(illustration) Todd Marshall; (inset photo) Mike Hettwer

That's my boy! Raptorex has proven to be a pint-sized version of its famous descendant, T. rex.

T. rex, Meet Your Great-Grandfather

Tyrannosaurus rex now has his "mini me." Paleontologists have discovered a creature that matched the famed dinosaur in nearly every respect--but was only one-fifth the size. The fossil, which the team has named Raptorex kriegsteini, may overturn previous thinking about the rise of the most famous predator among the "Terrible Lizards."

Just about every schoolchild can spot a T. rex--even a stuffed version. The huge head, the powerful jaws bristling with long teeth, the magnificent tail, and the stubby arms--all have been immortalized in photo, story, and screen. For decades, scientists have thought that these traits, along with the creature's enormous olfactory capability, evolved from a less specialized but similarly sized version of T. rex. One leading candidate has been Allosaurus, a comparably sized but clunkier version of T. rex that lived 50 million years earlier, in the late Jurassic period.

Raptorex changes all that. The 125-million-year-old fossil represents the long-sought blueprint for what would become T. rex, but "to our great surprise, seen in a human-sized dino," Paul Sereno of the University of Chicago in Illinois said at a news conference yesterday. The similarities include the massive "runner's legs" like those T. rex used to chase its prey, Sereno said. The little beast also had tremendously strong chompers. And just like T. rex, Raptorex's forelimbs were puny. Sereno explained that as the creature's head, teeth, and jaws grew larger and heavier--as its primary weapon for bringing down prey--its arms probably shrunk and in the process helped to balance the animal's large head and long tail over its legs. Based on a trait-by-trait comparison, Sereno said, Raptorex belongs to an early side branch of the evolutionary group that gave rise to T. rex about 60 million years later.

The fossil, described online today in Science, almost didn't come to light, Sereno and co-author Stephen Brusatte of the American Museum of Natural History in New York City told reporters. Raptorex's skeletal remains were found several years ago in a rock formation in northeastern China, but they were spirited away and later sold quietly to a collector. That collector, ophthalmologist Henry Kriegstein of Hingham, Massachusetts, eventually made the nearly complete specimen available for analysis by the researchers. In exchange, they named the creature for Kriegstein's father and mother, who are Holocaust survivors. Although Raptorex is currently residing at the University of Chicago, Sereno said it will be returned to a museum in China.

What could make the find even sweeter? "Soft tissue," says mass spectrometrist John Asara of Harvard Medical School in Boston. Asara, who has examined such samples from T. rex fossils, says "It would be great if a soft-tissue–containing Raptorex can be discovered." Analysis of protein sequences in soft tissue could allow researchers to sharpen the evolutionary relationship between various tyrannosaurs.

The Raptorex discovery, and others like it from China, has greatly improved scientific understanding of the evolutionary origins of T. rex, says paleontologist Matthew Lamanna of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania. Until recently, he says, "there were practically no reasonably complete fossils" of species that might be considered ancestors of T. rex. But Raptorex, he says, shows "that many of the anatomical hallmarks of T. rex first appeared in much smaller, geologically older relatives."

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Posted in Paleontology, Archaeology, Evolution