Early model. The most primitive tyrannosauroid ever found was decked out with downy "protofeathers" and a T. rex-like skull.

Tyrannosaurs Evolved Head First

Like reclusive celebrities, tyrannosaurs have risen to evolutionary stardom while keeping their origins shrouded in mystery. Now, the most primitive tyrannosauroid yet discovered has revealed the basic blueprint from which Tyrannosaurus rex and its kin evolved. The fossils, so well preserved that one even shows a "protofeather" fuzz covering the body, show that tyrannosaurs began evolving the deadly design of their heads before their bodies morphed into powerhouses.

Paleontologists have found about a dozen species of tyrannosaurs. Most lived late in the Cretaceous Period, which ended 65 million years ago. Isolated bones have been found from older and more primitive tyrannosaurs, but not all have been accepted as ancestors. The new specimens come from western Liaoning Province in China and were studied by Xing Xu and colleagues from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, along with Mark Norell of the American Museum of Natural History in New York City. The skull has many familiar attributes, including bones shaped like those that apparently helped later tyrannosaurs launch swift, bone-jarring ambushes. The team dubbed the new creature Dilong paradoxus, for "surprising emperor dragon."

Those surprises include features that distinguish D. paradoxus from its descendents, the team reports in the 7 October issue of Nature. Its small body, 1.6 meters long, will give researchers a chance to study which aspects of T. rex's anatomy are truly tyrannosaurian rather than due to gargantuan size. And compared with T. rex, D. paradoxus had relatively long arms. Maybe developing the head for attacking--a safer approach than hands-on grappling with prey--enabled D. paradoxus's descendants to grow bigger and handle bigger prey, speculates Oliver Rauhut of the Bavarian State Collection of Paleontology and Geology in Munich, Germany.

Another previously unknown feature of tyrannosauroids is the soft pelt of 2-centimeter-long fibers, called protofeathers. These have been found in more primitive ancestors outside the tyrannosaur group, but large tyrannosaurs appear to have sported reptilelike scales instead. Norell proposes that smaller tyrannosaurs needed fuzz to stay warm but that their larger descendants, like modern elephants, shed their insulation to keep from overheating.

"I think people are going to be tremendously excited about this," says Matthew Carrano, dinosaur curator of the Smithsonian Institution. "It's certainly going to clarify a huge amount about the evolution of tyrannosaurs."

Related site
More about the Tyrannosauridae from UC Berkeley's Museum of Paleontology