How the small tyrannosaurs turned into these menacing giants has long been a mystery because of a vast gap in the fossil record. But on Monday, scientists gathered at the Smithsonian National Museum of Natural History in Washington to reveal a newly discovered dinosaur that appears to be the long-sought missing link.
Fossils of the new dinosaur, dubbed Timurlengia eutoica, were found in the central Asian nation of Uzbekistan. The species appears to have been about the size of a horse, and without the absurdly huge head and the industrial-strength jaws of T. rex. But its brain case indicates that it was rather intelligent, like T. rex, and had many of that dinosaur's advanced sensory abilities, including the ability to hear low-frequency sounds.
The discovery strongly suggests that tyrannosaurs got smart before they got large.
"The skill set was the key qualification to apply for the job of top predator," said Hans-Dieter Sues, a paleontologist at the Smithsonian and a co-author of a paper describing the discovery, which was published Monday in the Proceedings of the National Academy of Sciences. "Our new beast certainly had very good hearing, certainly better than any other tyrannosaur."
Tyrannosaurs had been around since the Jurassic Period, but given their modest stature, they were far from being dominant. Their evolution into apex predators is hard to track because of the patchiness of the fossil record in the Cretaceous Period.
There are very few places on Earth where fossils of terrestrial animals from about 80 to 100 million years ago can be found. Where sediments from that era exist, they typically are marine sediments, which are useless for studying land-dwelling dinosaurs.
But the remote site in Uzbekistan, 17 miles from the nearest road, is a rare exception. Sues did field work there for 10 summers starting in 1997, driving around in beat-up Soviet-era military vehicles, buying mutton from desert nomads and keeping spirits up with plenty of vodka. The site produced a trove of fossils that were later analyzed in museums and laboratories around the world.
Timurlengia — named for the 14th century conqueror Tamerlane — lived about 90 million years ago. The scientists describe it as a forerunner, or remote cousin, of T. rex rather than a direct ancestor.
The fossil hunters did not find a full skeleton. Rather, they just have scraps of Timurlengia — remnants of many animals scattered in the bone beds.
Two years ago, Stephen Brusatte, a University of Edinburgh paleontologist who is the lead author of the new paper in PNAS, visited one of those Uzbekistan fossil collectors in St. Petersburg, Russia. Alexander Averianov, another co-author, suggested that Brusatte look at a fossilized brain case.
"That looks like a tyrannosaur," Brusatte said.
"Great! That's what Hans and I were thinking, but we weren't sure," Averianov responded, according to Brusatte.
Brusatte took the piece back to Edinburgh to study. He concluded that it was from a new species of tyrannosaur. Using a CAT scan, he determined that this tyrannosaur had anatomical features suggesting advanced sensory skills.
The sedimentary formation with the fossils was framed by easily dated marine sediments. That gave the scientists confidence that they were, indeed, looking into the little-understood gap in the fossil record.
“This is the first one – the first tyrannosaur – a clear, unequivocal tyrannosaur, represented by good fossils that come from that gap," Brusatte said. "It's hinting that tyrannosaurs weren’t really that big in the middle part of the Cretaceous, and they probably got super big at the end of the Cretaceous."
Lindsay Zanno, curator of paleontology at the North Carolina Museum of Natural Sciences, said in an email that the size of the animal described "supports current ideas about tyrannosaur evolution. That is to say, that they didn't beat out existing predators, they took advantage of extinctions in the top ranks." Zanno, who was not involved in the latest research, continued, "They grew rapidly once top predator niches were vacated ... ultimately evolving into the largest of all tyrannosaurs, the tyrant king T. rex."
Thomas Holtz Jr., a University of Maryland paleontologist who is familiar with the new find, described it in an email as a "glimpse" of the new tyrannosaur since so few bones were discovered. "But it can tell us something: For instance, like its giant later cousins, it has a good sense of agility and a large (for a dinosaur) brain," he wrote.
Many questions remain, though. Did Timurlengia have three claws like older tyrannosaurs, for instance, or only two like the bigger, advanced forms?
Brusatte acknowledged the limits of what can be inferred from a single species in a single location. The new paper concludes: "Timurlengia remains a single data point from a still murky interval in dinosaur history, and future discoveries from this gap will undoubtedly lead to a better understanding of how tyrannosauroids rose from marginal creatures into some of the largest terrestrial predators in Earth history."