About 420 million years ago, near the end of the so-called Silurian period, the last of a series of mass extinctions struck the world’s oceans. Some scientists have suggested these die-offs were caused by worldwide cold spells. But a new study hints that the extinctions—which mostly affected corals, colonymaking creatures called graptolites, and eel-like creatures called conodonts—may have instead been caused by changes in ocean chemistry, including reduced oxygen and elevated concentrations of toxic metals dissolved in the seawater. The evidence, the researchers say, includes 100-micrometer-wide, beaker-shaped fossils called chitinozoans, which haven’t been clearly linked to any particular species but are presumed to be the egg cases of marine creatures at the base of the ocean’s food chain. Analyses show that the proportions of malformed chitinozoans (abnormal fossil at left, typical fossil at right) in seafloor sediments that accumulated just before the die-off 420 million years ago were as many as 100 times higher than normal, the researchers report online today in Nature Communications. Those defects, including riotous growth of tissue, are very similar to those caused in modern-day aquatic creatures living in metal-polluted waters, the team notes. Toxic metals, including iron, copper, arsenic, and lead, were likely pulled from sediments into the water when low oxygen conditions rendered the elements more soluble. The one-two punch of low oxygen and toxic metals may have been a major contributor to this and other die-offs, the researchers propose. They also suggest that malformed fossils of chitinozoans could one day serve as keen markers for ancient changes in ocean chemistry.