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Fossil Mammal Resembling Dog-Hare Hybrid Found in Bolivia

Nicholas Bakalar
for National Geographic News
September 19, 2006
 
In paleontology, not all big finds happen out in the field.

A new species of ancient mammal has been discovered—in the fossil collection of the National Museum of Natural History in La Paz, Bolivia.

The animal, which has been assigned the tongue-twisting name Hemihegetotherium trilobus, is a member of an extinct group called notoungulates, a term that means "southern hoofed mammals."

The creature resembles a cross between a dog and a hare. It was about the size of a beagle, weighing between 20 and 25 pounds (9 and 11 kilograms), and probably looked something like a capybara, the largest modern-day rodent.

(Related video: "Anaconda Stalks World's Largest Rodent.")

Specimens of the creature's bones—including almost complete skulls and jaws and parts of the skeleton—have been in collections in various museums for more than 30 years.

"Normally, you think of finding these in the field," said Darin A. Croft, an assistant professor of anatomy at the Case Western Reserve University School of Medicine in Cleveland, Ohio.

"But for this animal, no one who specialized on this group had taken a close look. No one had had the time or the expertise to look at the detailed anatomy."

Croft found the bones in a sample drawer while visiting the museum during a 1999 paleontology conference in Bolivia.

He noticed at the time that the molars had three lobes, whereas other notoungulates' teeth had only two, so he decided to study the remains further.

Croft and colleagues describe their findings in the June issue of the Journal of Vertebrate Paleontology.

Island Continent

Named for the distinctive three lobes of its last molar, H. trilobus lived about 12 to 13 million years ago, a time when South America was an island continent.

Hegetotheres, the family the new species belongs to, lived in South America for almost 30 million years.

The family of animals went extinct only about 10,000 years ago, which means that humans probably encountered some of the creatures.

The specimens that Croft examined came from the Quebrada Honda region of southern Bolivia (map of Bolivia).

During South America's isolation, which ended about three million years ago, the only other mammals that lived on the continent were marsupials, sloths, armadillos, monkeys, rodents, and various groups of ungulates, or hoofed mammals.

All of the region's indigenous ungulates are now extinct. Camelids such as llamas and alpacas are not native to South America but arrived there from the north, as did some deer and horses.

The notoungulates as a group are a bit of a mystery.

"We're not even sure [which animals are] the closest living relatives," Croft said. "We don't know if they're like deer or closer to rodents. Some look like rodents, others look like horses."

According to Croft, over time notoungulates evolved to become open-habitat runners like hare and antelope.

The earliest members of the group had two separate leg bones, the tibia and fibula, just like humans.

But then the leg bones fused, unfused, and then fused again in a slightly different configuration.

"What we would expect," Croft said, "is that once these fused legs evolved, all the descendants would keep that character. But they didn't."

Croft plans to go back to the Quebrada Honda region this winter to conduct further research on ancient South American mammals and potentially shed more light on notoungulate evolution.

The work is being funded by the National Geographic Society Committee for Research and Exploration. (National Geographic News is part of the National Geographic Society.)

Bruce Shockey is a research fellow in the department of vertebrate paleontology at the American Museum of Natural History in New York City.

Shockey, who was not involved in the research, called the paper "significant," and said that it "reveals new knowledge of ancient life on a 'lost' continent."

He added that the authors "performed a rigorous analysis of the anatomy of the new hegetothere and compared it with the anatomy of other hegetotheres, ultimately providing a hypothetical family tree of hegetotheres."

"We Just Missed Them"

Scientists are baffled about why the notoungulates went extinct.

Although H. trilobus is known only from one site, there were hundreds of species of notoungulates in South America, as revealed by an abundance of their fossils.

"How can a group that was so successful for so long go extinct?" study author Croft asked.

"And it only happened a few thousand years ago. We just missed them."

One theory, Croft says, is that rodents had something to do with it.

The notoungulates were in South America before rodents, and when rodents arrived and increased in numbers, notoungulates declined.

But, Croft said, "we don't know if rodents outcompeted notoungulates, or whether rodents just filled a niche that notoungulates had vacated for other reasons."

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