New Dinosaur Found; Shows How Giants Got That Way

Christine Dell'Amore
National Geographic News
November 11, 2009

A new dinosaur found in South Africa has given scientists a glimpse into the evolution of sauropods, the biggest animals ever to have walked the Earth, a new study says.

The newfound, 20-foot-long (7-meter-long) dinosaur species is a close cousin to the common ancestor of all sauropods—gigantic, four-legged, long-necked, big-bellied plant-eaters.

Dubbed Aardonyx celestae, the 195-million-year-old dinosaur had a lot of sauropod-like features, such as a robust skeleton for holding up its heft. (See extreme dinosaur pictures.)

Unlike sauropods, though, the newfound species walked on two legs and only dropped down on all fours, the new research shows. This means that walking on all fours started earlier in sauropod ancestors than previously thought, researchers say.

"What's exciting about Aardonyx is it's showing us that transition period … it's the closest thing just before becoming the classic sauropod," said study co-author Matthew Bonnan, a vertebrate paleobiologist at Western Illinois University.

The research was funded in part by a grant from the National Geographic Society's Committee for Research and Exploration. (The National Geographic Society owns National Geographic News.)

Dinosaur's Gigantic Traits

Bonnan and colleagues found the new dinosaur species on a game reserve in central South Africa.

During the early Jurassic period, the region was a dry floodplain filled with verdant oases, similar to the modern-day Okavango Delta in Botswana. (See an Okavango picture.)

Much of the skeleton of the animal—thought to have been fewer than ten years old and still growing when it died—was incredibly intact, Bonnan said.

"The excitement just kept building, like, What are we going to find next?"

Further analysis showed that bones of the forearm—the radius and ulna—interlocked, which would have allowed Aardonyx to support weight on its hands when it occasionally dropped to all fours.

The dinosaur also had flat feet, each with large claws that pushed weight to the inside of the foot for stability—a sauropod characteristic, according to the study published today in the journal Proceedings of the Royal Society B.

This means that such traits, associated with gigantism, likely evolved in smaller ancestors to help them move around, according to Bonnan.

Paleontologists had already suspected that sauropod ancestors sometimes walked on fours, so finding Aardonyx "fits our previous discoveries really well," said Thomas R. Holtz, Jr., a vertebrate paleontologist at the University of Maryland.

Binge-Eating Dinosaurs

But Aardonyx struck a blow to another theory about what sauropod ancestors would have been like.

Like other early plant-eaters, sauropod ancestors had been expected to have "cheeks," said Holtz, who was not involved in the Aardonyx research.

But the new species lacked the expected sheets of jaw-connecting tissue. Instead, Aardonyx, like a sauropod, sported a gaping mouth that allowed it to suck in more food.

In Aardonyx, this binge-eating adaptation probably paved the way for sauropods to evolve their titanic sizes, researchers say.

Holtz, though, isn't sad to see the cheeky theory go.

"We've got a little complication that's thrown in there to remind us that evolution isn't purposeful," Holtz said. "In the history of life, certain patterns wind up being more successful, and others don't."

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