"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."
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 sizeit'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 generalistsable 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.
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