Case professor identifies extinct species of mammal

Usually, fossil-hunting is grunt work -- hand-sifting dirt in some parched badland, or eyeballing a thousand identical-looking rocks on a dusty hillside.

But sometimes it's just a matter of pulling open a drawer and recognizing what's inside.

One day in 1999, Darin Croft was browsing the fossil collection of Bolivia's national museum of natural history. The young paleontologist, whose specialty is the odd, mostly extinct mammals of prehistoric South America, was attending a scientific conference at the museum. He jumped at the invitation to see what secrets its specimen cabinets might reveal.

Like a lot of museums, the La Paz institution has a backlog of fossils gathered over the years but not yet identified. The flora and fauna entombed in those rocks are the relics of one of nature's more exotic experiments.

For most of 80 million years, South America was an island continent, its old connection to Antarctica severed and its future land bridge to North America not yet emerged from the sea.

The isolation let evolution run in strange, unique directions.

The cavalcade of warm-blooded creatures that roamed its ancient shores -- jumping rats, giant ground sloths, saber-toothed marsupials, armor-plated glyptodonts, horse- and camel-like litopterns -- arose nowhere else on the planet.

One fossilized skull caught Croft's eye. About the length of a beer can, it belonged to a group of small, hoofed South American mammals called hegetotheres (pronounced heh-JEET-oh-theerz).

But something didn't fit. Previously identified hegetotheres and their relatives commonly had a distinctive tooth -- a rear molar with two rounded bumps, or lobes, on its chewing surface. The pea-sized molars in the lower jawbone Croft held had three lobes. So did a half-dozen similar specimens belonging to the museum, all collected in the early 1980s from the Quebrada Honda site in southern Bolivia.

Whatever they were, the fossils represented an ancient animal no one had described before.

Seven years and a lot of research later, Croft, now on the faculty of Case Western Reserve University, has determined the 12-million to 13-million-year-old bones are from a previously unknown species of extinct South American mammal. He and his Bolivian colleague, Federico Anaya, named it Hemihegetotherium trilobus, (pronounced try-LOW-bus) in recognition of the creature's three-lobed tooth.

The identification of H. trilobus fills another small gap in the complex evolutionary history of South America. Charles Darwin gathered some of the first fossils from there nearly two centuries ago, but Croft and other scientists are still piecing together the story of where the continent's weird mammals originated, how they co-existed, and why most of them died out with the arrival of invaders from the north.

"We can follow evolution at work for tens of millions of years in the history of South American mammals," wrote the renowned paleontologist George Gaylord Simpson, who spent most of his life doing just that.

"We have been provided with an almost ideal experiment," Simpson wrote -- the continent's shift from "splendid isolation" to a vast melting pot when North American mammals began their southward trek across the newly formed Isthmus of Panama about 3.5 million years ago.

Despite that lure, relatively few paleontologists are working in South America compared with other parts of the world, perhaps because it's viewed as sort of a developmental dead end.

"A lot of the studies on evolution and relationships are driven by animals that are alive today," Croft said. "A number of these animals we're finding didn't leave living descendants. But we think they're really neat because they're a whole separate experiment in evolution."

The twisting evolutionary road leading to 12 million-year-old H. trilobus and South America's other peculiar prehistoric residents begins far deeper in the past. The first small, primitive mammals began to appear more than 200 million years ago, when dinosaurs still dominated the planet and the supercontinent known as Pangaea was starting to splinter.

As the restless land masses jostled for position over the millennia, linkages between them were made and broken. The shifting continental pairings opened new territories for some mammal populations and stranded others. The meteorite impact and massive volcanic eruptions that altered Earth's climate and doomed the dinosaurs 65 million years ago enabled the surviving mammals to flourish.

By 35 million years ago, drifting South America had lost its last connection with the rest of the world. The "archaic" mammals that had originated on the continent or migrated there from Antarctica or elsewhere before the break -- pouched marsupials, hoof-footed ungulates, and xenarthrans, whose members included sloths, armadillos and anteaters -- were on their own to develop without outside influence.

The isolation was broken only by the arrival of monkeys and rodents, which likely island-hopped their way to South America from gradually receding Africa.

Among the continent's most prolific residents were the notoungulates. This hoof-clad group of mammals encompassed everything from the beagle-size H. trilobus to the squat, rhinoceros-like Toxodon, which weighed more than a ton.

So far, H. trilobus is a bit of a puzzle. "It would have looked fairly dog-like," Croft said, but the appearance of its ears, tail and coat are unknown. Its feet had four toes, with small hoofs on each toe.

"We really don't understand well how these things moved and, behaviorally, what they were doing," the paleontologist said. "Was it mostly geared toward speed -- something like a hare, adapted to outrunning its predators? Or was it more inclined to run and hide, like a rodent? The skeletons of these animals are a combination between adaptation for running and for power, like digging."

The purpose of its unusually divided molars also isn't clear. It could have occurred merely by chance.

Or perhaps the extra lobe helped H. trilobus chew tough, abrasive vegetation. The site where the fossils were found was lower in altitude and probably wetter and greener 12 million years ago than today, since the upthrust of the Andes Mountains was still under way.

No one knows why H. trilobus and its fellow notoungulates gradually disappeared from South America. The last of them went extinct 10,000 years ago.

A widely held view is that the more advanced and predatory mammals streaming in from North America with the completion of the land bridge out-competed their southern cousins. Croft thinks South America's diversifying rodents may have contributed to the decline, by shouldering their way into ecological niches that notoungulates previously had to themselves.

Those and other mysteries are yet to be solved. Croft hopes to go to Quebrada Honda for more work on H. trilobus next year.

"South America has a pretty rich fossil record," he said. "What it lacks today is study. There are still a lot of holes to be filled in."

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