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New species fills in tyrannosaur gap: horse-sized dinosaur had brain, ears like later, bigger T. rex.

A fossil from a new species of dinosaur is helping to bridge a crucial 20-million-year gap in tyrannosaur evolution.

The key fossil is a 90-million-year-old, grapefruit-sized partial skull from Uzbekistan's Bissekty Formation. This tyrannosaur braincase, the first well-preserved one found from the mid-Cretaceous period, shows that, although still small, tyrannosaurs of the time already had brain and ear features of later tyrannosaurs. Researchers have dubbed the in-betweener Timurlengia euotica, meaning "well-eared." They describe the new species online March 14 in the Proceedings of the National Academy of Sciences.

The braincase sheds light on a long-standing mystery: how tyrannosaurs evolved from an "average Joe" horse-sized predator some 100 million years ago in the early Cretaceous to the huge apex predators they became 80 million years ago in the late Cretaceous. "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," says study coauthor Stephen Brusatte, a paleontologist at the University of Edinburgh. These advantages, he adds, may have helped tyrannosaurs become such successful--and eventually enormous--predators.

Analyzed against a database of other tyrannosaur skulls, the braincase shows that T. euotica's brain and ears "are almost identical to T. rex, just smaller," Brusatte says. In particular, the dinosaur's long cochlea, a part of the inner ear, is a signature of bigger, badder late Cretaceous tyrannosaurs. "The long cochlea would have meant better sensitivity to low-frequency sound," Brusatte says. That sensitivity would have enabled T. euotica to detect subtle or distant sounds, giving it clear advantages over other predators.

"Timurlengia fills an important gap in both time and evolution," says Lawrence Witmer, a paleontologist at Ohio University in Athens who was not involved in the study. "Charles Darwin couldn't have scripted it any better."

The next step is to determine if the braincase is typical of a mid-Cretaceous tyrannosaur, or just one oddball data point. "We've analyzed the heck out of each scrap of Bissekty tyrannosaur bone," Brusatte says, "so 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."

Caption: T. euotica's braincase indicates the dino's hearing was similar to that of its larger cousin, T. rex.

Caption: The horse-sized tyrannosaur Timurlengia euotica, illustrated here, roamed what is now Uzbekistan 90 million years ago. The dinosaur already had the supersenses of the giant tyrannosaurs that appeared millions of years later.

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Title Annotation:LIFE & EVOLUTION; Timurlengia euotica
Author:Geiger, Beth
Publication:Science News
Geographic Code:1USA
Date:Apr 16, 2016
Words:418
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