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 get bigger, swim faster and evolved into the powerful predators we know today.
In a paper published last month in the journal Current Biology, the scientists reported that they measured the fin sizes and body lengths of 500 extinct and living sharks and compared that information with data from the sharks’ evolutionary family tree. Their results show that when the ocean became very hot approximately 122 million years ago during the Cretaceous period, some sharks left their seafloor habitat and moved up into the open ocean. This ascent may have altered their fin and body structure, leading to changes in their size and ability to swim.
It’s a misconception that all sharks are like the bloodthirsty, powerful and simplistic beasts from “Jaws” that swim near the surface of the ocean (or in tornadoes and city streets, if you’ve seen “Sharknado”). Most sharks have always been benthic, meaning they feed on the bottom. Unlike their pelagic or open water relatives, demersal sharks do not need to constantly swim to breathe. They can rest on the sea floor.
However, the need to breathe may have simply been the prompt that moved some sharks higher in the water column. The authors argue that Cretaceous ocean floors may have become increasingly oxygen-poor in places. In order for the ancestors of many modern sharks to survive and eventually thrive, it was time to dig up the sea floor.
Clues to this habitat shift and what endured in which environment are seen in the changing pectoral fins of ancient pelagic and benthic sharks.
“Most sharks in open water tend to have elongated fins, and benthic sharks have stiffer fins,” said Lars Schmitz, a biology professor at Claremont McKenna College in California who authored the paper.
Fellow author Philip Stearns, a California-based shark researcher, compared the pectoral fins to airplane wings. “Long, narrow wings” — like those on a commercial jet, for example — “help your lift-to-drag ratio, so they lower your fuel costs,” he said. In contrast, “the short, stubby wings of fighter jets aren’t good for long-range travel, but they can cost you a pretty penny.”
The same is true for sharks: longer pectoral fins may have made swimming more efficient for larger-bodied sharks, an important adaptation for species whose breathing now requires constant swimming.
But it’s not just body size and fins that may have increased. Peak ocean surface temperatures in the Cretaceous period at about 83 degrees Fahrenheit may have affected the sharks’ speed. (For comparison, today’s average is 68 degrees.)
Sharks and other fish are similar to most animals, explained Timothy Higham, co-author and professor at the University of California, Riverside, “in that muscle function is very temperature dependent.” In other words, he said, “if your muscles warm up, they get better at fast contraction.”
Warmer temperatures and faster, faster muscles mean sharks “can beat their tail back and forth faster,” he said. This translates into increased speed, which, he added, may have caused the sharks to “expand into more open water habitats,” catching fast-swimming prey and avoiding other Cretaceous marine predators that are now extinct.
Which all sounds profitable. As ocean temperatures rise now 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 introduced important evolutionary adaptations in some sharks, but Dr Higham stressed that today’s rapidly changing climate is more likely to harm ocean life.
“Because other animals other than shark organisms were completely devastated,” he said. He added that while some sharks adapted to the oceans of the Cretaceous period, “it also caused the extinction of many other animals.”
Allison Bronson, a faculty member at California State Polytechnic University, Humboldt, who was not involved in the research, agreed.
“The spread of marine anoxic zones and changes in global climate, often accompanying ocean acidification, have led to the worst mass extinctions in Earth’s history,” she said, adding that “the pace of change is now truly unprecedented.”
