Researchers from Case Western Reserve University recently found the oldest fossils on the South American continent along the banks of the Ucayali River, an offshoot of the Amazon River.

The fossils, which are at least 41 million years old, are the teeth of mouse and rat-sized animals that experts say are most closely related to African rodents. They are from the suborder Caviomorpha, which means they are related to living species such as guinea pigs, chinchillas and New World porcupines.

The discovery confirms hypotheses that the early rodents of South America likely had origins in Africa, according to researcher Darin Croft, an anatomy professor at the university's School of Medicine.

Now, researchers are also able to confirm that rodents landed in the northern part of the continent and spread southward, which contradicts the theory of a northward expansion that was determined from the fossil record 20 years ago.

The findings, published online in Proceedings of the Royal Society B, make Amazon River cruises, which encounter the confluence of the Ucayali with the Amazon, all the more interesting.

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