A horse-sized tyrannosauroid may help explain how the small, toothy predators of the Early Cretaceous evolved into the colossal carnivores that dominated from 80 to 66 million years ago. The mid-sized Timurlengia euotica from Middle Cretaceous Uzbekistan is the long-awaited intermediate tyrannosauroid: It had yet to develop the size of T. rex and its close cousins, but it already boasted some sophisticated brain and sensory features. The findings are published in the Proceedings of the National Academy of Sciences this week.

Thanks to their gigantic size and keen senses, apex predators like Tyrannosaurus and Albertosaurus enjoyed a spot at the top of the food chain in both Asia and North America up until the end-Cretaceous mass extinction event. But how these latest Cretaceous giants (called tyrannosaurids) came to dominate is still unclear because of a frustrating 20-million-year gap in the tyrannosaur fossil record.

We know that they evolved from an ancestral lineage of basal tyrannosauroids, which originated more than 100 million years before T. rex. The group first appeared in the fossil record of the Middle Jurassic around 170 million years ago, but they remained second-tier predators for most of their history – rarely exceeding the mass of a horse.

Now, University of Edinburgh’s Stephen Brusatte and colleagues have discovered a new tyrannosaur species that bridges the gap – in both size and age – between the marginal hunters of the Early Cretaceous and the tyrannosaurids. Timurlengia euotica was recovered from the Bissekty Formation in the Kyzylkum Desert of Uzbekistan. It lived around 90 to 92 million years ago, and it weighed 170 to 270 kilograms (370 to 595 pounds).

When the team used X-ray computed microtomography scanning to visualize the internal structures of the well-preserved fossil braincase, they found the signature brain and inner ear features of the latest Cretaceous forms. The new genus is named after the 14th-century Central Asian ruler Timurleng, and the new species name means "well eared," a reference to the large inner ear.

"To be honest, I didn't quite know what to expect in a mid-Cretaceous tyrannosaur. It was such a big gap in the fossil record that really anything was possible," Brusatte tells IFLScience. "I would have predicted that they would have been bigger. Maybe not T. rex size, but bigger than a horse for sure."

Analyses into its evolutionary relationships revealed that Timurlengia euotica occupies an intermediate position in the family tree between the oldest, smallest tyrannosauroids and the largest, last-surviving tyrannosaurids. While Timurlengia is just one data point from a "still murky interval in dinosaur history," it suggests that enormous size developed rapidly during the latest Cretaceous. The success of Tyrannosaurus and its close kin was likely enabled by innovations that first evolved at a far smaller body size.

"The advanced brains and senses may have been some of the features that allowed tyrannosaurs to step into the large predator role, and become so successful at it," Brusatte adds. "Opportunity arose when the incumbent giant predators (the allosaurs) mysteriously went extinct."