But in this case, it was the tiny terror that gave rise to the larger, fiercer relation.
Raptorex kriegsteini, described this week in the journal Science, likely lived about 125 million years ago, during the Cretaceous period.
That’s almost twice as far back as the mighty Tyrannosaurus rex, according to study leader Paul Sereno of the University of Chicago.
Raptorex has all the main characteristics of its larger descendants such as T. rex—big head, nipping teeth, stubby arms, fast legs—but packed into a 9-foot (3-meter) frame.
This T. rex design in miniature “reveals a spectacular carnivore strategy,” according to Sereno, a National Geographic Society explorer-in-residence. (The National Geographic Society owns National Geographic News.)
Tiny T. rex “Evolutionarily Staggering”
The 150-pound (70-kilogram) Raptorex “was running things down, dispatching them with its powerful jaws, and clutching them with its two-fingered hands”—the same hunting strategy that apparently worked for 6-ton T. rex, Sereno said.
“That’s the pretty evolutionarily staggering thing,” he added. Raptorex is T. rex, but “scaled up, almost without change, a hundred times.”
The find runs counter to previous theories, which had said that T. rex’s stumpy arms were a relatively recent evolutionary development. As tyrannosaurs got larger, their arms simply didn’t scale up fast enough, and the limbs eventually became small in relation to the dinosaurs’ oversized bodies, the older theories say.
It’s still thought, however, that T. rex’s earlier ancestors—even before Raptorex—had relatively long arms.
T. rex-style Arms Not a Liability
The new dinosaur is “a very significant find” for understanding the evolution of tyrannosaurs, said paleontologist Thomas Holtz of the University of Maryland.
“We didn’t know where and when in the history of the tyrannosaurs this arm-shortening occurred,” said Holtz, who was not part of the study.
“Now the question is going to be, What were they doing [with those small arms]?” Holtz said. “There’s not much of a reach,” he added, speculating that the tyrannosaurs grabbed prey first with their jaws and then used their arms to help hold onto their quarry.
Study leader Sereno noted that it can be hard for people to appreciate the trade-offs that evolution inevitably entails.
“It would seem to a human that forelimbs are so useful, that only when you got to the size of a tyrannosaur and you could frighten everybody with a growl could you get rid of [forearms],” he said.
“But this common sense type of thinking almost never works with evolution,” Sereno said. In the tyrannosaurs, for instance, “long, heavy forelimbs are a significant burden and would seriously curtail agility in the hunt.”
Smuggled T. rex Ancestor Heading Home
The new findings are based on a nearly complete Chinese dinosaur skeleton, which was excavated in secret, smuggled into the United States, and sold at auction to private collector Henry Kriegstein.
Sereno said he convinced Kriegstein to donate the fossil back to science.
Although the exact location the dinosaur came from will never be known, the excavated block containing the dinosaur’s skeleton also included fish bones and clamshells that link it to Northern China’s Yixian fossil formation.
Raptorex kriegsteini, named after the collector’s father, an Auschwitz survivor, will eventually be shipped back to Northern China, where it will be displayed in a museum in Hohhot, the capital of China’s Inner Mongolia region.
“Fossils like these should be protected from smugglers, or there’s a chance they could disappear forever,” Sereno said.
Until that day comes, Sereno hopes the story of this fossil can serve as a model for saving—and learning from—smuggled dinosaurs.
“I think everybody involved with this Raptorex is a winner here,” he said.