Come On In, the Water's Fine

Dick writes about Earth and planetary science for Science magazine.

From a sweltering Jacuzzi to a tepid bath. When the world's oceans experienced such a drop in temperature almost a half-billion years ago, life exploded. Ancient arthropods like the trilobites and their neighbors were soon overwhelmed by new legions of shelled beings in the sea. That's the conclusion of a study appearing tomorrow in Science, which may finally solve the mystery of this unprecedented diversification of animal life.

The Great Ordovician Biodiversification Event--or more modestly, the Ordovician radiation--was an uncomfortably warm, possibly scalding, time. At least that's what some research had indicated. Scientists applied a thermometer to the period by measuring the ratio of heavy to light isotopes of oxygen in 470-million-year-old carbonate shells belonging to stalked animals called brachiopods. The ratio varies depending on the temperature of the water in which the shells formed, and the data indicated unreasonable ocean temperatures of up to 70°C.

Paleontologist Julie A. Trotter of Australian National University in Canberra wondered whether the fossils were the problem. Perhaps their oxygen ratio had changed over time. So she and her colleagues analyzed the more durable fossil mineral phosphate in the form of the toothlike remains of a now-extinct eel-like animal. Across the 44-million-year-long Ordovician period, they found ocean temperatures steadily declining from a steamy 42°C (5°C higher than human body temperature) to a plateau of about 30°C, which is typical of the modern sea surface on the equator.

Not coincidentally, the group contends, the attainment of a modern balminess in the Ordovician coincided with the heart of the animal explosion. Corals, for example, could survive only under the cooler temperatures, Trotter says.

The new temperatures seem reasonable, says geochemist John Eiler of the California Institute of Technology in Pasadena. "If you had to bet, you would say these guys have [the temperature] right." And "if this cooling really did happen,” says paleontologist Arnold Miller of the University of Cincinnati, “it had to have some effect on biodiversity." Miller suspects, however, that the radiation had more than one cause, and this one needs a lot more testing.

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