of a very small tyrannosaur,” Canoville said. Their findings also suggested that the dinosaur was most likely an agile hunter.

Thomas Carr, a paleontologist from Carthage College in Wisconsin who was not involved in the study, said the finding “lifts the lid on what the earliest tyrannosaurs in North America really looked like.”

Stephen Brusatte, a paleontologist at the University of Edinburgh in Scotland who was also not involved in the paper, said the discovery pushed the tyrannosaur mystery forward in time.

“When did these second-tier hunters become the bus-sized bone-crunchers that terrorize our imaginations?” Brusatte said. “It must have happened sometime between about 90 and 80 million years ago, and we’re going to need new fossils from this time to figure it out.”