Marine Fossil Record shows Extinctions Happened at Regular Times

Author: Sepkoski & Raup
Date: August 1983
Source: Conference at Northern Arizona University.
Based on the work of thousands of scientists, Seposki has listed 3500 families of sea animals with their time of appearance and disappearance in the fossil record of 250 million years.
Examination of this list shows that periodically--about every 26 million years--there's a decrease in the numbers of families of marine animals in the fossil record. The decrease in diversity represents a mass extinction.

The first mass extinction in Sepkoski's record was at the end of the Permian.

Nine other probable mass extinctions happened at spacings of 22 to 28 million years.

Three of the extinction events were very dramatic. Others were barely detectable.

Sepkoksi said, "I'm stuck for a mechanism for causing such a period. We are aware of no documented process with a cycling time of approximately 26 million years."


PeriodMillion Years b.p.% Extinction
36 16
end-Cretaceous 66 46
82 23
120 16
end-Jurassic 144 20
160 20
190 23
end-Triassic213 44
end-Permian248 60+

Percent of genera of marine animals that became extinct at various times.
(From Raup and Sepkoski, 1984; and Fox, 1987).