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Scott K. Johnson - Jun 30, 2015 2:59 pm UTC

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.

The researchers primarily looked at the relative abundance of the teeth and scales. Scale counts were pretty consistent, as were teeth counts before the extinction event, but immediately afterward teeth from ray-finned fish became more and more common.

Less than a third of sharks and rays went extinct at the end of the Cretaceous, and there’s no indication from the number of scales in these cores that the total population dropped off across the event. But ray-finned fishes appear to have grown much more populous after the die-off. Some ecological niches would certainly have opened up for them as the impact of losing more than 90 percent of plankton species with calcium carbonate shells rippled across the food web. Ammonites, for example, were significant players before they disappeared.

There are also hints of change in the sizes of the ray-finned fish teeth. Right after the extinction, larger teeth particularly increased in number, suggesting that the fish community was transforming a bit. And at least in the core from the South Pacific, there was a big spike in the number of the largest teeth that only lasted for a few million years right after the mass extinction—possibly the result of some evolutionary experimentation that didn’t stick long-term.

The researchers also note that the changes across the end of the Cretaceous are abrupt, lining up with the impact of the Chicxulub asteroid rather than shifting more gradually during the tremendous volcanic eruptions that began previously. That’s about what we see on land, too, but it’s still interesting given recent research pinning down the timing of all these events more accurately.

Overall, this reads a lot like the story of our mammalian clan. The researchers write that the end-Cretaceous mass extinction “appears to have been a major driver in the rise of ray-finned fishes and the reason that they are dominant in the open oceans today.”

PNAS, 2015. DOI: 10.1073/pnas.1504985112  (About DOIs).

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