Even the biggest bully on the block starts life as a tiny tot. That’s true of dinosaurs, too. The earliest ancestors of Tyrannosaurus rex lived about 170 million years ago and were no bigger than a human is today. But by the time of T. rex’s heyday, about 68 million years ago, the ferocious beast was up to 12 meters long, as long as a truck trailer. How did it get so big? New dino fossils from Uzbekistan may provide an answer: Along their evolutionary way, tyrannosauroids, as this early dino group is called, developed a keen sense of hearing and other senses that propelled them to apex predator status.

Attempts by paleontologists to understand how tyrannosauroids evolved into tyrannosaurs have long been plagued by an inconvenient time gap: 20 million years between the early and late specimens. The last known species before this gap, dated no later than 100 million years ago, were still only about the size of a horse. But the first tyrannosaurs in the fossil record after this gap, dated to about 80 million years, were already very large, up to 9 meters long. What happened in between?

The new finds are providing some answers. This week a team led by Stephen Brusatte, a paleontologist at the University of Edinburgh in the United Kingdom, reports on its study of a skull and other assorted fossils of a tyrannosauroid found earlier in the Kyzylkum Desert of Uzbekistan, in sediments dated to 90 million years ago. According to co-author Hans-Dieter Sues, a geologist at the Smithsonian Institution’s National Museum of Natural History in Washington, D.C., the fossils were unearthed by an international team including Uzbek and Russian scientists between 1997 and 2006, and later study revealed that they belonged to an intermediate tyrannosauroid species.

As the team reports online today in the Proceedings of the National Academy of Sciences, this gap-filling species is still only about the size of a horse. The means that the explosive growth that would lead to T. rex had still not taken place. But the skull is notable for its unusually complex inner ear apparatus, typical of later tyrannosaurs like T. rex, that would have given the modestly sized animal a keen sense of hearing. In particular, it has a long cochlear duct, a canal in the inner ear associated with the ability to hear the kind of low-frequency sounds that its vegetarian prey would have made as they crept through the underbrush. The team has christened the species Timurlengia euotica (Timurlengia for the 14th century Asian ruler Timurleng, known as Tamerlane in English, and euotica, Latin for “well-eared”).

“The brain and ears of Timurlengia look like miniature versions of the same structures in T. rex,” Brusatte says. “It may be that the earlier evolution of these features gave tyrannosaurs the perfect toolkit that came in handy when the opportunity presented itself for them to rise to the top of the food chain.” That ability to chomp on other dinos pretty much at will gave them the calories they needed to maintain larger sizes, the team suggests.

The new dino is “long-awaited,” says Thomas Carr, a paleontologist and tyrannosaur expert at Carthage College in Kenosha, Wisconsin. The “huge temporal gap” between earlier tryannosauroids and later tyrannosaurs, he says, “really is a black box. We don’t have a complete picture of how that transition occurred.” On the other hand, Carr cautions, the fossil record is still too patchy to pin the success of tyrannosaurs on any particular feature of their brains. Until there are more specimens, he says, it will be hard to tell “if it was novel anatomy or just dumb luck” that led to the later success of T. rex and its mighty relatives.