Aleks chose the Tyrannosaurus rex over the Stegosaurus and Triceratops. Presumably because it's fierce.
T. rex was a huge meat-eating dinosaur that lived during the late Cretaceous period, about 85 million to 65 million years ago. T. rex lived in a humid, semi-tropical environment, in open forests with nearby rivers and in coastal forested swamps. The seasons were mild.