for National Geographic News
A massive, ancient, rhino-like creature with two bony horns protruding from its nose and several species of distant elephant relatives are among a jackpot of fossils recovered from the highlands of Ethiopia.
The fossils help fill a huge gap in the evolutionary history of African mammals known as the "missing years," shedding light on the origin and distribution of the famed beasts that roam Africa today.
"Everything people think they know about African mammalsgiraffes, antelopes, lions, cheetahs, rhinosthey all are newcomers," said Tab Rasmussen, an anthropologist at Washington University in St. Louis.
Rasmussen is a co-author of a paper in tomorrow's issue of the science journal Nature that describes the newly discovered fossils. The research was supported in part by the National Geographic Society's Committee for Research and Exploration. The fossils are all from an era a few million years prior to when most of today's classic African mammals came down from Europe and Asia.
Some of the newly discovered fossil mammals represent the last holdouts of species thought to have gone extinct millions of years earlier while others mark the first evidence for the few truly classic African species that survived the Eurasian invasion and evolved into today's famed beasts, such as the elephants.
The Missing Years
The new fossils help fill in a gap in scientific knowledge about the timing and evolutionary dynamics that drove a major change in African mammalian communities near the Oligocene/Miocene boundary. The Oligocene epoch is from 33.7 million to 23.8 million years ago and the Miocene epoch spans from 23.8 million to 5.3 million years ago.
This knowledge gap, known as the "missing years," extends from 24 million to 32 million years ago. At that time, the Red Sea had not yet begun to rift open and Africa and Arabia were still joined as a single continent that was isolated from the rest of the world.
During that eight-million-year gap, African mammals grew, mutated, diversified, and died completely under the radar of scientific investigation. To put the gap in perspective, eight million years is longer than it took for fully modern humans to evolve from primates.
"It is an immense chunk of time to be missing," said John Kappelman, an anthropologist at the University of Texas, Austin. "There are lots of hypotheses of what happened to the mammals during those missing years. Did they go extinct? Did they continue through unchanged? How diverse were they?"
Kappelman is leading the investigation into the newly discovered fossil site that, for the first time, is giving scientists a window into the missing years.
The fossil site is located in rugged terrain at about 6,500 feet (2,000 meters) in elevation and is dated to 27 million years ago. The sediments that contain the fossils lie on top of 30-million-year-old basaltic lavas and are exposed among agricultural fields in streams and gullies in the Chilga region of Ethiopia.
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