Tyrannosaurs didn't used to be so scary. For millions of years, they were kind of puny, never bigger than a horse.
They probably lurked in the underbrush while larger dinosaurs like Allosaurus ruled the land as the top predators.
Jump forward to the end of the Cretaceous Era, and we see the astonishing creature we call Tyrannosaurus rex, a schoolbus-sized monster with a giant head, teeth as big as bananas and powerful jaws that could bite through bone.
How the small tyrannosaurs turned into these menacing giants has long been a mystery because of a vast gap in the fossil record.
But this week 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 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 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 analysed 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 fossilised 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."
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."
The discovery: Bones were found of a previously unknown member of the evolutionary branch that led to the huge tyrannosaur T. rex. This earlier dinosaur lived about 90 million years ago, south of what is now the Aral Sea. It looked roughly like a T. rex, but was only about 3m long and weighed only about 270kg at most. T. rex grew about four times as long and weighed more than 20 times as much.
Good timing: The discovery helps fill in a frustrating gap in the tyrannosaur fossil record. Before that gap, which began some 100 million years ago, the ancestral creatures were only about as big as a horse. Right after the gap, at about 80 million years ago, tyrannosaurs were multi-tonne behemoths like T. rex. The new finding shows the forerunners were still relatively small even just 90 million years ago. So the size boom happened pretty quickly.
Standard equipment: The inner ears of the newfound beast already had tyrannosaur features associated with good agility and hearing low-pitched sounds, which might have helped it detect prey or other animals at a distance. But the creature lacked the massive, bone-snapping teeth and large sinuses found in T. rex.
The name: The creature was dubbed Timurlengia euotic. The name honours the ancient Central Asian ruler Tamerlane and the large inner ears of the beast. The fossils include a braincase and bones from the neck, back, tail, feet and hands. The creature wasn't a direct ancestor of T. rex, but it indicates what such ancestors looked like.