At the end of the dinosaur age, Tyrannosaurus rex was a behemoth killer animal, up to 40 feet long and weighing several tons — the top carnivore in the food chain.

The very first tyrannosaurs, which arose about 100 million years earlier, were small, about the size of a person.

The evolutionary jump of tyrannosaurs from people- and horse-size to behemoths has remained a mystery. A recent fossil finding in Uzbekistan is providing paleontologists with a missing link in the lineage: They have discovered a tyrannosaur with many of the giant’s characteristics — but not its stature or heft.

“It has long been thought that tyrannosaurs were such successful predators, in part, because of their large brains and ears well-attuned to low-frequency sound,” said Stephen L. Brusatte, a paleontologist at the University of Edinburgh in Scotland and lead author of a paper published Monday in Proceedings of the National Academy of Sciences, describing the new dinosaur. “The new Uzbek tyrannosaur has basically the same brain as T. rex — same shape, proportions, etc. — just smaller.”