The fossil of Rhinodipteris, which lived in a shallow marine environment and gulped air.
Credit: Simon Couper
SYDNEY: A drop in global oxygen levels may have led to air-breathing marine animals 375 million years ago, said scientists, challenging the theory that it evolved in frolicking freshwater fish.
Low oxygen levels in the atmosphere were responsible for the evolution of fish who 'gulp' air from the atmosphere, according to the study published the journal Biology Letters, which analysed a recently discovered fossil.
About 375 million years ago oxygen levels dropped to just 12% of the atmosphere, compared to 20% today, which would also have caused a drop in the oxygen available from seawater.
Plunging oxygen levels forced animals to adapt
"This plunge in global oxygen levels would have been a strong selection pressure on lungfish and other animals, including the tetrapods - the fish-like ancestors of land animals," said John Long, a palaeontologist at the Natural History Museum in Los Angeles and one of the study's authors.
Until now, scientists thought tetrapods - the first four-limbed animals to emerge onto land 385 to 365 millions years ago - and air-gulping fish such as lungfish had evolved to breathe air in freshwater environments.
Oxygen levels are often low in fresh water because of rotting vegetation and a lack of mixing with the air.
New evidence challenges freshwater theory
The recently discovered fossil species of the lungfish Rhinodipteris, analysed in this study, challenges the freshwater theory. It lived 375 million years ago in a shallow marine environment and had adaptations common to today's freshwater lungfish.
Adaptations of Rhinodipteris include a larger mouth cavity and special ribs attached to the skull that held the mouth cavity open wider, which allowed the fish to take bigger gulps of air.
In addition, the world's oldest known tetrapod tracks were recently discovered in 395 million year old fossilised marine tidal deposits in central Poland.
The tracks, and the discovery of air-gulping evolving in a marine animal, suggest that tetrapods could have evolved in a marine environment before moving on to land.
More evidence needed
Study author Alice Clement, a palaeontologist from Australian National University (ANU), said the discovery of a lungfish fossil with these adaptations in a marine environment was unexpected because all previous theory pointed to a freshwater origin for air-breathing fish.
"It was a surprise," she said. "It made us sit back and think why this might have happened."
Clement said the next step is to search for additional fossil species with air breathing adaptations in marine environments to support the study's claims.
Adaptations good for sucking
However, Anne Kemp, a zoologist at the University of Queensland said that the adaptations described are not exclusively useful for breathing air and it is possible that they were used for some other function.
"All of those structures support a strong sucking action of the jaws," she said. "Lungfish suck food into the oral cavity, and they suck water in as well.
"So the new (fossil) fish could be like deep water marine lungfish, and like fresh water species, and use the structures … for several reasons, and air breathing may not have been one of them."
Air-breathing evolved many times
Zoologist Richard Barwick, a visiting fellow at ANU who has made several trips to the Gogo fossil site, said it's difficult to be certain about the purpose of adaptations that occurred nearly 400 million years ago. "It's a valid interpretation in terms of advancing a hypothesis," he said.
"Air breathing and air gulping has evolved independently a number of times in fish groups and this may well be an example of its early development."
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