Tiny Tyrannosaur's Mysterious Origins Divide Scientists

Years before the Mongolian president intervened in the auction of a tyrannosaur skeleton thought to have been illegally taken from that country, a fossilized dinosaur with similarly controversial origins followed a very different path.

Once removed from the rock, the bones of this dinosaur would, to some, reveal the existence of a new species of tiny but unmistakable predator. However, given few clues as to where these fossils came out of the ground, paleontologists have yet to resolve the debate about the dinosaur's true identity.

At a fossil show about nine years ago, a dealer approached Henry Kriegstein, a fossil collector and eye surgeon in Massachusetts, with photos of a block of rock that contained the remains of a small, meat-eating dinosaur curled up in a death pose. Enough of the fossils had been exposed for Kriegstein to suspect he was looking at a juvenile tyrannosaur, and he purchased the chunk of stone and fossil. [Image Gallery: Dinosaur Daycare]

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An illegal export?

The dealer told Kriegstein that he had bought the fossils from someone else and offered only the most vague details about its origin, Kriegstein said. "I knew it came from Asia, so I had suspicions it may have been removed illegally."

He was suspicious because Asian nations, such as Mongolia and China, don't allow the export of fossils excavated within their borders. In the case of the tyrannosaur, a species known as Tarbosaurus bataar, paleontologists say the specimen originated in Mongolia, where law makes fossils the property of the state and smuggling them out a crime.

Too important to keep

Kriegstein said he sent his purchase to a commercial paleontological company, Western Paleontological Laboratories in Utah, to have the fossils removed from the rock. While work was under way, the company sent Kenneth Carpenter, a paleontologist at the Denver Museum of Nature & Science, photos of the specimen.

"When they contacted me they had not finished the cleaning of the bones. It still had a lot of rock on it, but what I saw in the pictures was tantalizing. It looked different," said Carpenter, who is now curator of paleontology at the Prehistoric Museum at Utah State University-Eastern. [Image Gallery: Dinosaur Fossils]

Later, after seeing the fossils in person, Carpenter told Kriegstein in a letter that the fossils were important to science and should be donated to a museum.  

"I don't want to have an important specimen like that in my living room," Kriegstein said after receiving the letter.

He sent photos to Paul Sereno, a University of Chicago paleontologist he admired.

Dinosaur detective work

"This was not a composite of multiple specimens or a forgery carved by a desperate fossil dealer.  I could see a mini tyrannosaur," Sereno wrote.

Kriegstein agreed to donate the dinosaur to the University of Chicago, so Sereno could formally describe it, requesting that the new dinosaur be named after his father, Roman Kriegstein.

Figuring out where the dinosaur came from was crucial to understanding what it was. In research published in the journal Science in 2009, Sereno and colleagues concluded it was taken from the ground in the Yixian Formation of northern China based on characteristics of the fossils, the sandstone that entombed them, and the mollusks and fish bones contained with them. This origin would make the specimen about 125 million years old.

Based on an examination of the bones, they suggested the 9-foot-long (less than 3 meters) specimen was something extraordinary: A miniature ancestor and look-alike to the giant tyrannosaurs epitomized by Tyrannosaurus rex and its Asian cousin, Tarbosaurus bataar. This little dinosaur, which they classified as a young adult, shared their distinctive features, including the oversize head with powerful jaws, puny arms and lanky hind legs for running, but it evolved much earlier, Sereno’s team concluded.  

Keeping with Kriegstein’s request, Sereno named the dinosaur Raptorex kriegsteini.

The debate

But not everyone accepts his take. A re-analysis published in PLoS ONE in 2011 by other researchers challenged Sereno's conclusion regarding the age of the dinosaur and its origin, suggesting the fossils actually belonged to a juvenile Tarbosaurus from Mongolia.

Carpenter, whose letter prompted Kriegstein to donate the fossils, agrees to this latter assessment, citing features of the vertebrae that he says indicate the dinosaur had not yet matured.

"Of course, it depends on where it comes from, if it is from rocks much older than any tarbosaur, then Paul is essentially correct," Carpenter said.

Tarbosaurus specimens have been found only in the Nemegt Formation in Mongolia, a rock formation dated to the Maastrichtian Age, beginning around 70 million years ago — much younger than Sereno's estimate for Raptorex.

"That is why it would be crucial to find out where this specimen came from," Carpenter said.

At this point, he puts the chances of a definitive answer at "zip," saying, "Nobody is going to come forward and say, 'Yes, we are the ones who plundered the site.'"

Sereno has stood by his initial conclusion — "We are confident it is not a tarbosaur," he said — although he considers the fossils origin to be in dispute and has continued looking for more conclusive evidence to tie them to a particular place. 

"We've tried every line of attack," he said. Currently, he is hoping mollusks found with the specimen will solve the puzzle.

Sereno has made arrangements to send the fossils to a museum in China near the site where he believes they were discovered.

Follow Wynne Parry on Twitter @Wynne_Parry or LiveScience @livescience. We're also on Facebook & Google+.