Tiny T. rex? Big surprise!

Who would have thought Tyrannosaurus rex had such a murderous "mini-me" in its family tree?

Not Stephen Brusatte, a paleontologist at the American Museum of Natural History. "This was completely unexpected," he said.

And not University of Chicago paleontologist Paul Sereno, who along with Brusatte and other colleagues figured out that the tiny tyrannosauroid had virtually all the lethal weapons brandished tens of millions of years later by a behemoth 90 times more massive.

"From the teeth to the enlarged olfactory bulbs, the enlarged jaw muscles, the enlarged head, the small forelimbs, the lanky, running, long hindlimbs with thick-pressed foot for hunting prey - we see this all, to our great surprise, in an animal that is basically the body weight of a human," he told reporters.

The 125 million-year-old fossil dinosaur, unearthed in China and dubbed Raptorex kriegsteini, is "as close to the proverbial missing link on a lineage as we might ever get for tyrannosaurs," Sereno said.

The researchers laid out their conclusions in a paper published online today by the journal Science.

A T. rex expert who wasn't involved in the research, the University of Maryland's Thomas Holtz, agreed that the findings resolve some of the mysteries surrounding one of history's most fearsome predators. "It is unexpected, in a sense," he told me. "It really helps clarify what was previously a missing portion of the tyrant dinosaur family tree."

T. rex's tangled tale

Holtz, Sereno and many other paleontologists have been piecing together T. rex's family tree for decades. Before Raptorex, they knew that the species Tyrannosaur rex had similar-looking cousins in various parts of the world - Albertosaurus in Canada, for example, and Tarbosaurus in China. These creatures, known collectively as tyrannosaurids, weighed as much as 8 tons and occupied the top of the food chain when the age of dinosaurs ended 65 million years ago.

Scientists also knew that these giants sprang from much smaller, more primitive ancestors that existed 125 million years ago, designated tyrannosauroids (with an extra "o"). The little 'roids had some characteristics in common with the big 'rids, but not the traits that made T. rex so special - for example, the little arms or the long runner's legs.

Some experts suggested that those changes in body plan must have arisen less than 125 million years ago, and somehow served as the decisive factor that made T. rex as big and dominant as it was. As the rest of the tyrannosaur became super-sized, the forearms dwindled into useless appendages. Or so the theory went.

Sereno said Raptorex refutes that claim by showing that the killer body predated the super-sizing trend. The creature measured only about 10 feet (3 meters long) in life, compared with the 40-foot (12-meter) head-to-tail span for Tyrannosaurus rex. But it had all the characteristics that have made T. rex so memorable in the popular imagination.

"This is part of a beautifully designed predatory blueprint, and it certainly was one of the most successful," he said.

How Raptorex came to light

The surprising features were found in a nearly complete fossil skeleton that was excavated years ago in northeastern China, one of the world's top fossil hot spots. "The specimen was found perhaps in the dark of night, spirited out of China and possibly sold," Sereno said.

The dinosaur bones - still encased in a block of sediment - turned up at an Arizona fossil show about six years ago, and were purchased by Henry Kriegstein, an opthalmologist and private fossil collector from Hingham, Mass. Kriegstein told me he paid "on the order of tens of thousands of dollars" for the specimen and had it sent it off to experts in Utah for preparation.

At first, Kriegstein and the paleontologists who advised him assumed that the fossil was a juvenile Tarbosaurus. But they eventually came to the realization that the creature was nearly full-grown - and might even represent a new genus and species. That's when Sereno was called in to study the case.

"I immediately agreed that I didn't want to have it in my living room," Kriegstein said.

Kriegstein and Sereno also agreed that the species name would pay tribute to Kriegstein's parents, who were born in Poland, survived internment in Nazi labor and concentration camps and made their way to the United States. (The genus name, Raptorex, is a catchy title basically meaning "King of the Raptors.") Kriegstein donated the specimen to the University of Chicago, with the understanding that the bones would eventually be returned to a museum in China.

Deciphering the bones

Sereno and his colleagues analyzed the sediment from Kriegstein's purchase, as well as clamshells and fish bones found alongside the dinosaur bones, to determine where the specimen came from and how old it was.

The fact that the bones were still encased in their surrounding soil when Kriegstein acquired them argues against the possibility that the bones were thrown together to create a fraud, as has sometimes been the case, Sereno noted.

Researchers also took pains to confirm that Raptorex was nearly fully grown: They cut through a fossilized femur bone to check the growth rings, and concluded that Raptorex was 6 years old, nearing maturity. The way that various bones were fused together supported that assessment.

Detailed measurements of the skeleton showed that Raptorex was, in most respects, a scaled-down version of T. rex - "jaws on legs, as it were," Sereno said. The pint-size predator used its fast-running legs to run down its prey, and perhaps run away from rivals as well. Its jaws were the first line of offense. Although its clawed forearms were small, they were likely well-suited for grasping and manipulating Raptorex's victims once they were enmeshed in the powerful jaws.

Sereno guessed that Raptorex's "most delectable" prey might have been parrot-beaked dinosaurs, with other small dinosaurs, ancient birds and turtles filling out its diet.

How did T. rex get so big?

When Raptorex lived, 125 million years ago, there were far bigger predators to look out for - allosaurs and spinosaurs, for example. During the 40 million years that followed, however, Raptorex's descendants apparently bulked up dramatically and inherited the earth. So what happened? The results reported today indicate there was no dramatic evolutionary innovation that gave T. rex an edge.

To be sure, a big T. rex was more dangerous than a little Raptorex. For one thing, the teeth became deadlier, Sereno noted. "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. Most of these things are a natural consequences of body size," he said.

But the basic blueprint was already set tens of millions of years earlier. "We cannot say that this incredibly successful, scalable blueprint for a predator was responsible for their total domination" among predators in the time frame between 85 million and 65 million years ago, Sereno said.

So what happened to the little tyrannosauroid's bigger rivals? "We don't know what, and we wish we did," Holtz said. Maybe there was a mass extinction that hit the bigger dinosaurs harder, clearing the way for tyrannosaurs - just as the eventual demise of the dinosaurs cleared the way for mammals. Maybe Raptorex's descendants just gradually chewed away at the big guys. The fossil record doesn't provide a clear answer.

For whatever reason, Sereno says, "these other guys went extinct - a fact that always happens periodically with any lineage of dinosaurs - opening the way for the expansion of these predatory, long-limbed, short-forelimbed, big strong-jawed predators like Raptorex to expand in body size."

"And when they did, there was no turning back until the asteroid hit," Sereno said.

Raptorex doesn't bring all the mysteries surrounding T. rex and its ancestors to an end. There are lots more gaps to be filled in when it comes to the tyrannosaur timeline, and Sereno and Brusatte have heard rumors that more revelations about early tyrannosaurids may be on the way.

In the meantime, Raptorex's bones will remain with the University of Chicago for further study. Eventually, they'll be shipped to their once and future resting place in Inner Mongolia, following what Sereno hopes will become "a pathway for other important specimens" that have been spirited out of China and should by rights be returned.

Kriegstein, the fossil collector who got all this started, will never have the bones in his possession again. But he does have the honor of a scientific name ("kriegsteini"), the satisfaction of knowing he did the right thing ... and a full-size resin cast of the skeleton that now occupies a prominent place in his home.

More on dinosaurs:

• Gallery: The world's seven deadliest dinos
• Interactive: Are dinosaurs still alive?
• Was T. rex a chicken and baby killer?
• Teen dinosaurs hung out and got into trouble
• Hunt down "tyrannosaurus" on msnbc.com

This report was updated to correct a reference to fish (not bird!) bones found amid the Raptorex skeleton. Many thanks to Paul Sereno for pointing that out.

In addition to Sereno, Brusatte and Kriegstein, co-authors of the Science paper include Lin Tan of China's Long Hao Institute of Geology and Paleontology, Xijin Zhao of the Chinese Academy of Science's Institute of Vertebrate Paleontology and Paleoanthropology, and Karen Cloward of Western Paleontological Laboratories in Utah. The Raptorex find will be addressed in "Bizarre Dinos," a TV special due to premiere Oct. 11 on the National Geographic Channel.

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