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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