The remarkable discovery from an ancient lake bed in north-east China has allowed dinosaur experts to piece together a picture of a small but formidable hunter that was so finely tuned to killing they describe it as “Jaws on legs”.
The finding of the beast, named Raptorex kriegsteini, which lived 130 million years ago, has stunned palaeontologists because the skeleton resembles the larger Tyrannosaurs in every respect except its size. Measurements of bones recovered from the site reveal that the new species was one hundredth the size of T rex.
Analyses of the remains by researchers at the University of Chicago and the American Museum of Natural History in New York revealed the dinosaur to be a juvenile of five or six years old, measuring nearly three metres from nose to tail and weighing only 60kg. A similar aged T rex could weigh several tonnes.
Though smaller than its more celebrated descendant, Raptorex was the largest meat-eater of its time. It would have enjoyed a diet of parrot-beaked psittacosaurs, turtles, primitive birds and small, scampering dinosaurs that would have watered at the ancient lakes it lived near.
The exquisite and almost complete remains only came to light when an American eye surgeon, Henry Kriegstein, telephoned the researchers to say he had bought the fossil from a trader. Paul Sereno, at the University of Chicago, agreed to document the fossil -- and name it after the surgeon’s father -- on condition that the remains were returned to China.
The Chicago team has spent the past three years preparing and studying the fossil, which was lodged in a block of sediment removed from the Lujiatun lake beds in the Chinese north-east.
Writing in the US journal, Science, the researchers describe the delicate operation to clean and prepare the skeleton. The skull was sent through an x-ray scanner at Chicago hospital before moulds and casts of the bones were made. The x-rays revealed enlarged brain regions that suggest the creature had a highly evolved sense of smell.
The discovery overturns scientists’ thinking about how Tyrannosaurus rex evolved. Many of the most striking features of the beast, such as its puny forearms, were thought to be a trade-off with its enormous size, but Raptorex shows these features had already evolved more than 60 million years earlier.
“So much of what we thought we knew about Tyrannosaur evolution turns out to be simplistic or out-and-out wrong,” said Stephen Brusatte, a member of the team.
“The thinking has been that as Tyrannosaurs developed to a truly giant size, they needed to modify their entire skeleton so they could function as predators. Raptorex, the new species, really throws a wrench into this observed pattern. Here we have an animal that’s one 90th or one 100th the size of T Rex, but with all the signature features, the big head, the strong muscles and the tiny little arms.
“We can now say these features didn’t evolve as a consequence of body size, but rather they just evolved as a set of efficient predatory weapons,” Brusatte added.
Raptorex had powerful legs to run down its prey and a huge muscular jaw to dispatch them with. “This is a blueprint for a predator: Jaws on legs,” Sereno said.
The animal’s forearms were almost redundant in capturing its prey, perhaps explaining why they withered in size over millions of years.
Researchers now believe that Tyrannosaurs spent almost all of their time on Earth as small, flighty predators like Raptorex. As other large dinosaurs became extinct, it left the path clear for Raptorex to expand in body size and ultimately become the giant Tyrannosaurus rex. “When it did, there was no turning back until the asteroid hit,” said Brusatte.
Sereno said Kriegstein agreed to donate the fossil remains to science if the species was named after of his father, Roman. “The specimen was found perhaps in the dark of night and spirited out of China and ultimately sold. [Mr Kriegstein] contacted me and wondered if I would describe it. I said I would if it could be returned 100% lock, stock and barrel to science and ultimately back to China. I think he saw that there was a sense of immortality in having a dinosaur named after your family,” Sereno said.
