In a literal walk through time along the Ucayali River near Contamana, Peru, a team of researchers found rodent¹（啮齿动物） fossils at least 41 million years old -- by far the oldest on the South American continent. The remains² -- teeth -- showed these mouse- and rat-size animals are most closely related to African rodents³, confirming the hypothesis that early rodents of South America had origins in Africa, said Darin Croft, an anatomy⁴（解剖学） professor at the Case Western Reserve University School of Medicine and member of the research team.

This discovery supports the contention⁵ that rodents landed in the north and spread south. The rodents are from the suborder Caviomorpha, the group that includes living rodents such as guinea pigs, chinchillas, and New World porcupines⁶. The oldest fossils from this group are only about 32 million years old in central Chile and about 30 million years old in Patagonia, Argentina,. Taken all together, the pattern contradicts the theory of a northward⁷ expansion deduced from the fossil record 20 years ago.

The findings, which describe three new species, are published online in Proceedings⁸ of the Royal Society B.

"This really pushes back the date of the first South American rodents," said Croft, a paleontologist（古生物学者） who specializes in mammalian evolution.

Pierre-Olivier Antoine, a professor of paleontology in the Institute of Evolutionary⁹ Sciences at Montpellier University in southern France, asked Croft to join the team of scientists from France, Germany, Peru and Panama. Members first flew into the region in 2008, after reading Harvard Geology Professor Bernhard Kummel's 1948 description of the area.

Kummel mentions fossils along the Ucayali, a major tributary¹⁰（属国，支流） of the Amazon, but the team found no evidence that anyone had investigated them.

During three trips from 2008 to 2010, Antoine's group found the fossils in a portion of the riverbank exposed when the water level is low.

The geology along the river showed that layers of rock, including the fossil layer, had been pushed up in a rainbow-shaped fold, called an anticline. The layers that had once been above or below the fossils turned from horizontal to nearly vertical¹¹. Instead of digging down to the past, the scientists walked downstream from the fossil layer to go back in time, upstream to go forward in time.

Ash found among silt¹² particles 47 meters forward in time was dated at 41 million years ago using argon-argon radioactive dating, providing the minimum age of the fossils.

The date is supported by genetic¹³ studies of living African and South American rodents that show the animals are of common origin and estimate the animals arrived in South America during the Mid-Eocene Climatic Optimum, or about 40 million years ago.

At that time, other scientists estimate, an African rodent on a raft of vegetation could have reached Northeast Brazil in one to two weeks.

The characteristics of the teeth found reinforce the connection between the continents: the morphologies（形态学） are closest to those of African rodents.

The dental features indicate the rodents probably ate soft seeds and plant parts as many small rodents do today.

Pollen¹⁴ extracted from the fossilized mud that contained the teeth suggests these rodents lived in a rain forest, much like the rain forest there today.