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A tiny T. Rex with typical traits

At only 9 feet in length, Raptorex already had the powerful jaws, puny arms, and quick legs of its much larger and more famous descendants.
[Drawing by Todd Marshall]

When you think of Tyrannosaurus rex, you probably imagine a fearsome predator with a large skull and tiny forearms attached to a tremendous body. But, researchers have just unearthed a much smaller version of this prehistoric dinosaur in China, and it's no more than three meters tall. Yet, this new fossil still has all the same physical features of the large T. rex and it predates T. rex and larger versions of the carnivore by some 125 million years.

Weighing as little as 1/100th that of its descendant T. rex, 125-million year old Raptorex shows off the distinctive body plan of this most dominant line of predatory dinosaurs. Based on a fossil skeleton discovered in Inner Mongolia, China.
[Drawing by Todd Marshall]

This finding means that such specialized physical features huge skull, strong jaws, and tiny arms did not evolve as the prehistoric predators grew in size. Instead, it indicates that the physical features were always there, at all sizes of the dinosaurs, to help them catch prey and feed.

Paul Sereno and colleagues studied the new, small-bodied fossil, known as Raptorex kriegsteini, and estimated that it was a young adult, about five or six years old when it died. They studied the skull, teeth, nose, spine, shoulders, forearms, pelvis, and hind legs of the new fossil, and compared the features to larger evolutionary versions of tyrannosaurid dinosaurs.

The researchers conclude that the specialized feeding features of R. kriegsteini were simply scaled up with very few changes in its descendants, whose body masses eventually grew to 90 times the size of R. kriegsteini.