European scientists say they've discovered why a "dead zone" following the worst extinction of all time lasted so long -- it was simply too hot to survive.

Researchers have studied what happened after a mass extinction around 250 million years ago in the pre-dinosaur era wiped out nearly all the world's species.

Such mass extinctions are usually followed by a "dead zone" -- during which new species are not seen -- lasting some tens of thousands of years, but the event of 250 millions years ago was followed by a dead zone lasting a puzzling 5 million years, they said.

A study led by the University of Leeds with colleagues in Germany and China shows the cause of this lengthy devastation was a temperature rise to lethal levels in the tropics: around 120 to 140 degrees Fahrenheit on land at 104 degrees at the sea surface.

"Global warming has long been linked to the end-Permian mass extinction, but this study is the first to show extreme temperatures kept life from re-starting in Equatorial latitudes for millions of years," said Yadong Sun, who is completing his doctorate at Leeds.

Water temperatures near the ocean's surface of 104 degrees would be a near-lethal value at which marine life dies and photosynthesis stops, the researchers said.

Until now, climate modelers have assumed sea-surface temperatures cannot exceed 86 degrees.

The dead zone would have been a strange world, the researchers said, with no forests, only shrubs and ferns, no fish or marine reptiles in the tropics, and virtually no land animals because their high metabolic rate made it impossible to deal with the extreme temperatures.

Only the polar regions would have provided a refuge from the baking heat, they said.