Concentrations of selenium, a vital element for many organisms at the base of today’s ocean food chain, dropped substantially in seawater in advance of three of Earth’s largest die-offs, a new study suggests. Researchers analyzed the levels of various trace elements in hundreds of samples of carbon-rich shales that had been deposited in oxygen-poor regions of the ocean surrounding ancient continents during the past 3.5 billion years. They found that in the intervals preceding die-offs that occurred at the ends of the Ordovician, Devonian, and Triassic periods (about 443 million, 371 million, and 201 million years ago, respectively), only selenium dropped precipitously. In several pre-extinction intervals, selenium levels in seawater dropped to less than 1% of their modern value, the researchers will report in a forthcoming issue of Gondwana Research. Selenium is an essential part of certain enzymes and proteins for a broad range of organisms, from the sunlight-harvesting phytoplankton at the base of the food chain to the vertebrates that ultimately depend on them (such as the marine reptile Lariosaurus, shown, whose group died out with many others about 201 million years ago). So, the researchers argue, a major decrease in availability of the element would have had catastrophic effects on the ocean’s ecosystems—and thus may have caused, or at least played a major role in, the widespread die-offs. An initial slide in selenium concentrations may have been triggered by a decrease in atmospheric oxygen, which slowed erosion of that element and others from rocks on land, the researchers note. Then, the effect may have snowballed, with the levels of both selenium in seawater and atmospheric oxygen crashing. Other evidence from the rocks supports that notion, the researchers note: Before and during those mass die-offs, atmospheric levels of oxygen—a gas produced in prodigious quantities by phytoplankton—also dropped substantially, recovering only long after the mass extinctions had occurred.