By Will Dunham

WASHINGTON (Reuters) -At the twilight of the age of dinosaurs, an agile and vicious predator named Nanotyrannus prowled western North America, resembling a smaller version of Tyrannosaurus – about a tenth the body mass – but with several key anatomical differences.

That is the finding of new research concluding that Nanotyrannus – which for years has been at the center of a debate among paleontologists as to whether it was merely a teenage version of Tyrannosaurus – was, in fact, its own distinct dinosaur.

The researchers examined fossil specimens of Nanotyrannus unearthed in 1942, 2001 and 2006 in Montana dating to about 67 million years ago, determining that the individuals were mature and not juveniles based on traits in the bones including annual growth rings.

They a

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