Tyrannosaurus rex and related large carnivorous dinosaurs together form the family Tyrannosauridae. A long forgotten fossil skull in the collections of the Natural History Museum in London has now provided crucial clues to the early stages of the lengthy evolutionary history of these fearsome predators. Almost a century after its discovery, the specimen, named Proceratosaurus, has now been recognized as the oldest known relative of the Tyrannosauridae.

With the help of an ultramodern imaging technique, a team of researchers led by Dr. Oliver Rauhut from LMU Munich and Dr. Angela Milner from the Natural History Museum London, have been able to show that Proceratosaurus resembled its approximately 100-million-years younger descendant T. rex in a number of ways. The teeth, the jaws, and the structure of the cranial cavity of the two species have many features in common. Proceratosaurus weighed only about 40 kg, says Rauhut. Nevertheless, like the later tyrannosauroids, the animal obviously depended on its powerful biting apparatus. Later modifications of the jaw muscles and the overall structure of the cranium then gave rise to the perfect hunting weapon wielded by T. rex.

Among the dinosaur specimen housed in the collections of the Natural History Museum in London is an almost complete skull that was found in the West of England about 100 years ago. The fossil was initially misclassified, but was later recognized as representing an otherwise unknown genus, which was named Proceratosaurus. The skull has only recently been subject to detailed study by a team led by the palaeontologist Dr. Oliver Rauhut, who holds dual appointments in the Department of Geological and Environmental Sciences at Ludwig-Maximilians-Universität (LMU) in Munich and the Bavarian State Collection for Paleontology and Geology, and Dr Angela Milner, Associate Keeper of Palaeontology at the Natural History Museum in London.

This skull, which had been overlooked for so long, turns out to be a spectacular find. Proceratosaurus is the earliest known ancestor of the family Tyrannosauridae (named after its most famous representative Tyrannosaurus rex). Proceratosaurus and T. rex were both bipedal carnivores and each had a massive body, short and stubby forelimbs, a powerful tail, and sharp teeth set in a bulky skull. The best known members of the family, T. rex, lived during the late cretaceous period, although smaller species are known from the earlier Jurassic era.

Little is known about the origins and later evolution of this important group of dinosaurs. Proceratosaurus could now cast much needed light on the process. "It is quite astonishing that this fossil has received so little attention, since it is one of the best preserved dinosaur skulls in Europe," reports Rauhut. Parts of the skull that were still embedded in the rock matrix and these had to be exposed carefully by preparator Scott Moore-Fay at the Natural Histsory Museum in London; the team also used an advanced imaging technique to probe the detailed structure of the fossil.

"Computerized tomography is a wonderful method, because it offers us a non-destructive means of visualizing the internal structures of fossils," says Angela Milner, the researcher responsible for the specimen at the Natural History Museum, who personally took the fossil to Texas, where the tomographic scan was performed. Detailed studies of the resulting images and of the skull itself were subsequently carried out back in London.

The investigations uncovered a wide range of features in the cranial cavity, teeth and jaws that Proceratosaurus shares with the huge T. rex, despite the fact that the Proceratosaurus skull is about 100 million years older and much smaller. The Proceratosaurus cranium was about five times less massive than that of its mighty relative, and the intact animal appears to have weighed only about 40 kg. Mature specimens of Tyrannosaurus, in contrast, weighed in at up to eight tons.

Because the Proceratosaurus skull already displays characteristics that are typical of its later descendants, the powerful jaw with its slicing teeth was probably the animal's most important weapon. "It is likely that this hunting strategy developed first," says Rauhut. The basic tool kit was perfected in later tyrannosaurids: The skull became more robust and the jaw muscles larger and, overall, the body increased enormously in size. Proceratosaurus also confirms that the tyrannosauridae developed over a very long stretch of time, and gave rise to a great diversity of forms. Further members of the family surely await discovery."

The study was financially supported by the SYNTHESIS program of the European Union.