When the ocean got hot, the sharks bulked up

By Laura Baisas | Popular Science |

POPULAR SCIENCE - A giant spike in ocean temperatures about 93 million years ago may have helped sharks grow from stubby bottom dwellers into bigger predators. This surge in ocean heat in the Cretaceous period came from a gush of volcanic lava that sent carbon dioxide levels skyrocketing. This created a greenhouse effect that raised ocean temperatures. In response, some shark species evolved elongated pectoral fins that helped make them faster and move to a different part of the water column in order to eat. The findings are described in a study published last month in the journal Current Biology.

Earlier research published in 2018 found that a Cretaceous shark called Cretoxyrhina mantelli (pictured above) was large enough to eat a pterosaur. The fossil record not only contains evidence of how sharks interacted with other animals in their environment, but also what their teeth were like and how large they were. The team on the study published in 2024 took body length and fin measurements from over 500 fossilized and shark species to estimate how their pectoral fins have changed over time.

“The pectoral fins are a critical structure, comparable to our arms,” study co-author and University of California, Riverside (UCR) biology PhD student Phillip Sternes said in a statement. “What we saw upon review of a massive data set, was that these fins changed shape as sharks expanded their habitat from the bottom to the open ocean.”

The longer pectoral fins help make sharks movements more efficient. According to Sternes, their fins are long and narrow like the wings of an airplane to reduce the amount of energy needed for movement. The team also saw that the open-water sharks appear to have gotten faster, compared to the bottom dwellers.

“Shark muscle is very sensitive to temperature,” study co-author and UCR biologist Tim Higham said in a statement. “The data helped us make a correlation between higher temperatures, tail movement, and swimming speeds.”

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