A mass extinction of life on Earth may have occurred 10 million years before the largest known extinction took place around 250 million years ago, a Japanese scientist said Monday.

Yukio Isozaki, professor of life extinction history at the University of Tokyo, said he made the discovery along with Ayano Ota, a graduate student. They studied the fossils of fusulinidae, a unicellular organism, in a 40-meter-thick layer of limestone in Takachiho, Miyazaki Prefecture.

A study in southern China, which was a shallow sea some 250 million years ago, has led scientists to believe that mass extinction on Earth occurred in two stages. Isozaki said his findings help verify this two-stage hypothesis.

He said he will report the findings at an academic conference on Earth and planet science beginning next Monday in Tokyo.

The limestone found in the town bears traces of coral reefs that piled up near the surface of the sea during a period of some 10 million years prior to Earth’s largest mass extinction. In the largest extinction, 96 percent of all invertebrate life in seas — such as trilobites — disappeared between the Paleozoic and Mesozoic eras some 250 million years ago.

Isozaki said he and Ota paid close attention to fusulinidae, which became extinct after living in coral reefs or shallow seas during the Paleozoic era.

The study found that larger fusulinidae, measuring up to 1 cm in length, became extinct about 260 million years ago, he said.

It also found that smaller fusulinidae, measuring less than 1 mm, survived at that time, he said.

But the smaller fusulinidae became extinct about 250 million years ago, underscoring the two-stage theory of extinction. Scientists say they believe extinction occurred at those times due to a lack of oxygen caused by unusual volcanic activity.

Isozaki said he suspects such volcanic activity occurred twice and that larger fusulinidae were unable to cope with environmental changes brought on by the first wave.

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