By Helen Briggs BBC News Online

Fossilised nut: The two parts of the nut can be seen within the shell

A team from the University of Bonn found a burrow containing 1,800 fossilised nuts when digging at an open-cast mine near Garzweiler.

The winter food supplies were probably hidden away by a large hamster.

The rodent appears to have excavated a complex series of tunnels, the team reports in the journal Palaeontology.

Life in the Miocene

During the Miocene epoch, the area, west of Cologne, was very different from today.

The North Sea lapped at sand dunes fringed by a swamp forest. The climate was much warmer, supporting the growth of palm trees.

As far as we know, this is the oldest food cache in the fossil record Carole Gee

Mining excavations at Garzweiler are yielding clues to the past.

Palaeontologists came across mysterious piles of fossilised nuts when they visited an open-cast mine outside the town to study marine sediments.

Subsequent digs revealed long passageways of nut-filled tunnels but there was no sign of an animal inside.

The dimensions of the burrow suggest it had been carved by a large hamster or possibly a ground squirrel.

Secret hoarder

Both animals were known to live in the area at the time, says Dr Carole Gee of the University of Bonn.

"We narrowed the possibilities down to a hamster, because hoarding food is what hamsters do best," she told BBC News Online.

Dr Gee holding a resin cast of the tunnel

"The black-bellied hamster that lives today in Central Europe, for example, can store up to 90 kilograms of grain, peas and potatoes a year.

"This hamster likes to sort its food and stores, each one in a separate chamber. This is what we saw in the Garzweiler quarry."

The nuts were of one kind only, from a species of tree, related to the sweet chestnut, which grew widely in Central Europe in the Miocene Epoch.

The tree is found today only in Asia and on the north-western US coast.

Nuts fall off the tree in late summer or early autumn and the researchers believe they were collected by ancient rodents to eat during the winter.

"As far as we know, this is the oldest food cache in the fossil record," says Dr Gee, in a paper published in the journal Palaeontology.

Kangaroo rat

Two food stores from slightly younger rock deposits have been found in Nebraska.

Part of the skeleton of a kangaroo rat was found in one, and the tooth of a ground squirrel in the other.

Burrows dating back much further, to the Oligocene, dug by unknown vertebrates, have also been found in Nebraska and Wyoming.

These passageways, while older than the German hamster burrow, did not contain any food.

Photographs courtesy of Georg Oleschinski at the University of Bonn.