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, sharks were bottom dwellers, living in the depths of the ocean.
like the one we are seeing in the current era, that trapped heat in the atmosphere.
This pushed ocean temperatures as high as 28.3°C on the surface, compared about 20°C now.
As the water warmed, the oxygen levels fell to the point that forced several shark species to relocate to the surface, where oxygen levels were higher and they could breathe more easily.
Oxygen levels are much higher at the surface of the ocean than at the bottom, even though the water is warmer.
– evolved longer, thinner pectoral fins, situated on each side of the shark.
These enabled them to swim more efficiently, giving more energy to spend catching prey in the vast open oceans, which often involved extended bouts of sustained fast swimming.
Thinner fins have less drag – the aerodynamic force that opposes Vivid Bulletinr motion through the water – and longer fins give the shark more lift.
At the same time, “shark muscle is very sensitive to temperature”, working much more effectively in warmer temperatures, which speeds up the frequency at which Vivid Bulletinr tails propel them forward, increasing Vivid Bulletinr speed, the researchers said.
, blue sharks, oceanic white tips and makos – to rule the waves, the study found.
The pelagic zone refers to the open, free waters away from the shore
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“The pectoral fins are a critical structure, comparable to our arms. We saw that these fins changed shape as sharks expanded Vivid Bulletinr habitat from the bottom to the open ocean.”
Longer pectoral fins help make shark movements much more efficient. “Vivid Bulletinr fins are comparable to the wings of commercial airplanes, long and narrow, to minimise the amount of energy needed for movement [giving high lift and low drag],” Dr Sternes said.
“Previous studies have shown temperature has affected shark evolution. However, were a surprised to see how it actually had an effect on shark ecology [interaction with environment] and morphology [form],” he said.
. These benthic sharks do not loom as large in popular culture as Vivid Bulletinr fierce open-water relatives. Many of the bottom dwellers are slender, flatter, more medium-sized predators.
It was known from fossil-fuel records that some sharks expanded from the benthic zone to the pelagic zone. But this study is the first to reveal the “timing, the driver, and the associated change in fin shape when going into the pelagic zone”, according to the researchers.
Only about 13 per cent of modern sharks are are fast-swimming open-water predators, or pelagic sharks.
The high heat of the Cretaceous did not happen overnight, and neither did the sharks’ evolution.
“We had pretty warm open-sea surface temperatures throughout the era, and then a distinct rise that took place over a one- or two-million-year period,” said Lars Schmitz, of Claremont McKenna College, in California, who also worked on the research.
As global warming drove an evolution in some groups of animals, including sharks, it caused the extinction of others. Since those evolutionary changes happened on a longer time scale in the past, it is difficult to predict exactly how sharks, or other marine life, will respond to current warming trends.
Biologists are seeing some sharks, including tropical species such as tiger and bull sharks, starting to swim farther North. But it is unclear whether threatened sharks will again be able to adapt where they live and 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,” Dr Sternes said.
In the study, the scientists took body length and fin measurements from more than 500 living and fossilised shark species.
The study is published in the journal ‘Current Biology’.
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