A fossil classified as a diminutive Tyrannosaurus precursor may in-fact be a Tarbosaurus. Credit: Alejandra Vargas M./ La Nacion de Costa Rica / Newscom

Prominent palaeontologists are questioning whether Raptorex — the pint-sized supposed precursor to Tyrannosaurus rex — has been classified correctly.

The 3-metre-long fossil made headlines when its discovery was reported last year. But rather than a miniature blueprint for tyrannosaurs and other giant dinosaurs in the same family, some palaeontologists contend that it is a juvenile specimen of Tarbosaurus — a dinosaur closely related to, and living at the same time as, Tyrannosaurus.

Raptorex kriegsteini was classified by a team of researchers led by palaeontologist Paul Sereno of the University of Chicago, Illinois. The discovery was reported in Science in September 20091.

Peter Larson, president of the Black Hills Institute of Geological Research, a private fossil excavation and supply company based in Hill City, South Dakota, inspected Raptorex in June and decided to go public with his concerns after being contacted by Nature.

"Every feature about it says Tarbosaurus," he says. "It is exactly what I would have expected a baby Tarbosaurus to look like, including the correct number of tooth positions."

Every feature about it says Tarbosaurus. ,

Jørn Hurum, a palaeontologist from the University of Oslo who was behind the description last year of controversial primate fossil Darwinius Masillae, nicknamed Ida, has not personally had access to the fossil but agrees with Larson on the basis of his own reading of the Science paper. "Most evident was the skull structure that looked so much like a small copy of a Tarbosaurus skull," says Hurum.

Larson and Hurum also raise concerns about the reported origin and age of the fossil. On the basis of two other fossils — a fish vertebra and a freshwater clam — found alongside the dinosaur fossil, the paper says that the specimen of Raptorex is of Chinese origin and about 125 million years old. But the evidence is too vague, given that the fish and clam fossils are widespread in time and geographic area, say Larson and Hurum.

Larson suggests that the fossil could instead have come from known Mongolian Tarbosaurus beds dating from 70 million years ago. He believes that a more detailed analysis of the fossil matrix — including dating of any pollen associated with the fossil — is needed to resolve the issue.

Burden of proof

But at least one other palaeontologist who has inspected the fossil believes that Sereno is right about Raptorex. "I have not seen anything to justify the shadow of doubt," says Thomas Carr, who works on tyrannosaurid dinosaurs at Carthage College in Kenosha, Wisconsin. He says that there are specific physical characteristics, including the absence of a vertical ridge on the pelvis, that separate the specimen from other tyrannosaurids. He believes that the dating is reliable.

"The burden of proof is on Larson to demonstrate that the evidence presented is not valid," Carr adds.

The burden of proof is on Larson to demonstrate that the evidence presented is not valid. ,

Sereno told Nature that he stands by his conclusions, and noted that no one had "generated evidence" or "published a scientific paper" refuting them.

"I have not seen any features in the specimen that would specifically link it with Tarbosaurus; the rock type and fauna match what we find in the Chinese sequence; and fusion in the vertebral column indicates the dinosaur was reaching maturity," he says.

But he added that he was "happy to entertain more tests", although he was unsure how useful pollen testing would ultimately prove to be in pinning down a date.

Uncertain provenance

The issue is likely to cause a stir at the annual meeting of the Society of Vertebrate Paleontology (SVP), being held in Pittsburgh, Pennsylvania, on 10–13 October.

Philip Currie, a palaeontologist from the University of Alberta in Edmonton, Canada, who is set to become the new president of the SVP was the third outside scientist to inspect the specimen. He says there is "no question" that the fossil was "open to other interpretations".

The Raptorex fossil, which Sereno is planning to repatriate to China, was donated by a private fossil collector, Henry Kriegstein. He, in turn, acquired it at the Tucson Gem, Mineral and Fossil Showcase in Arizona from Hollis Butts, a fossil dealer in Japan. Sereno says that the information he received from the various dealers stated that the fossil came from an unspecified location in northern China.

"It is the whole problem of specimens being sold on the market where we don't really have accurate information," says Currie. "Fundamentally, if we don't really know where it came from and what the age is, then people are going to question the identification."

Major journals, he says, should not publish papers unless the exact location of the specimen in question can be pinpointed. This lines up with recommendations from the International Code of Zoological Nomenclature, which sets out how species can be classified.

But Sereno says that many papers are published for which the exact discovery site is unknown. In the case of Raptorex, not publishing would have risked the loss to science of a valuable new taxon, he says.

Sereno has an excellent reputation for naming dinosaurs. A paper published last month2 reported that 92% of the dinosaur species that he has named have remained unchallenged (see 'Top dinosaur hunters are worst at naming'). That formidable record has meant that palaeontologists have been hesitant to question Sereno's findings, Larson says.

Larson praises Sereno for giving other palaeontologists access to the fossil, but says that it could end up being repatriated to the "wrong country" — Larson says he has been informed by Butts that the specimen was originally sold as a juvenile Tarbosaurus from Mongolia.

Butts could not be reached for comment. But if the specimen is indeed from Mongolia rather than China, it is unlikely to be a new genus, says Larson, as Mongolia has no known dinosaur-producing beds dating from 125 million years ago.