Tiny tyrannosaur was still every inch the killer

A miniature version of Tyrannosaurus rex, the size of a human being, has been discovered after an extraordinary fossil that had been almost lost to the black market was recovered for science.

T. rex, which lived 60 million years later than its smaller cousin, shared its body shape in almost every detail, with an outsized skull, powerful jaws and teeth, athletic hind legs built for pursuing prey and puny forearms.

Raptorex kriegsteini, however, was only a fifth as long as its more celebrated successor, 100 times lighter and only 3m (9ft) from head to tail and 65kg (10st 3lb). T. rex grew to 13m and 7 tonnes and at the hip was more than twice as tall as a person.

The “miniature” creature’s anatomy has overturned standard explanations for the evolution of the tyrannosaurs, the group of giant predators to which T. rex belonged, which dominated the final 20 million years of the Cretaceous period immediately before the extinction of the dinosaurs. Tyrannosaur features such as huge heads and tiny arms were generally thought to have been adaptations that emerged as the theropod dinosaurs grew in size. The discovery indicates that this body shape had long been successful on a smaller scale, and simply expanded as predatory dinosaurs got bigger.

“We really thought that these features evolved in the course of gaining large body size,” said Paul Sereno, of the University of Chicago, who led the research. “We can now say this was a blueprint for a predator, jaws on legs, as it were, that was one of the most successful of the Mesozoic.”

His colleague, Stephen Brusatte, of the American Museum of Natural History in New York, said: “In short, much of what we thought we knew about tyrannosaur evolution turns out to be simplistic or out-and-out wrong.” While T. rex had “a head larger than a man is tall”, Raptorex “was about my size, but with all the features of a tyrannosaur”.

R. kriegsteini is also a triumph for Dr Sereno because of the deal that he brokered to save the specimen for science. The fossil was originally unearthed in mysterious circumstances in the Yixian Formation of inner Mongolia in China, but was then smuggled out of the country and sold to Henry Kriegstein, a private collector.

Mr Kriegstein approached Dr Sereno to analyse it three years ago and he agreed on the condition that it was donated “100 per cent lock stock and barrel” to science and returned to China. The specimen, which bears the Kriegstein name, will be displayed at a museum in inner Mongolia once the research team has made a detailed analysis.

Dr Sereno said: “I think he realised there was a sense of immortality in having a dinosaur named after your family, in this case his father, who had survived World War Two. Many of the family members didn’t, they were European Jews, and this was a way of recognising the contribution his parents made in their lives.”

Raptorex kriegsteini, which lived about 125 million years ago, is described in the journal Science. “It’s almost horse-like in the way it’s compacted so it’s able to run fast,” Dr Sereno said.

“This is really characteristic of the later tyrannosaurs. This indicates an animal that really ran down its prey, and it used its skull, the ripping incisors and the powerful jaw muscles, to dispatch the prey.”

The undersized arms were secondary to the jaws as offensive weapons, and they atrophied over time, Dr Sereno suggested. The scientists also suggest that tyrannosaurs may later have grown so large — to become the only large predators in North America and Asia — because competitors, such as Allosaurus, became extinct.

“As best as we can tell, it looks as if these other guys went extinct, opening the way for the expansion of these predatory dinosaurs to expand in body size,” Dr Sereno said.

“When they did, there was no turning back until the asteroid hit, because they had it down pat.”


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