The remains -- teeth -- showed these mouse- and rat-size animals are most closely related to African rodents, confirming the hypothesis that early rodents of South America had origins in Africa, said Darin Croft, an anatomy professor at the Case Western Reserve University School of Medicine and member of the research team.
This discovery supports the contention 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 porcupines. 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 northward expansion deduced from the fossil record 20 years ago.Kummel mentions fossils along the Ucayali, a major tributary 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 vertical. 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 silt 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.
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