Image caption One of the recently discovered fossil rodents hinted at a grassland ecosystem

Two of the earliest rodents, including the earliest chinchilla, have been discovered in Chile.

The finds add weight to the suggestion that grasslands appeared in South America 15 million years before anywhere else on Earth, after a period of global cooling.

The rodents have been described from two fragmentary fossils of lower jaws.

Both lived near a volcanic chain 32.5 million years ago that was buried in volcanic debris.

The structure of the animals' cheek teeth have been interpreted as being adapted to a diet of plants found in open, dry environments.

The work appears in American Museum Novitates .

Chewing in Chile

"You don't always realise the importance of what you have when you find these fossils" Dr John Flynn of the American Museum of Natural History, who has for 26 years led expeditions in the Tinguiririca River Valley, told BBC news.

You don't always realise the importance of what you have when you find these fossils Dr John Flynn, Frick Curator of Fossil Mammals, AMNH

Dr Flynn and his colleagues were surprised to find rodents, as out of the hundreds of specimens collected from their field site so far, these were the first.

"It's often not until we get them back to the lab, but if you split open a rock and you find a jaw there, you get the 'a-ha, eureka!' moment," he said.

However, what really got him and his co-authors excited was the presence of "hypsodont" teeth - a specific type of extremely tall cheek teeth where the enamel extends far down past the gum line.

The fossils were found in volcanic sediment, which is extremely hard to work with as it encases the fossils. But although this limited the range of analyses the team could perform, the presence of hypsodonty meant they could extrapolate the habitat in which it lived.

Hypsodonty is interpreted to be an adaptation to eating grassy plants and is found in modern day grazing animals. As grass contains silicious, glassy particles, the higher crowns enable tooth wear to be less severe. "It's basically an adaptation to eating food that functions like sandpaper".

Dr Flynn explained how the rodents had "convergently" evolved this tooth type at the same time as other animals during this period. "We have a whole series of extinct groups that become hypsodont at that time as well. We even see it in tiny marsupials," he explained.

Image caption The Tinguiririca River Valley where the discoveries were made

Gnawing doubt

"Here's another case where we find its predecessors had low crowned teeth, but these animals have become high crowned, so it seems to be compelling evidence for an ecosystem change... We know today that hypsodonty is correlated primarily with grassland habitats, so it's a reasonable inference that it was grassland then," he said.

Other palaeontologists, however have remained sceptical about the claim.

"Palaeontologists have long been interpreting the vegetation context in which the oldest hypsodonts lived by means of indirect evidence," commented Dr Luis Palazzesi of the Museo Argentino de Ciencias Naturales, whose forthcoming research into pollen records of plants counters Dr Flynn's conclusions.

Whether the first hypsodonts were feeding on grasslands, or actually evolved within rainforests, remains to be debated by palaeobiologists.