Brains came before brawn for tyrannosaurs. The iconic Tyrannosaurus rex, the star of Jurassic Park, emerged as a huge and cunning predator some 80 million years ago. But how it evolved has long been a mystery, because of a gap in the fossil record that began 100 million years ago.
Now, new remains that include a skull fragment show that this dinosaur started out smaller, the size of a bear, and evolved its big brain before its huge stature.
The first tyrannosaurs evolved about 170 million years ago, when they split from the group of feathered predators that later gave rise to birds. But they remained in the shadows of larger beasts, such as allosaurs and spinosaurs. They emerged as giants during a 20-million-year gap in the fossil record, when the dinosaur age was coming to an end.
A tyrannosauroid fossil called Timurlengia euotica dating back 90 million years fills that gap, helping us understand how T. rex was able to become so big and so dominant, says Stephen Brusatte of the University of Edinburgh, UK.
The vital clues emerged two years ago when Alexander Averianov of the Russian Zoological Institute in St Petersburg showed Brusatte a braincase that was among many fossils his group had collected in the remote Bissekty formation of Uzbekistan.
“I’ve studied a lot of tyrannosaurs and a lot of braincases, and after a while I recognised it as a tyrannosaur,” Brusatte says. On further scrutiny, the team concluded it was a new species.
CT scans revealed that Timurlengia’s brain was large for a dinosaur and that it had structures in the inner ear used for balancing and for hearing low-frequency sounds. Earlier tyrannosaurs lacked those traits, which could have helped T. rex in its niche as an apex predator.
Yet with a body mass of some 200 kilograms, Timurlengia was much smaller than T. rex, which could weigh over 5 tonnes.
The fossils support earlier proposals that T. rex evolved “head first” – brain before bulk.
But how it took over as apex predator remains unclear, although Brusatte says it may have taken advantage of the mysterious disappearance of other large predators. “It looks like the allosaurs go out and tyrannosaurs opportunistically take over,” says Brusatte.
“This is cool stuff,” says Thomas Holtz of the University of Maryland in College Park. But he notes that the remains of Timurlengia are too fragmentary to reveal other details like the length of its arms. He hopes further fossil discoveries will fill in the gaps.
The number of tyrannosaur species known has more than doubled in the past 15 years, says Mark Norell of the American Museum of Natural History in New York. “It’s a great time to be working on tyrannosaurs.”
Other Uzbek fossils collected by Averianov’s team have already yielded important insight into other dinosaurs. They include the oldest horned dinosaurs and the first duck-billed dinosaurs, both of which lived alongside T. rex, says Holtz.
Journal reference: PNAS, DOI: 10.1073/pnas.1600140113
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