How did the Tyrannosaurus rex become one of the fiercest dinosaurs the planet has ever seen? By developing keen senses prior to the growth spurt that allowed it to grow to be as much as 13 feet tall and weigh close to seven metric tons, according to a new study.

According to lead researcher Dr. Steve Brusatte from the University of Edinburgh’s School of GeoSciences and his colleagues, the new remains of a horse-sized dinosaur recently unearthed in Uzbekistan shed new light on how and when tyrannosaurs became such fearsome predators.

As they explained in Monday’s edition of the Proceedings of the National Academy of Sciences, the fossils belonged to a newly identified species, Timurlengia euotica, that lived approximately 90 million years ago and which had much smaller bodies than T. rex and other tyrannosaurs.

Its discovery also fills a 20-million-year-gap in the tyrannosaur fossil record, the authors noted, showing how these horse-sized, 250 kilogram, long-legged, sharp-toothed, fast-running dinosaurs went on to evolve into the massive, intelligent carnivores that dominated the Earth about 66 million years ago. The secret, they said, was that tyrannosaurs developed brains before brawn.

“The ancestors of T. rex would have looked a whole lot like Timurlengia, a horse-sized hunter with a big brain and keen hearing that would put us to shame,” Dr. Brusatte said in a statement. “Only after these ancestral tyrannosaurs evolved their clever brains and sharp senses did they grow into the colossal sizes of T. rex. Tyrannosaurs had to get smart before they got big.”

Tyrannosaurs stayed small until late in their evolutionary history

Previously, experts were uncertain how tyrannosaurs became such massive and cunning hunters, the researchers explained. With the discovery of Timurlengia euotica, however, they can now see that their predecessors already had skull features indicative of highly-developed brains.

Early tyrannosaurs lived about 170 million years ago and were only a little bigger than humans, but over the next 100 million years, they began experiencing a growth spurt of epic proportions. By the end of the Cretaceous Period, they evolved into massive creatures such as the T. rex and the Albertosaurus, which could weight in excess of seven metric tons, the authors said.

Timurlengia euotica would have lived about 80 million years after tyrannosaurs first appeared, which Dr. Brusatte’s team said demonstrates that they only grew to their massive sizes near the tail end of their evolutionary history. Prior to that, they were smaller, highly intelligent hunters that likely preyed on duck-billed dinosaurs and other plant eaters, the researchers noted.

“The middle Cretaceous is a mysterious time in evolution because fossils of land-living animals from this time are known from very few places,” said co-author Professor Alexander Averianov of Saint Petersburg State University. “Uzbekistan is one of these places. The early evolution of many groups like tyrannosaurs took place in the coastal plains of central Asia in the mid Cretaceous.”

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Image credit: Todd Marshall

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