Summary
Each year, the discovery of new shark species underscores how little we know about ocean biodiversity (1). This is alarming not only because human pressures threaten sharks more than other marine lineages (2) but also because their fossil record suggests that they were largely resilient to extinction in the past (3), with some extant species persisting for tens of million years (4). On page 1105 of this issue, Sibert and Rubin (5) report an unexpected finding: a wholesale extinction of shark lineages in the pelagic ocean, the largest ecosystem on Earth, about 19 million years ago. Their discovery suggests that some extinctions in the open sea of the past may have been cryptic. More puzzling is that this event in the early Miocene seems to have been hiding in an interval of geologic time that was previously unremarkable. How did they find it, and what does it mean?
