Tyrannosaurus rex and its relatives were North America's dominant predators in the late Cretaceous period, about 99 million to 65 million years ago, but a new analysis of a toothy fossil skull suggests that the early history of this group includes smaller meat-eating ancestors that date as far back as 170 million years ago.

The skull belongs to the only known specimen of Proceratosaurus, which now represents the oldest known relative of T. rex and its cousins, extending the evolutionary history of tyrannosaurs back to the middle of the Jurassic period, say paleontologist Angela Milner of The Natural History Museum in London and her colleagues. The skull has belonged to the museum since the 1940s and had not been linked with tyrannosaurs previously. The team recently reexamined the skull directly and, via CT scans done at the University of Texas at Austin, found it had all the classic markings of a theropod dinosaur (the carnivorous group that gave way to modern birds) and many tyrannosaur features in its teeth, jaws and skull cavities.

Proceratosaurus was bipedal and weighed between 28 and 36 kilograms—making it small compared with T. rex (which weighed about 8,000 kilograms) and most other Cretaceous tyrannosaurs. It probably measured about three meters long (only the skull is preserved, so the length is an estimate). Like other theropod dinosaurs, Proceratosaurus had four ferocious-looking and serrated snout teeth, D-shaped in cross-section. It also sported a nose horn.

Proceratosaurus is part of a large group called tyrannosauroidea that includes the larger Cretaceous tyrannosaurs, along with some smaller versions and their immediate ancestors. Until the new work by Milner and her colleagues, including Oliver Rauhut of the Bavarian State Collection for Paleontology and Geology in Munich, there was no strong evidence for tyrannosauroids dating back to the middle Jurassic. The results were published online November 4 in the Zoological Journal of the Linnaean Society.

"The origin of that [larger] group goes back to the middle Jurassic with this animal now," Milner says. "As well as being earliest tyrannosauroid, it's also the earliest coelurosaur. That tells us that a lot of the evolution of theropod dinosaur groups happened a lot earlier in time that we really thought."

The analysis shows a close evolutionary relationship among Proceratosaurus, which was found in England, and other recently described tyrannosauroids found in China, including Guanlong, a 160-million-year-old feathered specimen described in 2006 by Xu Xing of the Chinese Academy of Sciences in Beijing; Dilong, a 130-million-year-old specimen described in 2004 by Xu; and Raptorex, a 125-million-year-old specimen described this year by Paul Sereno of the University of Chicago.

"The fact is that [Proceratosaurus] puts a new time frame on the evolution of more derived theropod dinosaurs," Milner says.