One of the great mysteries of Earth's history is how tyrannosaurs - which were never bigger than a horse for millions of years - evolved into the giant, predatory Tyrannosaurus rex at the end of the Cretaceous Era. Now, a new study claims to have solved this mystery, and the key lies in a 90-million-year-old grapefruit-sized partial skull that was retrieved form Uzbekistan's Bissekty Formation.
The partial skull is a T. rex relative called Timurlengia and it is the first well-preserved tyrannosaur braincase from the mid-Cretaceous period. It reveals that despite their small size at the time, they possessed brain and ear features that match those of later tyrannosaurs, shedding light on the 20-million-year gap in tyrannosaur evolution.
"Our study is the first to show that the sophisticated brain and hearing of big tyrannosaurs evolved in smaller-bodied species, long before tyrannosaurs got giant," said Stephen Brusatte, co-author of the study, claiming that these advantages may have helped tyrannosaurs become such giant predators.
The team took the fossil and analyzed it against a database of other tyrannosaurs, showing that its brain and ear are nearly identical to the T. rex's, only smaller. In addition, the dinosaur's cochlea, a part of the inner ear, has a length that is comparable to the bigger, more dangerous Late Cretaceous tyrannosaurs.
"The long cochlea would have meant better sensitivity to low-frequency sound," Brusatte explained, giving them an advantage over other predators.
The braincase also points to a high intelligence level, much like the T. rex.
"The skill set was the key qualification to apply for the job of top predator," said Hans-Dieter Sues, another co-author of the paper. "Our new beast certainly had very good hearing, certainly better than any other tyrannosaur."
The team now plans to continue research in order to determine if the braincase is representative of a typical mid-Cretaceous tyrannosaur or simply a chance finding.
"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 superbig at the end of the Cretaceous.
"The thing that could move us forward is the discovery of new specimens in other middle Cretaceous rock units in other parts of the world," he concluded.
The findings were published in the March 14 issue of the Proceedings of the National Academy of Sciences.
