After the Dinosaurs Died, Earth Experienced the Age of Fish

The fossil record shows how ray-finned fishes took over the planet’s oceans

fish teeth
A collection of fish teeth and shark scales from the Early Cenozoic period. Elizabeth Sibert with Yale University via Scripps Institution of Oceanography

Non-avian dinosaurs weren’t the only victims of the mass extinction that marks the end of the Cretaceous period. Roughly three-quarters of plants and animals on Earth also succumbed to the catastrophic environmental effects of a massive impact by an asteroid or comet (scientists are pretty sure that was the trouble). But one group of animals appears to have triumphed in the aftermath: fish.

Michael Balter reports for Science that the extinction of animals competing for food made room in Earth’s seas for ray-finned fishes, so named for the boney spines or rays that support their fins. Today ray-finned fish make up more than 95% of all fish species, Balter writes.

Specifically, the extinction of ammonites let fish take over the oceans. Named after the Egyptian god Ammon, because many had spiral-shaped shells shaped like the god's rams’ horns, ammonites are the long-lost relatives of modern day squid, octopuses and other cephalopods. They were voracious eaters of tiny plankton and prolific breeders. Paleontologists today see evidence of ammonites’ success in the fossil record — they are some of the most abundant fossils found in rocks dating from about 240 million years ago up until 65 million years ago. Ultimately, the ammonites died out because plankton populations crashed during the extinction event.

Researchers recently compared the ratio of fish remains to those of sharks. While sharks numbers apparently stayed steady after the end of the Cretaceous, "the size and diversity of ray-finned fish populations took off," Balter writes for Science. Thus began the "New Age of Fishes," according to the researcher’s paper published in Proceedings of the National Academy of Sciences. "New" is attached to the name because the first Age of Fish already took place during the Devonian Period, which lasted from 417 to 354 million years ago. During that time the first fish, including sharks, started to appear.

This New Age of Fishes lasted for at least the first 24 million years of the Cenozoic, which followed the Cretaceous. Today, ray-finned fish are still the most diverse vertebrates on land or in the water. So while other animals may loom larger in the public consciousness, fish are still doing well, evolutionarily speaking.

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