For the last 20 million years of the Age of Dinosaurs, giant tyrannosaurs sat high on the top of the global food chain thanks to their enormous size and sharp senses.
But it wasn't always that way.
The giant carnivores that ruled the planet from 85 to 65 million years ago evolved from an ancestral lineage of tyrannosaurids that were much smaller -- about the size of a horse, paleontologists say.
These mini-tyrannosaurids also lacked the distinctive brains and specialized ears attuned to low frequency sounds that helped their more modern relatives hunt so effectively.
The earliest tyrannosaurids begin to show up in the fossil record about 170 million years ago, or almost 100 million years before T. Rex first stomped across the Earth. Paleontologists say that their ascent to the colossal giants of the late Cretaceous was one of the seminal events in dinosaur evolution.
But exactly how this change developed has long been a mystery. That's because the fossil record of tyrannosaurids has had a frustrating gap that lasted for about 20 million years between roughly 100 and 80 million years ago.
But now, at last, some clues to the how the kings of the dinosaurs evolved have started to emerge.
A paper published Monday in PNAS describes the recently discovered Timurlengia euotica, a species of tyrannosaurid that lived about 90 million years ago, helping to fill in "a major gap in the evolutionary history of tyrannosaurids," the researchers wrote.
The newly described dinosaur was discovered in Kyzylkum Desert in Uzbekistan. It was also the size of a horse, but the authors say it had an inner ear similar to that of later tyrannosaurids, as well as a brain that was similar in structure, although smaller than that of T. rex.
This suggests that this dinosaur lineage developed its keen hunting senses first, and its giant size later.
"Although it is currently only a single data point, Timurlengia indicates that tyrannosaurids remained small-to-medium-sized well into the Middle Cretaceous," the authors wrote.
Although the find is helpful, the authors said they still hope to learn more about this murky interval in dinosaur history.
"Future discoveries from this gap will undoubtedly lead to a better understanding of how tyrannosaurids rose from marginal creatures to some of the largest terrestrial predators on Earth," they wrote.