How did evolution produce the T Rex? This new discovery provides a clue

Tyrannosaurus rex is an icon, a dinosaur known to nearly everyone on the planet. It doesn’t get much more awesome than a 13-metre long, seven-ton superpredator that could bite through the bones of its prey.

T. rex may be the undisputed king of the dinosaurs, but how did evolution produce such a marvellous creature, the biggest predator ever to live on land? It’s been a mystery for a long time, but a new species of tyrannosaur from Uzbekistan – a smaller and earlier cousin of T. rex – provides some valuable clues.

Meet Timurlengia euotica, a horse-sized tyrannosaur that lived about 90m years ago when Uzbekistan was a sweltering maze of forests and rivers bordering a vast inland sea.

Small and smart

The bones of Timurlengia were collected during a decade of field expeditions to Uzbekistan’s desolate Kyzylkum Desert, one of the driest areas of the world, led by my colleagues Alexander Averianov and Hans-Dieter Sues. They invited me to help study the fossils, and we have described the new species in a paper in Proceedings of the National Academy of Sciences.

Timurlengia is particularly important because it is the first tyrannosaur known from the middle part of the Cretaceous period. Previously this was a dark interval of tyrannosaur history: a 20-30 million year gap in the fossil record concealing the moment when tyrannosaurs switched from fairly marginal hunters living in the underbrush to the colossal tyrants that fuel our nightmares.

Like pieces of a jigsaw puzzle, we have various bits of Timurlengia’s skeleton, including part of the snout and jaws, some teeth, various vertebrae of the neck, back and tail, and fragments of the hands and feet. These bones tell us that Timurlengia was about 3-4 metres long and weighed about 170-270 kilograms, roughly the size of a big horse.

Timurlengia would have been a nasty critter, but nowhere near the brutish size of T. rex. In fact, it wasn’t at the top of the food chain at all. It was still living in fear of other, more primitive carnivorous dinosaurs called allosaurs, which were the apex predators of the day.

But there’s also another part of Timurlengia’s skeleton that we were able to study: the braincase, the fused bones at the back of the skull that surround the brain, ear, and sinuses. We put it into a CT scanner, which allowed us to digitally peer inside and see what the brain and sensory organs looked like.

This gave us quite a surprise: Timurlengia had the same type of brain and ear as the giant tyrannosaurs such as T. rex. It was very smart, and had an ear attuned to hearing low frequency sounds. Previously, these features were thought to be unique to the big tyrannosaurs, part of that toolkit of predatory superpowers that evolved as they turned into giants.

Evolution story

So our new Uzbek tyrannosaur helps to tell a story, about how evolution turns seemingly ordinary animals into extraordinary freaks of nature. It goes something like this.

Tyrannosaurs originated around 170m years ago during the Jurassic period, as human-sized, fast-running stalkers who used their long arms to grab prey. For about 80m years they stayed this way, far from spectacular, but eking out a living in the shadows.

Then some of these small tyrannosaurs developed sophisticated brains and senses, probably to help them better track their prey. Little did they know that, eventually, these neurosensory features would come in handy, when the allosaurs went extinct around 80-90m years ago and a new niche at the top of the food pyramid suddenly opened up. Their intelligence and sharp senses made tyrannosaurs perfectly equipped to swoop into the top-predator role.

And swoop they did. Very quickly the human-to-horse-sized tyrannosaurs grew into supersized monsters, longer than a bus and weighing more than a ton. Their heads became giant killing machines and their arms, now unnecessary, shrunk down to nubbins. By 80m years ago these mega tyrannosaurs were terrorising what is now North America and Asia, spreading into all ecosystems on land, displacing smaller predators, and eating whatever they wanted.

It would remain this way for another 15m years or so, until the day, when T. rex was at the peak of its success rampaging across western North America, that a 10-kilometer-wide asteroid fell out of the sky and the world changed in an flash.