With its massive frame, bone-crunching jaws, and adorably diminutive forearms, Tyrannosaurus rex is the most iconic of all dinosaurs. This spectacular late Cretaceous predator measured around 13 meters long from head to tail, and weighed seven tons, making it one of the largest meat-eaters ever to roam the Earth.
But tyrannosaurs like T. rex were not always supersized apex hunters, as evidenced by the newly-discovered species Timurlengia euotica, which lived 90 million years ago in what is now Uzbekistan's Kyzylkum Desert. The animal's fossilized remains, described in a study published today in the Proceedings of the Academy of Sciences, reveal that smaller hunters like Timurlengia pioneered many of the adaptations that made later tyrannosaur incarnations so successful.
"The new species—Timurlengia—is still relatively small, only about the size of a horse, but it has the advanced brain and senses of the colossal latest Cretaceous apex predators," Steve Brusatte, a paleontologist at the University of Edinburgh and lead author of the new study, told me via email.
"This tells us that tyrannosaurs got smart before they got big," he added. "They evolved their signature brains and keen senses at relatively small body size, and these features later came in handy when tyrannosaurs had the opportunity to rise to the top of the food chain."
This finding is particularly significant in light of the dearth of fossils from the period in which tyrannosaurs diversified from small, sleek hunters to the gargantuan creatures of carnage we know so well today. Timurlengia is the lone survivor from this important time; a missing link nestled right in the middle of a gaping chasm in the tyrannosaur fossil record. For that reason, the animal provides a rare window into this murky transitional phase in tyrannosaur evolution.
"Timurlengia is the first tyrannosaur from a very frustrating gap in their fossil record, between about 100 and 80 million years ago," Brusatte said. "Before the gap tyrannosaurs are small: human-sized up to horse-sized. They are not apex predators. Then after the gap they are all big, top-of-the-food chain animals over ten meters long and a ton in mass. So their supersizing happened within the gap."
Having acquired this valuable new piece of this evolutionary puzzle, Brusatte and his colleagues were able to compare Timurlengia's morphological features to their giant cousins. They found that while this animal weighed only about 400 to 600 pounds, it was still an adept hunter that depended on deadly speed rather than bulky strength. As such, it would have been faster than T. rex, and likely terrorized the duck-billed dinosaurs with which it shared the hot, lush environment of mid-Cretaceous Uzbekistan.
But while it differed in size and strategy from the big guns, Timurlengia helped lay down the evolutionary groundwork for the tyrannosaur family's exceptional sensory intelligence.
"Timurlengia is small and doesn't have such a big or robust skull, or such thick teeth," Brusatte explained. "But it has the characteristic brain and ear of big tyrannosaurs. So this tells us that these neurosensory features evolved early in tyrannosaur evolution, and then were later co-opted by the giant tyrannosaurs into successful hunting weapons for a mega-predator at large size."
"Basically, they were fortunate that their ancestors evolved these neurosensory features, because they came in handy when tyrannosaurs had the opportunity to rise to the top of the food chain," he said.
That said, the tantalizing question of why tyrannosaurs evolved into such nightmarish, awesome giants during the late Cretaceous is still open for debate. Paleontologists know the transition occurred rapidly, and that the extinction of these beasts was even more sudden, following the impact event that killed off all the non-avian dinosaurs 66 million years ago.
One popular theory suggests that tyrannosaurs capitalized on niches left open by top predator lineages that went extinct—for instance, the Allosaurus clan. But ultimately, it will require more fossil evidence to know for sure what drove these animals to utterly dominate their ecosystems with both brawn and brains, until a freak planetary collision wiped them all out.
Fortunately, Uzbekistan may be a promising place to look for fossils with the potential to fill out these gaps. "People have been looking in Uzbekistan for awhile, but it was harder to work there during the Soviet years," Brusatte told me. "After the fall of communism these large international fieldwork projects were possible, and my friends in St. Petersburg and Washington organized a remarkable series of expeditions."
"This was real-deal, hard-core field paleontology: intrepid expeditions into the middle of a desert, sand and dust and heat, picking up fossils from the desert floor," he said. "Because of their work Uzbekistan is now one of the best places in the world for middle Cretaceous dinosaur fossils!"
Hopefully, this relatively under-explored region will yield many more tyrannosaur fossils to come, which will help paleontologists further reconstruct the factors that played into their remarkable heft. After all, T. rex may be considered to be the reigning king of the dinosaurs, but even kings depend on those that paved the path before them.