Oldest Rodent Cache Found—Filled With Fossil Nuts

John Pickrell in England
for National Geographic News
December 10, 2003
Paleontologists in Germany have uncovered the world's oldest
underground pantry. The unusual fossil was spotted by accident in an
open pit mine near the city of Cologne. Thought to be more than 17
million years old, the complex burrow system is packed with more than
1,800 fossilized nuts.

The burrow may have been dug by an extinct species of large hamster or ground squirrel storing food for a winter cold spell, researchers concluded. They described their find in a recent edition of the science journal Palaeontology.

"It's relatively easy to find out the shape and morphology of animals from fossils, but to find out what their behavior was is much more difficult," commented Jerry Hooker, mammal paleontologist at London's Natural History Museum. Hooker said small mammal fossils alone are quite unusual, making this burrow a rare and exciting find.

The food larder, which is suggestive of a winter cold spell, may also hint at previously unknown climatic changes taking place in the otherwise warm region during the period between 23.8 million and 5.3 million years ago, known as the Miocene epoch.

Long History of Digging

The oldest fossilized vertebrate burrows discovered are known from deposits in Wyoming and Nebraska associated with the Oligocene epoch, the period between 33.7 million and 23.8 million years ago.

While the Cologne fossil is the oldest food store yet discovered, two other ancient larders have been found in slightly more recent deposits in Nebraska. In those cases, partial remains of rodents were evidence of one-time occupants, says study co-author and paleobotanist Carole Gee of the University of Bonn. In the absence of such remains in the current fossil, Gee's team have had to use other evidence as the smoking gun to lead them to the animal occupant.

Gee and University of Bonn co-author Bianka Petzelberger first spotted a strange cluster of nuts in the wall of the Garzweiler open pit mine while on a field trip to examine marine sediments. Gee collected a few nuts for later analysis, and the pair headed off to another site. But the odd "ball-shaped" accumulation—found 100 meters (328 feet) down in the deep pit—had peaked Gee's curiosity. "I wasn't able to concentrate on anything else during the rest of the trip," she said.

Complex Burrow System

Gee and Petzelberger returned with Gee's husband, University of Bonn vertebrate paleontologist Martin Sander, less than a week later. "I'd convinced myself it was a [larder] of fossil nuts," said Gee, "and I knew we only had a few weeks' time, as the giant coal excavator would [soon] pass through the area and destroy everything with it."

The effort paid off. When the team began to dig the fragile nuts tunnel system out of the soft sand rock they discovered a complex tunnel system—traced out by nut-filled tunnels and teardrop-shaped pockets. The shape of the tunnels and chambers were only discernible by the arrangement of nuts, as sandy sediment the same color as the surrounding rock had filled all the remaining space.

The researchers estimated that 1,800 or more fossilized nuts filled the chambers. The nuts all came from a type of Castanopsis, or chinkapin tree, modern representatives of which are found in Northwest United States and Asia today.

This and other evidence, such as the width of the tunnels, hinted at the type and size of the burrow's former occupant or occupants.

"Burrowing animals make tunnels only big enough to squeeze through. It would otherwise be a waste of time and much effort," said Gee.

Hamster and ground squirrel remains were known from neighboring sites of a similar age, making those species likely contenders.

"We narrowed the possibilities down to a hamster because hoarding is what hamsters do best," she said. For example, the black-bellied hamster (Cricetus cricetus) that lives in central Europe today is known to store up to 90 kilograms (200 pounds) of grain, peas, and potatoes within a year. In contrast, ground squirrels eat excessively before the months get cooler in order to build up fat reserves for winter hibernation.

Climate Enigma

That left one final mystery, said Gee. Hamsters and other rodents today usually build food stores like this to see them through winter months with scarce food resources. However, research on animals and climate in the Miocene of Central Europe indicates that conditions would have been warm and humid at this time.

The burrow system was found in what would have been beach dunes on the coast of the ancient North Sea, where resources may have been scarcer than in nearby forests. Otherwise, "there could have been a wet and a dry season just as severe," as a winter in terms of resource scarcity, suggested Hooker.

"Food hoarding takes a lot of energy…and must give the food hoarder some sort of advantage to make it worthwhile," said Gee. It is also possible that an annual cold season was just developing, but has not yet been reflected in other evidence, she said. "This remains an unsolved, but very interesting, puzzling aspect."

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