Mysterious mass extinction The term “shark” inspires predictable images of stealthy and streamlined marine predators that are key components of modern ecosystems. Studying shark teeth buried in deep sea sediment, Sibert and Rubin reveal that current shark diversity is a small remnant of a much larger array of forms that were decimated by a previously unidentified major ocean extinction event (see the Perspective by Pimiento and Pyenson). The extinction led to a reduction in shark diversity by more than 70% and an almost complete loss in total abundance. There is no known climatic and/or environmental driver of this extinction, and its cause remains a mystery. Modern shark forms began to diversify within 2 to 5 million years after the extinction, but they represent only a minor sliver of what sharks once were. Science , aaz3549, this issue p. 1105 ; see also abj2088, p. 1036