Scientists have uncovered new evidence which supports the theory that a sudden ice age killed off the dinosaurs.

The team from Holland and Italy discovered fossilised cold water plankton dating from 65 million years ago in an area which had previously been a warm sea.

The find, in modern day Tunisia, ties in with the hypothesis that an asteroid struck the Earth at around this time, causing a dramatic plunge in temperature which lasted for thousands of years.

The ice age forced many species into extinction, including the dinosaurs. The ancestors of today's mammals survived to later emerge and fill the empty niches left by their former rivals and predators.

Evidence of a giant asteroid crater dating back to around the right time has been found in Mexico's Yucatan Peninsula.

But the discovery of Dinoflagellates and Benthic Formanifera plankton is the first time fossilised evidence of the climate swing has been found.

Matthew Huber of Indiana's Purdue University, who worked on the study said: "The fossils indicate that something suddenly made the water cold enough to support these tiny critters."

"We theorise that the meteor strike produced huge quantities of sulfate particles, such as are often blown high into the atmosphere during a volcanic eruption, and these particles shielded the Earth's surface from sunlight. The decrease in solar energy ultimately caused a long cold spell, called an 'impact winter,' that persisted for years."