Tyrannosaurus was one of the largest carnivorous dinosaurs, but for most of their history, tyrannosaurs and their ancestors were rather small, according to a phylogenetic review in a recent issue of Science. The best-known tyrannosaurs are late Cretaceous members of the tyrannosaurid family, such as Albertosaurus, Daspletosaurus, Gorgosaurus and Tyrannosaurus itself, which were varying degrees of ginormous. But based on new fossil discoveries, the tyrannosauroid superfamily originated in the middle Jurassic and stayed small — human-sized — for 80 million years. (Also, they didn’t all have those little two-fingered arms.) It was only when the existing large carnivores — carnosaurs like Allosaurus, for example — got out of the way that tyrannosaurs were able grow larger and take over the large-predator niche.
(Remember, tyrannosaurs are coelurosaurs, not carnosaurs: they’re more closely related to ostrich dinosaurs, birds and, well, velociraptors than they are to other big meat-eaters like Allosaurus and Megalosaurus.)