How did T. rex get such keen, predatory senses? New tyrannosaur offers hints.

There's a new dinosaur in town, and it holds tantalizing clues into the evolution of tyrannosaurs.

One of the most fearsome, massive members of this dinosaur family, Tyrannosaurus rex is thought to have been such a successful predator thanks to its size, intelligence, and keen senses. But little was known about how T. rex came to have those features.

It turns out tyrannosaurs probably became huge rather quickly at the end of their history, but their unique, strong sense perception evolved earlier, according to a paper announcing the discovery of a new tyrannosaur species, published Monday in the journal Proceedings of the National Academy of Sciences.

"Although T. rex is very familiar to us, as dinosaurs go it's actually really bizarre," Lawrence Witmer, a paleontologist at Ohio University who was not part of this study, tells The Christian Science Monitor in an interview. But paleontologists had yet to get a good glimpse into the evolution of the beast's unique features, thanks to a spotty fossil record.

Timurlengia euotica, the newly-discovered tyrannosaur, lived 90 million years ago, right smack in the middle of a 20-million-year gap in the fossil record of tyrannosaurs. And in such a position in the tyrannosaur family tree, the new fossils, although fragmentary, reveal long-sought insights into how its gigantic, fearsome tyrant lizard relative came to rule the dinosaur landscape.

Timurlengia was about the size of a horse, not much larger than the human-sized tyrannosaurs that kicked off the lineage some 170 million years ago. T. rex was closer to the size of a tractor-trailer truck and appeared on the scene around 80 million years ago. So these predatory dinosaurs likely evolved to be so huge in just about 10 million years. 

"Because T. rex is the most famous dinosaur, we often think of tyrannosaurs as if they were always the biggest and meanest of the bunch, but they only achieved this status late in their evolutionary history," says Lindsay Zanno, head of the Paleontology Research Lab at the North Carolina Museum of Natural Sciences, who was not involved with the study, in an email to the Monitor. But tyrannousaurs, she says, did not beat out the existing predators so much as outlast them. "The truth is that tyrannosaurs were little more than understudies in ecosystems long dominated by other dinosaurian megapredators before seizing the opportunity to become the starring act themselves."

Despite the size differences between Timurlengia and T. rex, the new specimen has some of its later cousin's more distinct features. Some of the same structures seen inside T. rex skulls thought to give the massive predator heightened senses appeared in the Timurlengia specimens unearthed in Uzbekistan. 

These structures were previously only seen in the bigger, later tyrannosaurs, says study lead author Stephen Brusatte in a phone interview with the Monitor. But "now we know they evolved in earlier, smaller tyrannosaurs and later on were co-opted to become part of the predatory arsenal of T. rex."

Evolving perceptive hunters

Some of the fossils found were part of the prehistoric animal's skull, particularly the braincase. The researchers CT scanned the fossils and were able to construct a model of what the inside of the animal's skull looked like to investigate its sensory structures.

Although Timurlengia's braincase was around the size of a grapefruit, and T. rex's braincase was larger than a basketball, Dr. Brusatte says, "it turns out that inside of that small skull it has a brain and an ear that look just like miniature versions of the ones in T. rex."

These sensory structures, says Dr. Zanno, set the stage for T. rex's ancestors to fill the ecological niches vacated by earlier Cretaceous predators. "Brusatte and his team have uncovered evidence that tyrannosaurs already had the advanced sensory toolkits they would need to become the most feared predators on the planet and were primed and at the ready for taking over ecosystems when their competitors bit the dust," she says.

Hunting with their ears

One of T. rex's unique abilities was its hypersensitive hearing. A bone in its inner ear, the cochlea, was remarkably long. And in animals alive today, that specialization is associated with the ability to hear low frequency sounds well. Timurlengia had a long cochlea too.

"You can imagine hearing would be very important for predators, especially for a very big predator that has to eat a lot of food," Brusatte says. "It was part of the toolkit that makes these things such great predators."

And now researchers know this sense evolved before tyrannosaurs dominated the landscape.

But Timurlengia's hearing structure isn't a perfect match for T. rex's, Dr. Witmer points out. Sinuses in the middle ear aren't quite as specialized as those in the later dinosaur's head. "It really is transitional, or intermediate between earlier tyrannosaurs and other dinosaurs, and these really specialized late Cretaceous dinosaurs like T. rex itself," he says of Timurlengia.

Pieces of the puzzle

Although scientists credit the animal's keen senses to its later success, Brusatte cautioned against assuming those same senses lead to the later tyrannosaurs' large size too. There's just not enough fossil evidence to make that conclusion, he says.

Furthermore, this specimen is not a complete skeleton of Timurlengia. In addition to parts of its skull, the researchers found parts of its spine and a few pieces of its claws. "It's like a detective finding a single clue at a murder site," Brusatte says. It can answer some questions, but leads to more. 

"The new species only begins to fill in this significant temporal and evolutionary gap in the tyrannosaur fossil record," David Evans, a vertebrate paleontologist and curator at the Royal Ontario Museum who was not part of this study, writes in an email to the Monitor.

"Its contribution to our knowledge of sensory adaptation in tyrannosaurs will become clearer with more detailed studies of the brain cavity and sensory systems of more primitive tyrannosaurs in particular, and with more quantitative analytical approaches."

Timurlengia's discovery could lend insight not just to Cretaceous ecosystems, but to ones in our current geologic period, which thanks to human activity is experiencing a mass extinction, particularly of top predators. "By studying how extinction shaped ecosystems in the past," says Zanno, "we can better prepare ourselves for what's coming down the pike in the future."