T-rex got smart before becoming big.

• Position: Evolution

Fragments of a newly identified dinosaur fossil from Uzbekistan have shed more light on how Tyrannosaurus rex and other fearsome carnivores evolved from their diminutive forebears--by getting smart before they got big.

The tyrannosaurs of the late Cretaceous period (80,000,000 to 66,000,000 years ago) are among the biggest carnivores to have walked the Earth, but their predecessors--who lived as far back as 170,000,000 years ago, in the Jurassic period--generally were no bigger than a horse. They also had proportionally smaller heads and longer arms, and lacked the sense of smell and ability to hear low-frequency sound of later tyrannosaurs.

A 20,000,000-year gap in the fossil record, from 100,000,000 years ago to the time of the giant tyrannosaurs' appearance, has made it difficult to trace how the keen-sensed giant carnivores evolved, but fossil fragments found in the Kyzylkum Desert belong to a newly described horse-sized dinosaur that lived 92,000,000 to 90,0000,000 years ago, slotting neatly into this gap, scientists report. 'This is what we'd been waiting for," says Stephen Brusatte, a paleontologist at the University...