For mammals, the mass extinction event at the end of the Cretaceous was the crisis that opened the door to evolutionary success. With so many species gone, like the dominant dinosaurs (minus the ancestors of birds), opportunities were plentiful. Our small, furtive ancestors made the most of those opportunities, giving rise to the diversity of mammals around today.
Perhaps the ray-finned fishes—which include almost every fish you can think of apart from sharks and rays and make up almost half of all modern vertebrate species—found similar opportunities. Researchers knew that this group of fish only took off in the last 100 million years (so since the mid-Cretaceous), but the early details were fuzzy. Scripps Institution of Oceanography’s Elizabeth Sibert and Richard Norris set out to tighten up that history by picking through seafloor mud for tiny fish teeth.
Those seafloor muds came from deep drilling in multiple locations in the Atlantic and Pacific Oceans. Samples of Italian limestone that had been laid down in an ocean long since closed up by plate tectonics rounded out the collection. In all the samples, which spanned from the late Cretaceous (about 75 million years ago) to the mid-Eocene (about 45 million years ago) the researchers sifted out teeth shed by ray-finned fishes and scales belonging to sharks or rays. Both types of fossils are plentiful, as they resist dissolving away on the ocean floor.