Paleontologists estimate that 99% of all species that ever existed have vanished from the planet. To understand the process of extinction, paleontologists have measured the lifetime of species—especially species that leave lots of fossils behind. Mollusks (a group of invertebrates that includes snails and clams) leave some of the most complete fossil records of any animal. Michael Foote, an evolutionary biologist at the University of Chicago, and his col- leagues inventoried fossils of mollusks that lived in the ocean around New Zealand over the past 43 million years. They cataloged every individual fossil from each species, noting where and when it lived. Foote and his colleagues found that.
