When picturing a scaled, toothy Tyrannosaurus rex stomping across North America, building nests and eating other dinosaurs, it's easy to forget that scientists have gleaned everything we know about T. rex from clues left more than 60 million years ago. While the presence of fossils in Alaska suggested that this prehistoric ancestor of birds was already on the evolutionary path from cold-blooded reptile to warm-blooded avian, there wasn't a way to take T. rex's temperature—until now.