By Stephen Beech via SWNS
Sharks survived a major ocean temperature spike more than 90 million years ago – by growing longer fins, according to new research.
The ocean apex predators we know today evolved from stubby bottom dwellers during a “dramatic” episode of global warming, say scientists.
A massive outpouring of volcanic lava about 93 million years ago sent carbon dioxide levels soaring, creating a “greenhouse” climate that pushed ocean temperatures to their hottest.
Now, American scientists have found that some sharks responded to the heat with elongated pectoral fins.
The discovery was made by the research team taking body length and fin measurements from more than 500 living and fossilized shark species.
Study first author Phillip Sternes said: “The pectoral fins are a critical structure, comparable to our arms.
“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.”
Mr Sternes, a biology doctoral student at the University of California, Riverside, says longer pectoral fins help make shark movements much more efficient.
He said: “Their fins are comparable to the wings of commercial airplanes, long and narrow, to minimize the amount of energy needed for movement.”
The findings, published in the journal Current Biology, also show that the open-water sharks got faster compared to bottom-dwelling sharks.
Study co-author Professor Tim Higham, also of the University of California, Riverside, said: “Shark muscle is very sensitive to temperature.
“The data helped us make a correlation between higher temperatures, tail movement, and swimming speeds.”
Most living shark species are still bottom dwellers, occupying what scientists refer to as the “benthic zone”.
The researchers said that benthic sharks do not loom as large in popular culture as their fierce open-water relatives.
Many of the bottom dwelling sharks are slender, flatter, more medium-sized predators.
Only about 13 percent of modern sharks are fast-swimming open-water predators.
The research team believe that breathing may have become difficult for their ancient relatives.
They explained that oxygen levels near the bottom during the Cretaceous period likely dropped as the heat increased.
Modern sea surface temperatures average about 68 degrees Fahrenheit.
But during the Cretaceous they were much warmer, reaching an average of about 83F.
The high heat of the Cretaceous did not happen overnight, explained the researchers, and neither did the evolution of sharks.
Co-author Professor Lars Schmitz, of Claremont McKenna College, said: “We had pretty warm open-sea surface temperatures throughout the era, and then a distinct spike that took place over a one- or two-million-year period.”
He says that as global warming drove an evolution in some species, including sharks, it caused the extinction of others.
Because those evolutionary changes happened on a longer time scale in the past, the researchers say 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 further north.
However, it is unclear whether threatened sharks will again be able to adapt where they live and survive the rapidly increasing heat.
Sternes added: “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.”
