How sharks survived a major temperature spike in Earth’s history

Sharks that live in different parts of the ocean, and their respective pectoral fins. Credit: Phillip Sternes/UCR.

Millions of years ago, the sharks we know today as top ocean predators evolved from small bottom-dwelling species during a period of intense global warming.

Around 93 million years ago, massive volcanic eruptions released large amounts of carbon dioxide into the atmosphere, creating a greenhouse effect that significantly raised ocean temperatures.

Researchers at UC Riverside discovered that some sharks adapted to these changes by developing longer pectoral fins.

This discovery, published in the journal Current Biology, was made by measuring the body length and fin sizes of over 500 living and fossilized shark species.

Phillip Sternes, a UCR biology doctoral student and the paper’s first author, explained, “The pectoral fins are crucial, similar to our arms.

We found that these fins changed shape as sharks moved from living on the ocean floor to swimming in open water.”

Longer pectoral fins help sharks move more efficiently, much like how long and narrow wings help airplanes fly with less energy.

Sternes noted, “Their fins are like airplane wings, designed to minimize the energy needed for movement.”

The research also showed that open-water sharks became faster swimmers compared to their bottom-dwelling counterparts.

Tim Higham, a professor in UCR’s Department of Evolution, Ecology, and Organismal Biology and co-author of the paper, said, “Shark muscle is very sensitive to temperature. We found a connection between higher temperatures, tail movement, and swimming speeds.”

Today, most shark species still live near the ocean floor, in what scientists call the benthic zone. These benthic sharks are not as well-known as their open-water relatives, which are often depicted as fierce predators. Benthic sharks are generally smaller and flatter, with about 87% of modern sharks living in this zone. Only about 13% are fast-swimming open-water predators.

The researchers believe that breathing may have become difficult for ancient sharks as oxygen levels near the ocean floor dropped during the Cretaceous period’s heat spike. Modern sea surface temperatures average around 68 degrees Fahrenheit, but during the Cretaceous, they were much warmer, averaging about 83 degrees. This warming did not happen overnight; it took place over one to two million years.

As global warming drove some animals, including sharks, to evolve, it caused the extinction of others.

Because these changes happened over a long period in the past, it is challenging to predict how sharks or other marine life will respond to the current rapid warming trends.

Biologists are observing some tropical sharks, like tiger and bull sharks, swimming farther north.

However, it remains unclear if threatened shark species will be able to adapt quickly enough to survive the rapidly increasing temperatures.

Sternes commented, “The temperature is rising so fast now, there is nothing in the geologic record that we can use for a true comparison.”