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> The mass Devonian extinction in the oceans may have driven partially air-breathing animals onto land by using estuaries:
> 
> https://phys.org/news/2018-06-stable-isotopes-earliest-tetrapods-euryhaline.html
> 
> "A team of researchers from several institutions in France and China has found evidence that some of the earliest creatures to walk on land likely emerged from estuaries or deltas. In their paper published in the journal Nature, the group describes studying certain stable isotopes in fossil specimens to determine the salinity in which they lived. 
> 
> "Back in 1929, a team of researchers discovered the fossilized remains of Ichthyostega, a tetrapod that was believed to be among the first creatures to walk on land. Since that time, similar types of remains have been found in places like Greenland and China. Study has shown the creatures were able to live both on land and in water—they had four legs, tails for swimming and gills. But until now, scientists reported difficulty in figuring out if the water they came from was fresh or salty (suggesting an ocean existence). In this new effort, the researchers tested 51 ancient fossilized tetrapod bones as a new way to find the answer to this question.
> 
> "The team studied sulfur and oxygen isotopes. Seawater has more sulfur-34 compared to sulfur-32 than freshwater. Since both wind up in the bones of creatures that live in water, the researchers studied the ratios in the fossilized bones. They found that the ratios fell closer to seawater. But in studying oxygen isotopes, they found that the creatures were also exposed to freshwater. The evidence suggests that the tetrapods lived part of the time in seawater and part of the time in freshwater. Such places today include estuaries and river deltas. To further bolster their theory, they tested modern creatures that live in such places and found a near match.
> 
> "Adding to the story, the fossilized remains have been dated back to approximately 365 million years ago, which was towards the end of the Devonian Period—just prior to the mass extinction of ocean dwelling creatures. The ability to live in both fresh water and sea water, the researchers note, would have given the tetrapods a leg up, so to speak—they would have been able to survive in both types of water and sometimes on land. "
> 
> 
> Comment: why did seagoing animals have the ability to breath air in advance of the mass extinction? Good ,luck or God?