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Tiny T. rex cousin

A new fossil find from China has filled another branch on the tyrannosaurid family tree. The fossil is about 95 million years older than Tyrannosaurus rex, but is clearly a relative of the thunderous carnivore from the Cretaceous. The find gives paleontologists a better idea of when and how the branch of meat-eating dinosaurs that would eventually lead to T. rex evolved.

In northwestern China, researchers recently found a new species of tyrannosaurid, called Guanlong wucaii, that is much older and much smaller than its better-known cousin, the Tyrannosaurus rex. Image by Zhang Zongda/IVPP; courtesy of Greg Erickson.

Most tyrannosaurid fossils have been found in Cretaceous sediments, and before this find, which dates to 160 million years ago, the oldest fossil discovered was 130 million years old. Given where T. rex is on the evolutionary tree, paleontologists have long suspected that T. rex and other tyrannosaurids from the Cretaceous had much older ancestors that would have lived during the Jurassic, says Peter Makovicky, assistant curator of paleontology at the Field Museum in Chicago, Ill. But, he says, the record has been spotty to date. “This is an important find. It’s nice to have that [expectation] confirmed.”

That confirmation came from two nearly complete specimens of the new tyrannosaurid, called Guanlong wucaii — meaning “crowned dragon of the five colors,” referring to the colorful rocks of the fossil-rich Xinjiang province in northwestern China in which the specimens were found. As described in the Feb. 9 Nature, G. wucaii would have been about 3 meters (10 feet) long from nose to tail and stood about 1.1 meters tall at the hip — about one-quarter of the size of T. rex.

Like its larger cousin, G. wucaii was bipedal, with “serrated, meat-eating teeth” and “big, recurved claws,” says Greg Erickson, a paleontologist at Florida State University in Tallahassee, who was part of the research team. G. wucaii and T. rex also had similar “nasal features and hips.” However, the new species’ arms were longer and thinner than T. rex’s and it was more agile, similar to many Jurassic-aged predators, Erickson says.

G. wucaii also had a “most unusual feature, a crest, that was very delicate, probably very colorful,” Erickson says. The cranial crest, which was fully preserved in both fossil specimens, was about 1.5 millimeters thick. “There’s no doubt it was for sexual display or species recognition,” he says. Crests such as this have been found in other meat-eating dinosaurs at the time, but not in the tyrannosaurid line and in very few dinosaurs after the Jurassic, Makovicky adds. “Crests may have been an evolutionary fad,” he says.

Comparing the bones and the “mosaic evolution” of features in T. rex to its older cousin helps researchers to understand growth patterns and thus the dinosaurs’ lives, Erickson says. For example, based on the specimens, researchers determined that G. wucaii grew more slowly than T. rex but reached adulthood and died earlier, with a lifespan less than half that of T. rex. Thus, over time, he says, tyrannosaurids grew larger and more quickly.

“If you want to learn how any creature, including man, came to be the way it is, you have to learn about their lineages,” Erickson says. “This find helps us figure out how T. rex grew to be giants.”

Megan Sever

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