The ray-finned fishes, so called because their fins are supported by bony spines or rays, make up more than 95% of all fish species. They come in all shapes and sizes, from the showy lionfish (pictured above) to the delicious Atlantic salmon. Yet paleontologists have been unsure when and why ray-finned fishes exploded into such prominence, in large part because the preservation of fish fossils is a very hit-or-miss affair. Now, researchers have taken a new approach to the problem: They looked at marine sediments taken from deep-sea cores at six sites around the world, including the Atlantic and Pacific oceans. To figure out when ray-finned fish numbers took off, they calculated the ratio of fossilized teeth from ray-finned fishes to the fossilized scales from another major group of fish: sharks. As they report online this week in the Proceedings of the National Academy of Sciences, this ratio shows that sharks well outnumbered the ray-finned fish at the end of the Cretaceous, about 66 million years ago. That was when dinosaurs, ammonites, and most marine reptiles went extinct, probably because of a large asteroid hitting Earth. After the extinction event, the ratio of these ray-finned fish remains shot up dramatically, quickly outnumbering those of sharks. Although the sharks also survived the end of the Cretaceous, their numbers appear to have remained flat, whereas the size and diversity of ray-finned fish populations took off. The researchers suggest that the mass extinction, especially of ammonites (which probably competed with fish for food), allowed the ray-fins to exploit new ecological niches and launched what the authors call a “new age of fish.”