The fossil, about nine feet long and weighing just 150 pounds, has been named Raptorex kriegsteini.
The Raptorex, which lived about 35 million years before T. rex first appeared, has the same oversize head, sharp teeth, long legs and puny arms of the T. rex. But the Raptorex's size stands in stark contrast to the "king of the dinosaurs," which was roughly five times longer and nearly 100 times heavier.
Raptorex evolved at "punk size," said University of Chicago paleontologist Paul C. Sereno, who co-authored a paper on the subject for the journal Science. "Basically our bodyweight. And that's pretty staggering, because there’s no other example that I can think of where an animal has been so finely designed at about 100th the size that it would eventually become."
Scientists originally thought T. rex's features evolved as the result of its large size. Now the Raptorex has them thinking otherwise, reports the New York Times:
"The thought was these signature Tyrannosaurus features evolved as a consequence of large body size," Stephen L. Brusatte of the American Museum of National History, an author of a paper describing the dinosaur published online by the journal Science, said at a news conference announcing the discovery. "They needed to modify their entire skeleton so they could function as a predator at such colossal size."
The new dinosaur, which has been named Raptorex kriegsteini, “really throws a wrench into this observed pattern,” Mr. Brusatte said.
The fossil was found nearly complete in ancient lake deposits in northeastern China and purchased (as seems to be all the rage these days) by a collector, Henry J. Kriegstein.
Kriegstein then alerted Sereno, who with five others authored the paper.
The fossil was deemed to be that of a young adult, since the fused nature of many of its bones suggested it was near the end of its growth period.
But its forelimbs -- as puny as T. rex's -- indicate, along with its other features, that the Raptorex's physical shape was optimized for its role as a predator: "Jaws on legs, as it were," he tells the Times.
As time wore on, that successful blueprint simply scaled in size.
Here's a video of Sereno discussing the discovery.