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.
Oxygen plunge left ancient fish gasping for air
if the oxygen levels were so reduced and fish had to breath air at what point did they start the process and if one fish species did it why didn't all fish species have to do it as the report indicates oxygen fell in the seawater
my experience with today's fish species is they are extremely adversely affected by a fall of available oxygen in seawater
i think the theory may have a couple of holes in it
because genetic variety is,
because genetic variety is, well, variable and useful traits will not necessarily be held, nor be available, to all members of all species, that's how evolution works, not by the naive Lamarkian methods you've mentioned in passing.
For the losers, It's a bit like holding lots of confederate dollars when the North wins the civil war in the USA.
BTW
so shallow marine mudflats instead of shallow fresh water habitat, not really a big deal
LIVE LUNGFISH IN QUEENSLAND
Research is great , but the present is an issue in Australia,
http://www.envlaw.com.au/paradise.html