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New Dinosaur May Link S. American, Aussie Dinos

James Owen
for National Geographic News
June 10, 2008
 
A rare fossil found in Australia suggests dinosaurs were able to traverse the vast prehistoric continent of Gondwana much later than thought, scientists report.

The hundred-million-year-old fossil belonged to a two-legged meat-eater, or theropod, that is closely related to Megaraptor namunhuaiquii, a giant, big-clawed carnivore from Argentina, says a team led by Nathan Smith of the University of Chicago's Field Museum.

The discovery could help redraw the world map during the dinosaur era, researchers add.

That's because the newfound Australian dinosaur shows that animals could travel across Gondwana during the Cretaceous period, about 145 to 65 million years ago.

This in turn suggests that Gondwana's Southern Hemisphere landmasses broke up later than traditionally thought.

Strange Forearm

The study is based on the unidentified theropod's arm bone, which was discovered at Dinosaur Cove in southeastern Australia in 1989.

The fossil has unique features that solidly link it to the South American Megaraptor that was first described in 1998, Smith said.

"Megaraptor has a huge hand with a big [clublike] claw and a very strange forearm, so if you had to pick one bone to refer to, then the ulna [arm bone] might be that bone," Smith said.

The length of the fossil bone, 7.6 inches (19.3 centimeters), suggests the dinosaur was about half the size of Megaraptor.

This size difference could be because it is a smaller species or because it was a juvenile, Smith said.

The still-nameless Australian specimen is described this week in the journal Proceedings of the Royal Society B.

Animal Exchange

Previously scientists thought that Australian animals were isolated from life on other Gondwana landmasses during most of the Cretaceous because of geography and climate, the study authors said.

"What we now have is demonstration that there must have been some kind of [animal] exchange between Australia prior to about a hundred million years ago," Smith said.

(See a prehistoric time line.)

The new study also supports alternative models for the break up of Gondwana.

Traditionally it was thought that Africa and South America separated from eastern Gondwana—which included Antarctica, Australia, India, and Madagascar—some 138 million years ago.

The alternative models show Africa separating first.

The new study is not the final say on the matter, Smith emphasized.

But "I think in the future we are going to start seeing more [Australian fossils] that really demonstrate close affinities with other animals in South America."

(Related: "Giant Dino Found in Fossil 'Lost World'" [October 16, 2007].)

Little Evidence

Yet the Australian husband-and-wife team who led the excavation of the theropod fossil aren't convinced by the findings.

Patricia Vickers-Rich, a paleontologist at Monash University in Victoria, said "too much is being interpreted based on a single bone."

The Australian dinosaur fossil record is "very scanty," she said, so "rather sweeping generalizations about biogeography are made based on very little evidence."

Tom Rich, curator of vertebrate paleontology at Museum Victoria, echoed his wife's comments.

He added that other Australian dinosaurs from the same period seem to be more closely allied with those from Asia than South America.

The reason for this is a "complete mystery to me," he wrote in an email, since a map of the world of that time would show "Australia was much further from Asia than it is today."

But, he said, "fossils should not be identified on the basis of geography if one is going to do meaningful paleobiogeographic reconstructions—otherwise one will be going around in a very tight logical circle."

The Riches said that more theropod bones from the Dinosaur Cove area are currently being studied. "There could be half a dozen different theropods," Tom Rich said, though the condition of the fossil bones may not allow individual species to be identified.
 

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