Altogether, the pieces reveal a slender tyrannosaur that roamed Cretaceous Mongolia about 86 million years ago and was about 13 feet long—or about the size of juvenile T. rex that would stalk North America 20 million years later.

A newly identified mid-sized dinosaur from Mongolia dubbed the "Dragon Prince" has been identified as a pivotal forerunner of Tyrannosaurus rex in an illuminating discovery that has helped clarify the famous predator's complicated family history.

Discovery leads to revised tyrannosaur family tree, shedding light on T. rex's ancestry.