Tyrannosaurus rex was an obligatory inclusion in every book and documentary about dinosaurs I saw as a kid. It was the tyrant king of all dinosaurs, the supreme predator of the end-Cretaceous, but for all its majesty no one could explain where it had come from. Along with its kin—such as Albertosaurus and Tarbosaurus—Tyrannosaurus simply seemed to be the culmination of a trend towards larger size and ferocity among predatory dinosaurs, but plotting the succession of giant theropods during the course of the Mesozoic did not seem to provide many clues about the origins of the tyrannosaurs.
As summarized in a new Science review by a team of tyrannosaur experts, however, new discoveries made in the last decade have finally placed Tyrannosaurus in its proper evolutionary context. In the past year alone, no less than six new tyrannosauroids have been either discovered or identified from previously-known specimens, and this growing knowledge of tyrannosaur evolution has confirmed that the largest predators of Late Cretaceous North America started off small. The first tyrannosaurs were not derived from already-large Jurassic predators such as Allosaurus, but instead were relatively small coelurosaurs, with small heads and long arms, which evolved during the Middle Jurassic more than 165 million years ago. Proceratosaurus, a crested dinosaur from England once believed to be closely related to Ceratosaurus and other early theropods, was just recently found to be one of the first tyrannosauroids. At a glance, Proceratosaurus and similar tyrannosaurs would have looked more like "raptors" than like their more famous relatives. Exceptionally-preserved specimens of the Early Cretaceous tyrannosauroid Dilong from China show that, like their relatives among the coelurosauria, these dinosaurs were covered in feathery dino-fuzz.
After almost a century of uncertainty, it was finally confirmed that enormous Late Cretaceous tyrannosaurs evolved from small, elaborately-ornamented coelurosaurian ancestors. The approximately 80 million years between the first tyrannosaurs and the radiation of truly giant forms is still relatively sparsely known, though. The recent discovery of the long-snouted genus Xiongguanlong and the announcement of the miniature tyrant Raptorex have illustrated that there was no single, slow evolutionary march towards the Tyrannosaurus rex body form. Instead there was a radiation of relatively small genera which preceded the development of large body size, and there are probably a number of strange Late Jurassic and Early Cretaceous tyrannosaurs waiting to be found.
There is more to recent tyrannosaur research than just filling out evolutionary trees, though. Not only is Tyrannosaurus rex the most famous of all dinosaurs, but thanks to numerous specimens and decades of scientific study it is also the most extensively studied. Bite forces, brain anatomy, running speed, growth rates, bone microanatomy, biogeography and other aspects of its paleobiology have all been—and continue to be—extensively investigated. The abundant remains of some of its close relatives, such as Albertosaurus, have even allowed paleontologists to see how different the last tyrannosaurs were; paleontologists could hardly wish for better fossilized records of these dinosaurs. Research will continue, and new discoveries will continue to revise our understanding of tyrannosaur evolution, but it is wonderful that a more complete history of the tyrannosaurs is beginning to come together.
Brusatte SL, Norell MA, Carr TD, Erickson GM, Hutchinson JR, Balanoff AM, Bever GS, Choiniere JN, Makovicky PJ, & Xu X (2010). Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms. Science (New York, N.Y.), 329 (5998), 1481-1485 PMID: 20847260