land vertebrates evolved from air-breathing fish

http://phys.org/news/2014-01-evidence-animals-evolved-ability-air.html

so it appears that ability to breath air evolved before the ability to move on land and in a rather peculiar way because air initially entered through 'spiracles' rather than nostrils and then nostrils evolved later while the spiracles evolved into ear channels!

Originally posted by humy
http://phys.org/news/2014-01-evidence-animals-evolved-ability-air.html

so it appears that ability to breath air evolved before the ability to move on land and in a rather peculiar way because air initially entered through 'spiracles' rather than nostrils and then nostrils evolved later while the spiracles evolved into ear channels!

Nobel prize discovery! If they had one for evolutionary biology that is. So this living fish has to be close to the surface and breathes through snorkels! I wonder what evolutionary advantage it gave the originators of that ability. Obviously later, it led to land animals but what good was that development for the first versions of that organ? Avoiding deep water predators? Maybe being able to live in much shallower waters not habitable for ordinary fish?

Thinking about that, if they lived in the shallows, they would perhaps have also had to develop some kind of melanin protection from the sun, if they lived all their lives within a few feet of the surface.

Just thinking about the possibilities is mind boggling. The conversion of the spiracles into ear channels for instance!

Originally posted by sonhouse
Nobel prize discovery! If they had one for evolutionary biology that is. So this living fish has to be close to the surface and breathes through snorkels! I wonder what evolutionary advantage it gave the originators of that ability. Obviously later, it led to land animals but what good was that development for the first versions of that organ? Avoiding deep ...[text shortened]... possibilities is mind boggling. The conversion of the spiracles into ear channels for instance!

what evolutionary advantage it gave the originators of that ability

I kept wondering about that. It could have been a way to deal with living in areas of ocean that had unusually low oxygen content or even no oxygen because perhaps, just like in many eras in post natural history but unlike the modern day, there may have been vast areas of ocean with severe oxygen deficiency. But I don't know how that hypothesis fits with the evidence of what was going on in the oceans at the time when they were evolving.

Originally posted by humy

what evolutionary advantage it gave the originators of that ability

I kept wondering about that. It could have been a way to deal with living in areas of ocean that had unusually low oxygen content or even no oxygen because perhaps, just like in many eras in post natural history but unlike the modern day, there may have been vast areas of ...[text shortened]... s fits with the evidence of what was going on in the oceans at the time when they were evolving.

Water contains less oxygen than air does. So the advantage is that obtaining oxygen is easier as long as the organism is always close to the surface.

Originally posted by KazetNagorra
Water contains less oxygen than air does. So the advantage is that obtaining oxygen is easier as long as the organism is always close to the surface.

My current reasoning on this, whether right or wrong, goes like this:
The rate at which a unit of O2 defuses through a unit surface area of lung/gill lining into the blood stream from external water/air is determined in part by whether the external media is water or air. Air may always have a higher concentration of O2 but, providing some water is fully saturated with O2, even though that water would have a lower proportion of O2, the rate at which a unit of O2 defuses from water through a unit area of gill lining into the blood stream should be about the same as the rate at which a unit of O2 defuses from air through a unit area of lung lining into the blood stream. This is just because of the way the physics of diffusion of gasses works.
So, this is why I assume that, providing the water is fully saturated with O2, there shouldn't be much advantageous difference whether the organism breaths air or water if the organism always stays close to the surface of the water.
Of course, water can be very far from fully saturated with O2, hence my hypothesis.