Some of the mass extinctions in Earth's long history resulted from catastrophic events such as asteroid impacts or massive volcanic eruptions. But two researchers who have studied the fossil remains of coral-like marine creatures during two of the biggest of these die-offs argue that prolonged environmental stress might have wiped out many species more quietly. The findings could help scientists predict the effects of climate change on today's marine life.
The demise of the dinosaurs and other species 65 million years ago is well known, but it was preceded by two even bigger mass extinctions. The Great Dying, as scientists call it, ended the Permian period 250 million years ago by killing off 90% of marine creatures and 70% of the land dwellers. The Triassic extinction about 200 million years ago exterminated 20% of Earth's marine life and half the terrestrial species--while allowing dinosaurs eventually to flourish. In both cases, there's no evidence for an asteroid impact or volcanic conflagration, so the causes of the events remain unresolved.
It's possible those two great extinctions actually occurred gradually over millions of years and were caused by steady deterioration of the global environment. At least, that's what the research--the most comprehensive of its kind--by paleontologists Catherine Powers and David Bottjer of the University of Southern California in Los Angeles suggests. After combing through 396 collections of fossil bryozoans--bottom-dwelling animals that resemble corals and live in colonies--the researchers report in this month's issue of Geology that the number of species declined steadily and slowly. Powers says she thinks their fossil record should be "a good proxy for studying the environmental effects of these mass extinctions."
Powers says that chemical analyses done elsewhere suggest the bryozoans' decline accompanied a gradual increase in the carbon dioxide content of the oceans, possibly caused by undersea volcanic eruptions. Regardless of the source, the increase could represent a bellwether for adverse environmental changes, such as increasing acidification of seawater or declining oxygen levels. Powers says the next step in the research will be to pinpoint when the bryozoans started declining, to see if the start of the trends can be isolated. That would give scientists better ideas about when to look for specific triggering events, such as plate tectonics or volcanic eruptions, in the geologic record. So far, she says, the declining bryozoan diversity can only be analyzed in blocks representing millions of years.
The paper shows how long-term environmental stress could have led to the two mass extinctions, says paleoecologist Margaret Fraiser of the University of Wisconsin, Milwaukee. The data suggest that "large-scale environmental and ecological changes have occurred in Earth's history and were facilitated by increased CO2 levels," she says, which gives researchers "much to learn about the present and future from the geologic record."