But the remains of ancestors of modern-day elephants and other animals, unearthed by the team of U.S. and Ethiopian scientists 27 million years on, provide some answers.

"We show that some of these very primitive forms continue to live through the missing years, and then during that period as well, some new forms evolved - these would be the ancestors of modern elephants," said Dr John Kappelman, who headed the team.

The find included several types of proboscideans, distant relatives of elephants, and fossils from the arsinoithere, a rhinoceros-like creature that had two huge bony horns on its snout and was about 7 feet high at the shoulder.

"It continues to amaze me that we don't have more from this interval of time. We are talking about an enormous continent," said Kappelman, who is based at the University of Texas at Austin.

Scientists had thought arsinoithere had disappeared much earlier but the discovery showed it managed to survive through the missing years. The fossils from the new species found in Ethiopia are the largest, and at 27 million years old, the youngest discovered so far.

"If this animal was still alive today it would be the central attraction at the zoo," Tab Rasmussen, a paleontologist at Washington University in St Louis, Missouri who worked on the project, said in a statement.

Many of the major fossil finds in Ethiopia are from the Rift Valley. But Kappelman and colleagues in the United States and at Ethiopia's National Science Foundation and Addis Ababa University concentrated on a different area in the northwestern part of the country.