Tyrannosaurus rex is by far the most iconic of all the Tyrannosaurids, a group of apex predators that includes Albertosaurus and Tarbosaurus. Although T-rex certainly ruled the last 20 million years of the Cretaceous period, its predecessors were much smaller. A horse-sized Tyrannosaur discovered in Asia is a perfect example. It lived 90 million years ago and had the highly developed brain and keen senses that later allowed T-rex to be such a feared predator. Scientists think the massive body size came later in the Cretaceous period.