Thursday, July 5, 2007

Fossils Shed Light on Africa's "Missing Years"




Fossils Shed Light on Africa's "Missing Years"
John Roach
for National Geographic News

December 3, 2003
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 mammals—giraffes, antelopes, lions, cheetahs, rhinos—they 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.

"What's interesting is it is smack dab in the middle of this interval and provides this window on what did happen," said Kappelman.

In addition to shedding light on what happened to African mammals during the missing years, the fossils provide clues to the evolutionary dynamics that allowed some mammals to survive the continental merger of Afro-Arabia with Europe and Asia about 24 million years ago.

Writing in an accompanying commentary on this research in Nature, Jean-Jacques Jaeger, an evolutionary scientist at the University of Montpellier in France, said that these newly discovered fossils add insight to the "dynamics of the faunal interchange between Afro-Arabia and Eurasia."

African Mammals

William Sanders, a study co-author from the Museum of Paleontology at the University of Michigan in Ann Arbor, said that among the more interesting finds at Chilga were five species of proboscideans, which are distant cousins to modern elephants.

The team found several species of archaic proboscideans called Palaeomastodons previously known from 32-million-year-old coastal sediments in Fayum, Egypt. These ancient species were lying side by side with more advanced forms called deinotheres and gomphotheres, which were not previously known to exist until 20 million years ago.

"The first thing significant about this is that it greatly extends the fossil record of all these proboscideans," said Sanders. "For example, the deinothere record is now a third again longer."

In addition, scientists had thought the deinotheres and gomphotheres evolved in response to the invasion of immigrants from Eurasia after the continental merger. The new finds indicate that these species evolved for independent reasons prior to the merger.

Another unusual mammal found at Chilga is the arsinoithere, which looks like a larger version of a modern rhinoceros, but actually evolved independently of rhinos, said Rasmussen.

The species was previously known from sediments older than 32 million years but scientists did not know if the arisoitheres had survived the missing years. The rhino-like creature has not shown up in any of the early Miocene sediments, dated to around 23 million years.

"We are able to show [the arsinoithere] continued through the interval," said Kappelman. "It also increased in size—it's much larger than its forebears." The arsinoitheres from Chilga were about 7 feet (2 meters) tall and weighed over 5,000 pounds (2,270 kilograms).

Merger and Extinction

The discovery that several primitive mammals such as arsinoitheres were living in Africa as late as 27 million years ago when several scientists thought they went extinct millions of years earlier, may offer new answers to the question of why and when the primitive mammals perished.

"We are down to two maybe three million years to when we first get Eurasian forms coming in [to Africa]," said Kappelman. "Did some go extinct before the influx, or was it head-on competition that drove them to extinction? That is what we don't know yet."

The similarity of the Chilga fossils to older Afro-Arabian fossils found along the continental margins, such as in Egypt, suggest a low diversity among the endemic mammal forms and that many of them were generalists—able to adapt to a wide array of habitats.

One theory, said Kappelman, is that after the continental merger more specialist mammals moved into Africa and out-competed the low-diversity generalists. The exception is the proboscideans, which were diverse and went on to colonize the rest of the world.

Other theories suggest that changes in climate or ecology were ultimately responsible for the demise of some species, such as the arsinoitheres and palaeomastodonts, said Sanders.

"Diversity certainly helps survival, though, because it gives a group potentially more evolutionary pathways to exploit and one or more of those might steer a lineage clear of competition," he said.

More fossils closer in age to the continental merger are needed to fully evaluate the competing hypotheses as to what caused some of the original Afro-Arabian mammals to go extinct.

This research is supported by the National Geographic Society, the National Science Foundation, the Leakey Foundation, and the Ethiopian Ministry of Culture.

1 comment:

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