Mulling over mastodon mass extinctions.

Mulling over mastodon mass extinctions

At the end of the Pleistocene epoch, about 10,000 years ago, mastodons, mammoths and many other large mammals that roamed North America suddenly died off. Two theories have been proposed to explain these Pleistocene extinctions. One holds that extreme seasonal shifts in temperature were responsible; the other is that humans, by hunting the animals, had a hand in their demise.

While the cause of the extinctions is still a matter of debate, some scientists have been cooking up promising methods for testing these ideas. These tools, as well as a new, third extinction theory, were discussed last week at the meeting of the Geological Society of America (GSA) in San Antonio, Tex.

At a GSA meeting three years ago, Daniel C. Fisher and Paul L. Koch of the University of Michigan in Ann Arbor concluded that humans had been hunting as well as scavenging mastodons. They based this theory on the finding that all the scavenged animals had died during one season, the fall, whereas the non-butchered mastodons had died at the end of the winter (SN: 11/12/83, p.312).

Specifically, they based their conclusions on the thickness of growth bands on the animals' tusks, assuming that thin groups of bands corresponded to winter growth and thick groups to summer. The problem with this approach, says Koch, is that the thickness of the bands might be controlled by the animals' reproductive cycles. So the researchers searched for a method that would more reliably reflect the environmental temperature and not the animals' biology.

Since mastodons could keep their own temperatures relatively constant, the researchers reasoned that the changes in oxygen isotope ratios measured in the tusks must reflect changes in season -- oxygen-18 is more prevalent in North American rainwater in the summer and oxygen-16 in the winter. They found that the tusk ratio of oxygen-16 and oxygen-18, which the animals ingested when they drank water, corresponded to the thickness of the growth bands, confirming, they conclude, the seasonality of the deaths.

This "is the first time anyone's been able to measure [prehistoric] seasonality in a continental region," says Koch.

Fisher cautions that while these findings are consistent with the hunting hypothesis, they certainly do not prove it. The next step, adds Koch, is to use the same oxygen isotope method on tusks to see if the seasons really did become more extreme, as the climate theories suppose.

The researchers are also working on another way to test both hypotheses. Fisher notes that before the mastodons went extinct, their body size became smaller. Each theory could account for this, but in different ways: In a more severe climate, the animals would become sexually mature later in life, and so tend to grow more slowly, whereas hunted mastodons would mature at an earlier age so that they could reproduce more often, and hence would stop their growth sooner.

In studying the patterns of growth bands on mastodon tusks, Fisher has devised a way to gauge the age at which an animal became sexually active. By comparing the life cycles of the last living mastodons with those of their ancestors, he hopes to nail down the cause of the extinction.

In the meantime, Robert G. Brakenridge at Wright State University in Dayton, Ohio, has thrown another theory into the pot. He proposes that the Vela supernova (SN: 6/20/81, p. 391), which appeared at about the time of the extinctions, emitted a burst of gamma and X radiation that destroyed 35 to 80 percent of the earth's ozone layer. This loss of ozone, he says, could have allowed harmful ultraviolet light from the sun to penetrate the atmosphere, killing off the plants upon which the mastodons, mammoths and other large mammals grazed.

The supernova also would have increased the production of carbon-14 in the atmosphere, according to Brakenridge. He hopes his idea will prompt other scientists to look for traces of this increased carbon-14 in the geologic record.

Because there is so much uncertainty in all the calculations that go into this model, comments Fisher, it may be hard to evaluate. "But I'm perfectly happy to put it on the stove and let it cook with everything else," he says.