T. Rex, Meet Your (Small) Ancestor

Sept. 17, 2009 — -- Tyrannosaurus rex was a giant killing machine. At the end of the dinosaur age, 65 million years ago, it ruled the earth, running down prey, tearing them apart with its 9-inch long teeth. A typical T. rex was 40 feet long from head to tail and weighed 5 or 6 tons.

But now a team of scientists reports finding an ancestor, 60 million years older. They call it Raptorex kriegsteini,and they say it was just as efficient a killer -- with one major exception.

By T. rex standards, it was tiny -- probably shorter than today's adult human beings and weighing less than 175 pounds. Its body was much the same, except that it weighed a hundredth as much. If "Jurassic Park" were made today, it would need an extra rewrite.

"To me, the most interesting and important thing about this new fossil is that it's completely unexpected," said Stephen Brusatte of New York's American Museum of Natural History, one of the authors of a paper in this week's online edition of the journal Science. "It's becoming harder and harder to find fossils like this that totally throw us for a curve."

To paleontologists, T. rex is a triumph of evolution -- the top predator of the dinosaur era because it was such a proficient killing machine. If you follow Darwin's theory of natural selection, T. rex dominated the landscape because of its large teeth, its crushing jaw -- and the powerful legs that could outrun most prey. T. rex grew in size because it was better than other predators.

But then why did they find the same traits in Raptorex, which came ages before?

"Raptorex is really a pivotal moment in history, where most of the biologically meaningful features about tyrannosaurs came into being," said Paul Sereno of the University of Chicago, lead author of this week's paper. "And the surprising fact is that they came into being in such a small animal."

Sereno says Raptorex weighed only about as much as he does. But its body served its needs so well that there are only minor changes between Raptorex and T. rex.

"The jowls stick out from the side of the skull. You get more of a brow. The neck becomes more like that of a fullback," he said, "but there's very few things we can point to that really strike us as biologically meaningfully different ways of going about eating meat, and running around."

"Raptorex is an excellent example of a transitional fossil, combining the small body size of more primitive tyrannosaurs with many of the characteristic features of T. rex," said Matt Lamanna, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh.

Lamanna, who was not involved in the research, reviewed the paper in Science at the request of ABC News. He said Raptorex fills an important gap in the lineage of T. rex.

"Until about five years ago, there were practically no reasonably complete fossils of species that might be considered tyrannosaurid ancestors," he said.

Raptorex is in remarkable shape for its age. Almost all the bones are in place; the end of the tail is about the only significant part missing. Scientists can tell that it was about 5 to 6 years old, based on their examinations of other dinosaurs. (As in human beings, there are bones that are separate at birth but gradually fuse together as the body matures.)

The dinosaur was the Cretaceous equivalent of a young adult. It probably did not die violently. Luckily for the scientists, it was quickly buried in the silt near the prehistoric lake where it lay.

Raptorex kriegsteini, 5 feet high, as it may have looked in life. (Drawing by Todd Marshall.)

"This really turned on its head our ideas about tyrannosaurs," said Sereno. "We thought that many of those features appeared as the animal was becoming huge, or soon after, losing its strong forelimbs and so forth. Not so. These all came together 125 million years ago, some 60 or 70 million years before Tyrannosaurus rex, in an animal that weighed no more than me or you."