Ancestor of Kings: Early progenitor of T. rex had a crest

Paleontologists have unearthed remains of the oldest known dinosaur of the tyrannosaur clan. About 160 million years ago, the agile, 3-meter-long predator roamed what is now northwestern China. Its fossils bolster a recent theory about the evolutionary origins of the fearsome meat eaters that appeared later.

Dubbed Guanlong wucaii, which in Mandarin means “crowned dragon from the five-colored rocks,” the newly described creature gets its genus name from the distinctive, 6-centimeter-tall crest that runs along the top of its snout, says James M. Clark, a vertebrate paleontologist at George Washington University in Washington, D.C.

The dinosaur shares several traits with Tyrannosaurus rex, which lived about 95 million years later. Both beasts sported front teeth with U-shaped cross sections, nasal bones fused for strength, and an ornamented skull.

In the Feb. 9 Nature, Clark and his colleagues describe two sets of nearly complete remains representing G. wucaii. One set is of an adult estimated to be 12 years old, and the other is of a juvenile about 6 years of age. Although both creatures sported a crest, on the younger dinosaur that feature was small. In both the adult and the juvenile, the bony structure is largely hollow and doesn’t appear to be directly connected to the creature’s nasal passages.

The crest was too fragile to have served an offensive or defensive function and “only makes sense in terms of visual signaling,” says Clark. The structure might have distinguished one sex of the species from another, been a sign of sexual maturity, or enabled G. wucaii to recognize other members of its species at a distance. Some scientists have speculated that the bizarre anatomical features of many other dinosaurs served similar purposes (SN: 8/13/05, p. 103: Available to subscribers at Just for Frills?).

Tyrannosaurs were noted for their short snouts, sturdy skulls ornamented with knobs and ridges, and puny, two-fingered arms. T. rex stood 4 m tall at the hips, stretched nearly 13 m from its snout to the tip of its tail, and had teeth the size of bananas.

Paleontologists once thought that tyrannosaurs had evolved from large predators that lived at the end of the Jurassic period, 145 million years ago. However, a decade ago, scientists began to speculate that T. rex and its cousins evolved much earlier, from a line of small, meat-eating coelurosaurs. Fossils show that these creatures shared many skeletal features with tyrannosaurs.

The remains of G. wucaii, the first tyrannosaur relative known to be more than 130 million years old, support the latter theory, says Thomas R. Holtz Jr., a paleontologist at the University of Maryland, College Park.

Not only did G. wucaii share traits with the tyrannosaurs, but it also had coelurosaur-like features, including a relatively long snout, bladelike teeth on the sides of its jaws, and long arms with three-fingered, grasping hands, says Holtz.

Although T. rex and the other large, relatively recent tyrannosaurs were undoubtedly the rulers of their ecosystems, G. wucaii didn’t reign in its era. It had to be agile not only to catch prey but also to escape from predators about twice its size that lived in the area, says Clark.