Nature: The discovery of intact plankton fossils in California, New Jersey, and Tanzania may yield clues to the history of ocean acidification. The coccolithophores found in 56-million-year-old sediments are unusual because most single-celled organisms do not leave intact fossils. Using scanning electron microscopes, paleoceanographers Paul Bown and Samantha Gibbs plan to compare the fossils with samples of the modern species. Differences in size, shape, thickness, and growth rates will help determine levels of ocean acidification and plankton’s ability to adapt to changes. Of key interest is a period of rapid ocean warming that occurred 55 million years ago called the Paleocene–Eocene Thermal Maximum, which is considered similar to the warming currently occurring.