When the ocean got hot, the sharks bulked up

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.”

Most of today’s living shark species are still bottom dwellers. They live in what scientists refer to as the benthic zone close to the ocean floor. Benthic sharks like leopard sharks and reef sharks aren’t quite the pop culture villains as their open-water relatives. They are typically more slender, flatter, and more medium-sized predators. Only about 13 percent of sharks are fast-swimming open-water predators like great whites.

Currently, sea surface temperatures are an average of 68 degrees Fahrenheit. In the Cretaceous period, they reached an average of roughly 83 degrees. The team believes that breathing may have become difficult for bottom dwelling ancient shark relatives, since oxygen levels probably dropped as the heat increased. Importantly, this increase in temperatures didn’t happen all at once and neither did the sharks’ evolution.

“We had pretty warm open-sea surface temperatures throughout the era, and then a distinct spike that took place over a one- or two-million-year period,” study co-author and Claremont McKenna College paleobiologist Lars Schmitz said in a statement.

This ancient global warming spurred evolution in sharks and some groups of animals, but extinction in others. Since those changes occurred over such a longer time scale, it is hard to predict exactly how sharks or other marine life will respond to our planet’s current warming trends. Some sharks–including species found in the tropics like tiger sharks and bull sharks–are swimming farther north. However, it is unclear whether threatened sharks will adapt again to survive the rapidly increasing heat.

“The temperature is going up so fast now, there is nothing in the geologic record I am aware of that we can use for a true comparison,” Sternes said.