It is widely believed that there has been about a dozen mass extinctions during the history of life on Earth, the most devastating of which saw 84% of the planet's species disappear.

But research by geologist Professor Andy Gale of the University of Greenwich and palaeontologists from the Natural History Museum, published recently in the American journal Paleobiology, is now casting doubt upon whether these mass extinctions took place.

"Large gaps in the fossil record are often cited as evidence of mass extinctions. But there are other explanations for this lack of fossil evidence which do not point to a catastrophic annihilation of large numbers of species, says Professor Gale.

"During the Cretaceous period (146 to 65 million years ago), dominated by dinosaurs, there were periods of intense global warming which saw dramatic rises in sea levels so severe that the oceans flooded Europe, turning it into an archipelago of little islands. This forced shallow marine species and land animals to migrate from their usual habitats.

"Once the sea level dropped again these species migrated back with it, and the fossil record laid down in sedimentary rock during those periods of high sea level was largely destroyed over time by wind, rain and glacial erosion," continued Professor Gale.

"The interruption in the fossil record during these periods was caused by species migration and the loss of the fossil record of that migration, and not by a mass extinction."

Evidence of these "pseudoextinctions" can be seen in the fossil record of the chalk cliffs at Dover in Kent and Beachy Head on the Sussex coast. Many of the shallow water organisms which disappeared from the fossil record in these cliffs, 100 to 95 milllion years ago, reappeared millions of years later when sea levels fell.

"It initially appears from the fossil record contained within these cliffs that marine species during this period suffered extinction rates of up to 70%," says Professor Gale.

"But the reality is that the reappearance of the majority of these species indicates that they were simply displaced to other areas by rising sea levels, and then returned, without suffering significant extinction. Taking this into account, the extinction rate during this period falls to 17%, a level that inevitably occurs at any time.

"Based on the fossil evidence we have analysed, we believe that there was no mass extinction 100 million years ago, as previously thought," concludes Professor Gale.

"It is also probable that many of the other mass extinctions that were thought to have taken place during the history of life on Earth either did not happen or have been greatly exaggerated."

Peer reveiwed publication and references: Paleobiology, 27 (2), 2001, pp 241-253

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