Rodents tell a geologic tale

From Ottawa, at a meeting of the Society of Vertebrate Paleontology

The discovery of previously unknown rodent species that lived in Chile millions of years ago suggests that mountains in the southern Andes first rose to significant heights at least 18 million years ago.

By measuring the proportions of radioactive isotopes in ash deposits, scientists can estimate the date but not the height of a particular volcanic eruption. Therefore, geologists haven’t been able to determine when the southernmost portions of South America’s Andes, as a whole, rose to their current heights, says Jill Wertheim, a paleontologist at the University of California, Santa Barbara. Fossils could provide the answer.

Wertheim and her colleagues studied fossils of small animals from a part of that area that’s now hundreds of meters high. More than 20 million years ago, however, the region would have sat close to sea level along the Pacific coast, she notes.

In rocks laid down as sediments as early as 18 million years ago, the researchers found the fossils of 20 species of rodents that haven’t been reported anywhere else in South America. “That’s a large number of new species,” says Wertheim, who notes that 10 of those species belong to new genera. While some of the species that suddenly appeared are related to the agoutis and spiny rats that live in South America today, others belong to extinct lineages, says Wertheim.

Geographic isolation by the growing mountains would be a likely explanation of rapid evolution of rodent species along the coast. So, the appearance of the new rodent species about 18 million years ago indicates that the southern Andes at that time became too tall for the animals to cross, Wertheim speculates.