Great extinction came in phases

The rocks at Meishan stretch over the Permo-Triassic (PT) boundary

The greatest mass extinction recorded in Earth history did not occur as a result of one single cataclysmic event.

A joint UK-Chinese team tell Nature magazine the disaster that befell the planet 250 million years ago must have happened in phases.

Their conclusion is based on the abundance of "organic fossils" found in rocks at Meishan in southern China.

These suggest there were at least two episodes to the mass die-off that saw up to 95% of lifeforms disappear.

"And this fits with a growing body of literature that now points to a complex sequence of changes on Earth," Richard Pancost, from the University of Bristol, UK, told the BBC News website.

Complex route to death

Some scientists have proposed the idea that the "great dying" at the boundary of the Permian and Triassic Periods could have occurred quite abruptly - the result of environmental changes brought on by the impact of a giant space rock.

It is a similar argument to the one put forward to explain the demise of the dinosaurs at the much later date of 65 million years ago.

Trilobites were one of the groups wiped out in the extinction

A geological structure, known as the Bedout High, in the seabed off what is now Australia, has even been suggested as the possible crater remains from the impactor.

But it is an argument that has struggled to find favour.

The prevailing theory is that several factors - including supervolcanism and extensive climate warming - combined over thousands of years to strangle the planet's biodiversity.

Earth may well have been hit by extraterrestrial objects, but it is unlikely there was some killer punch from space.

Ring of existence

The new data from China supports this view. It is based on the traces left in rocks by cyanobacteria.

These photosynthetic, mostly single-celled organisms existed in vast blooms in the Permian oceans. They are one of the major groups of phytoplankton, which form the basis of the marine food chain.

PERMO-TRIASSIC EXTINCTION The greatest of all Earth's mass extinctions occurred about 250 million years ago About 95% of marine species and three-quarters of all families on the Pangean (above) landmass perished End Permian rocks can be seen today in places such as China, Italy and Pakistan Chief suspects include sea-level fluctuations, volcanic activity, space impacts and melting methane-ice in sea sediments

However, the phytoplankton not eaten by higher organisms would have fallen to the seafloor over time to be incorporated into the sedimentary rocks we see today. And chemical components in their cell membranes have left telltale signs of their past existence.

Specifically, a lipid molecule, known as 2-methylhopane, has left ring structures in the Meishan rock.

"These ring structures are the 'hydrocarbon skeleton' - that is how we would refer to them - and they can be preserved for a very long time," explained Dr Pancost.

The research team sees two peaks of abundance in the Chinese rocks which are believed to indicate periods immediately following biotic crises in the oceans - times when the collapse of higher marine lifeforms allowed the cyanobacteria populations to boom.

"What we think happened was that the grazing pressure changed," explained Dr Pancost. "A lot of the fauna that went extinct went through larval stages that would have fed on the phytoplankton.

"Changes in the faunal assemblages would have changed predation patterns, and this led to the phytoplankton prospering."

Land of turmoil

The Permian-Triassic mass extinction killed off about 95% of all marine species and about three-quarters of all land families.

It is the boundary at which the famous water-dwelling arthropods known as the trilobites were wiped out.

The Permian saw the creation of the Pangean supercontinent, and the geological evidence suggests this landmass experienced huge volcanic turmoil.

The Siberian Traps were built during the period - millions of cubic kilometres of basalt lavas were spilled on to the Earth's surface.

"This is the first time that we know what's happening to the very base of the food chain at this time. We had no handle on plankton populations before this because they don't fossilise," commented Dr Paul Wignall of the University of Leeds, who studies the Permian-Triassic extinction.

"It's very interesting, because to affect the base of the food chain takes a lot of doing. It shows a world in crisis."