Prehistoric Life 2016 Dinosaur Extinction

Percent extinction per 1-m.y. interval

Mean

FIGURE I 6 Variation in intensity of extinction for the last 600 million years broken up into 1-million-year intervals. [After David Raup.] The Big Five are out on the right tail; Pi refers to the Pleistocene extinction rate, which is far below that of a mass extinction, even though climate and sea level changed drastically.

have not figured out how the ice ages spared so many species, though perhaps the ice advanced slowly enough to allow some to migrate to warmer climes, while others may have been preadapted for cold or survived in ecological refuges. Some think that the arrival of skilled aboriginal hunters on a virginal North American continent has much to do with the Pleistocene mammal extinction, but many disagree, pointing to the millennia of coexistence of humans and large mammals in Africa.

If we accept Raup's conclusion that species living over a wide area can be killed off only by stresses with which they are unfamiliar, and that those stresses must occur too rapidly for migration or adaptation, it follows that rare, sudden, and global catastrophes must also exist—otherwise there is no way to explain the several mass extinctions that mark the geologic record. This is worth repeating: To accept that global mass extinctions have occurred is also to accept that global catastrophes have occurred, a conclusion that is the antithesis of strict uniformitarianism.

The five largest mass extinctions in terms of percent of species killed—the Big Five—are shown in Table 2. Note that the record for intensity is held not by the K-T but by the end-Permian extinction.

Until recently, paleontologists believed that extinction came in two forms: a regular, low-level, background extinction, and the much more destructive mass extinctions. Over the last few years, they have had the benefit of databases meticulously compiled by such paleontologists as John Sepkoski, also of the University of Chicago. He and Raup studied the fossil record of over 17,000 extinct genera of marine animals, and several times that many species. Their database shows that the mean duration of a genus is about 20 million years and that of a species is about 4 million years.

tABLE 2

The Big Five Mass Extinctions tABLE 2

The Big Five Mass Extinctions

Extinction episode

Age (million years before present)

Estimated species extinction %

Cretaceous-Tertiary

Was this article helpful?

0 0

Post a comment