It sounds like something out of a Hollywood movie script, but it really happened: Shark evolution researchers say increased ocean temperatures more than 100 million years ago may have caused sharks to grow larger, swim faster and become the powerful predators we know today.
In a paper published last month in the journal Current Biology, scientists reported that they measured the fin size and body length of 500 extinct and living sharks and compared that information with data from the shark evolutionary family tree. Their results show that when the ocean got too hot about 122 million years ago during the Cretaceous period, some sharks abandoned their seafloor habitat and moved up into the open ocean. This ascent may have changed their plumage and body structure, which led to changes in their size and ability to swim.
It’s a misconception that all sharks are like the bloodthirsty, powerful, simple beasts of “Jaws” that swim close to the surface of the ocean (or in tornadoes and city streets, if you’ve seen “Sharknado”). Most sharks have always been benthic, meaning they are bottom feeders. Unlike their pelagic or open water relatives, benthic sharks do not need to constantly swim to breathe. They can rest at the bottom of the sea.
The need to breathe, however, may have been just the impetus that moved some sharks higher in the water column. The Cretaceous ocean floor, the authors suggest, may have become increasingly oxygen-poor in some places. For the ancestors of many modern sharks to survive and eventually thrive, it was time to remove the sea floor.
Clues to this habitat change, and what endured in which environment, are seen in the changing pectoral fins of ancient pelagic and benthic sharks.
“Most open-water sharks tend to have elongated fins, and benthic sharks have sharper fins,” said Lars Schmitz, a professor of biology at Claremont McKenna College in California, who authored the paper.
His co-author Phillip Sternes, a California-based shark researcher, compared pectoral fins to wings on an airplane. “Long narrow wings” — like those of a commercial airplane, for example — “help your lift-to-drag ratio, so it lowers the cost of fuel,” he said. In contrast, “the short, sharp wings of fighter jets are not good for long-distance travel, but they can cost a dime.”
The same goes for sharks: Longer pectoral fins may have made swimming more efficient for larger-bodied sharks, an important adaptation for species whose breathing now requires continuous swimming.
But it is not only the size of the body and legs that may have increased. Cretaceous ocean surface temperatures peaking at about 83 degrees Fahrenheit may have affected the shark’s speed. (For comparison, today’s average is 68 degrees.)
Sharks and other fish are similar to most animals, explained Timothy Higham, a co-author and professor at the University of California, Riverside, “in that muscle function is highly dependent on temperature.” In other words, he said, “if your muscles warm up, they get better at contracting fast.”
Warmer temperatures and faster, faster muscles meant the sharks “could beat their tail back and forth faster,” he said. This translates into increased speed, which, he added, may have then caused the sharks to “expand into a more open-water habitat,” catching fast-swimming prey and avoiding other Cretaceous marine predators. which are now gone.
Which all sounds useful. With ocean temperatures now rising due to global warming, could we see similar changes in today’s sharks? In other words, can sharks get even bigger and faster?
Global warming millions of years ago may have brought about important evolutionary adaptations in some sharks, but Dr. Higham pointed out that today’s rapidly changing climate is more likely to result in damage to ocean life.
“Because the other animals, the non-shark organisms, were absolutely destroyed,” he said. He added that while some sharks adapted to the Cretaceous oceans, “it caused many other animals to become extinct.”
Allison Bronson, a faculty member at California Polytechnic State University, Humboldt, who was not involved in the research, agreed.
“The spread of anoxic marine zones and changes in global climate, often accompanied by ocean acidification, have resulted in the worst mass extinctions in Earth’s history,” she said, adding that “the pace of change is now truly unprecedented”.