At the end of the dinosaur age, Tyrannosaurus rex was a behemoth killer animal, up to 40 feet long and weighing several tons — the top carnivore in the food chain.
The very first tyrannosaurs, which arose about 100 million years earlier, were small, about the size of a person.
The evolutionary jump of tyrannosaurs from people- and horse-size to behemoths has remained a mystery. A recent fossil finding in Uzbekistan is providing paleontologists with a missing link in the lineage: They have discovered a tyrannosaur with many of the giant's characteristics — but not its stature or heft.
"It has long been thought that tyrannosaurs were such successful predators, in part, because of their large brains and ears well-attuned to low-frequency sound," said Stephen L. Brusatte, a paleontologist at the University of Edinburgh in Scotland and lead author of a paper published Monday in the Proceedings of the National Academy of Sciences describing the new dinosaur. "The new Uzbek tyrannosaur has basically the same brain as T. rex — same shape, proportions, etc. — just smaller."
The finding, he said, is a new indication that tyrannosaurs got smart before they got big.
"It is one of the closest cousins of T. rex and tells us that tyrannosaurs evolved sophisticated brains and senses before they became colossal apex predators," Brusatte said.
When the first tyrannosaurs evolved, about 170 million years ago, they lived in the shadows of larger meat-eaters like Allosaurus. For tens of millions of years, tyrannosaurs remained small.
And because few rocks of the age 100 million years to 80 million years old are now lying exposed or at the surface anywhere in the world, few clues exist to explain how the early tyrannosaurs evolved into large animals like T. rex.
"The fossil record just stops," Brusatte said. "We don't know what goes on in that time."
Before the gap, all tyrannosaurs were small. After the gap, none were. Tyrannosaurus rex and its relatives weighed 1 ton or more.
Uzbekistan is one of the few places with geological formations that fall in that 20-million-year gap. Beginning in 1997, Hans-Dieter Sues, chairman of the paleobiology department at the Smithsonian's National Museum of Natural History, and Alexander O. Averianov, a senior scientist at the Russian Academy of Sciences, organized fossil-collecting expeditions to the Kyzylkum Desert in northern Uzbekistan.
The paleontologists uncovered some scattered bones of what appeared to be tyrannosaurs, but the key finding came in 2004 when they found the part of the skull surrounding the brain.
"The braincase proved to be the Rosetta stone for the whole thing," Sues said.
Brusatte, an expert on tyrannosaurs, was visiting Averianov a couple of years ago to study some other fossils. "He pulled open a box and pulled out this object about the size of a grapefruit," Brusatte recalled. "And he handed it to me and said, 'You know, what do you make of this?' "
Brusatte joined an international team of researchers including Sues and Averianov in the analysis of the new tyrannosaur. Named Timurlengia euotica, the dinosaur lived about 90 million years ago, right in the middle of the fossil gap. (Timurlengia was named after a Central Asian warlord, Timur; euotica roughly means "well eared.")
Based on a few scattered bones, the scientists estimated that Timurlengia was about the size of a horse, like earlier tyrannosaurs, weighing about 600 pounds, with long legs and blade-like teeth. Lithe and fast, it probably chased down plant eaters like early duck-billed dinosaurs also found in the region.
But a CT scan of the braincase showed the shape of the brain was similar to the later tyrannosaurs, and an inner ear structure that was tuned to low frequencies.
"We were very surprised it already had this sensory organization associated with T. rex and related animals," Sues said.
Timurlengia does not explain why tyrannosaurs got big, but Sues said he suspected that an unusually warm climate, one of the warmest in Earth's history, a played an important role. Allosaurus and its relatives did not adapt and died out, and then tyrannosaurs grew in size to replace them.
So far, Timurlengia is just one clue. It is possible, for instance, that larger tyrannosaurs had already evolved elsewhere. "Like at a murder scene," Brusatte said, "one clue is better than none."
A tyrannosaur from China named Xiongguanlong, imprecisely dated to 100 million to 120 million years ago, is another important clue.
"At a glance, Xiongguanlong looks to my eye like an advanced tyrannosaur," said Thomas Carr, a paleontologist at Carthage College in Kenosha, Wisconsin, who is not a member of the Timurlengia research team but who has examined the braincase fossil.
He expected Xiongguanlong and Timurlengia to be closely related, but a detailed analysis of Xiongguanlong using CT scans has yet to be published.
"This is what this work is all about, completing that evolutionary narrative," Carr said. "At the end of the day, there's much more to discover. It's still a great leap from Timurlengia to T. rex."