Researchers have described two new ancient species of South American rodents, including the oldest known chinchilla, in a study published last week in American Museum Novitates, a peer-reviewed scientific journal of the Museum. The research, which was led by Ornella Bertrand, a recipient of the Museum’s Annette Kade Graduate Student Fellowship, substantiates what might be the earliest grasslands in the world.
Bertrand worked closely with Museum Curator John Flynn, whose research teams discovered the rodent fossils on an expedition to the Tinguiririca River valley in the Chilean Andes in the late 1980s. In this area, near the border of Chile and Argentina, Flynn and his colleagues have discovered hundreds of mammal specimens representing more than 25 species. These fossils, including the new rodent specimens, suggest new information not just about the evolutionary history of mammals, but also about the geological history of the Andes mountains.
The researchers determined that these mammal fossils were buried in volcanic debris 32.5 million years ago. Although the fossils were buried in horizontal layers, today, they are tilted steeply upward, indicating that they were pushed up by the tectonic forces forming the rising mountain range. The Tinguiririca fossils’ position in the rock column and age suggest that this part of the Andes, once thought to be around 100 million years old, actually rose much later. The team’s research throughout this region suggests that the uplift began between 15 and 18 million years ago.
Rather than living on mountain slopes, the ancient rodents may have inhabited a grassy plain. This idea is based in part on evidence from the ancient chinchilla’s fossilized teeth. Its back or “cheek” teeth have an extra-long crown, or layer of enamel, an adaptation called hypsodonty that in modern ecosystems is associated with grazing animals, such as horses, cows, and goats. Hypsodonty is thought to make grazers’ teeth more resistant to the grit they encounter as they nibble abrasive, dirt-covered grass throughout their lives. Based on the age of the fossils and the chinchilla’s grass-ready teeth, as well as similar adaptations in other mammals from the same fossil fauna, Bertrand and colleagues propose that the South American rodents inhabited a grasslands environment 15 million years before grasslands emerged elsewhere in the world.
The two rodents belong to a group known as the caviomorphs, a subgroup of rodents that emerged and diversified in South America while it was an isolated island continent and breeding ground for many unique plants and animals. Researchers named the ancient chinchilla Eoviscaccia frassinettii after the late Daniel Frassinetti, their long-time collaborator and former head of paleontology at Chile’s National Museum of Natural History. They called the other fossil, which is related to modern agoutis, a group of rat-like creatures, Andemys termasi—Andemys for “mouse of the Andes,” termasi referring to the nearby summer resort town of Termas del Flaco. This fossil is now on display in the American Museum of Natural History’s exhibition Extreme Mammals, which is currently being shown at The Field Museum in Chicago.
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Photo caption: The fossilized jaw of the oldest chinchillid rodent, Eoviscaccia frassinettii, which is related to the modern chinchilla. © AMNH/M. Ellison