Modern sharks, those apex predators of the open ocean, might have actually evolved from bottom-dwellers during a significant episode of global warming millions of years ago. A new University of California, Riverside (UCR) study published in Current Biology has unveiled the adaptive fin morphology of sharks in response to historical climate changes.

Approximately 93 million years ago, massive volcanic eruptions released enormous amounts of carbon dioxide, creating a greenhouse climate that dramatically raised ocean temperatures. This environmental upheaval caused some shark species to adapt to the rising temperatures by elongating their pectoral fins. This evolutionary progression changed them from stubby bottom dwellers to proficient open-water swimmers.

The study involved precise body length measurements and fin dimensions from over 500 living and fossilized shark species.

“The pectoral fins are a critical structure, comparable to our arms,” said Phillip Sternes, a UCR biology doctoral student and the paper’s first author. “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.”

Evolution of shark fins and swimming efficiency

The shark species most affected by the warming temperatures during the Cretaceous period were those that transitioned from benthic (bottom-dwelling) or benthopelagic (near the bottom) habitats to the pelagic (open ocean) zone. Two primary orders experienced significant evolutionary changes due to the elevated sea surface temperatures: Lamniformes and Carcharhiniformes.

Lamniformes, which include modern species such as the great white shark and mako shark, were among the first to expand into the pelagic zone during the Early Cretaceous. Carcharhiniformes, which include tiger sharks, bull sharks, and blue sharks, followed in the Late Cretaceous.

Other shark species that showed adaptations to the pelagic zone at later stages include the cookiecutter sharks, which transitioned during the early Paleogene period; the whale shark, which, despite being a relatively slow swimmer, adapted to open water more recently; and the spined pygmy shark.

Sharks’ elongated pectoral fins help them move more efficiently, similar to the wings of commercial airplanes. These fins are long and narrow to minimize the energy needed for movement. This adaptation improved their swimming efficiency, resulting in increased swimming speeds.

The researchers also found the open-water sharks got faster compared to their bottom-dwelling cousins.

“Shark muscle is very sensitive to temperature,” said Tim Higham, a professor in UCR’s Department of Evolution, Ecology, and Organismal Biology, and a paper co-author. “The data helped us make a correlation between higher temperatures, tail movement, and swimming speeds.”

Despite the evolutionary success of pelagic sharks, most living shark species remain bottom dwellers, occupying the benthic zone. Unlike their open-water relatives, these benthic sharks are generally slender, flatter, and medium-sized predators. Currently, only about 13% of modern sharks are fast-swimming open-water predators.

According to oceanographers’ research, oxygen levels near the ocean floor dropped during the Cretaceous period, making it difficult for ancient bottom-dwelling sharks to breathe. This environmental stress and high sea surface temperatures most likely fueled the evolutionary shift.

“Modern sea surface temperatures average about 68 degrees Fahrenheit (20 degrees Celsius). In the Cretaceous they were much warmer, reaching an average of about 83 degrees (28 degrees Celsius). The high heat of the Cretaceous did not happen overnight, and neither did the sharks’ evolution,” Sternes said.

The study underlines that millions of years have passed between sharks’ evolutionary changes. Currently, biologists observe that some tropical shark species, like tiger and bull sharks, are migrating farther north due to warming trends. However, this go-around, the rapid pace of contemporary climate change raises concerns about the ability of threatened sharks to adapt.

This has Sternes concerned.

“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.”