An AMNH scientist digs into the fossil record to discover why ammonites, a highly successful group of mollusks, perished after an asteroid strike 65 million years ago, while their cousins the nautilids became unlikely survivors. The answer—ammonites’ vulnerability to ocean acidification—highlights the unpredictable nature of mass extinction events. When an environment changes dramatically, winners and losers are hard to foresee. As today’s oceans become warmer and more acidified, some of the most abundant and diverse organisms may be unable to adapt. Others may be surprisingly resilient.