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The concentration of oxygen in water tends to be around 6.5-8 mg/L. In the atmosphere, it is 21%, one liter of air weighs roughly 1.25 gram, so, oxygen concentration is 0.25 g/L or 250 mg/L. So, lungs can access 50x higher concentration of oxygen than gills can. This suggests, sort of, that maximum oxygen uptake possible should have a significant increase when lungs evolved. Is that trend observed and a topic that is often talked about?

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The assumption in the question was corroborated by this article here, and is correct. The downvote is unnecessary, it is a valid question, and was easily answered with strong scientific consensus.

As a precondition for the coevolution of eurythermy and life with high energy turnover, oxygen availability had to be maximized and the capacity of oxygen supply mechanisms selected high (Fig. 2). Such preconditions were met with atmospheric oxygen levels always above a minimum of 13%–15% during animal evolution (Fig. 4). Moreover, such preconditions were most ideally met in terrestrial environments since breathing air rather than water implies a reduction by one order of magnitude in the energy cost of ventilation at 30–35 times higher levels of ambient oxygen and about 103 times lower viscosity of the respiratory medium. However, new and largely different ventilatory systems had to be developed in air (lungs instead of gills in vertebrates) in most cases (except for terrestrial crabs) to exploit the excess amount of oxygen. At the same time, thermal conductivity in air is more than 20 times less and specific heat about four times less than in water, reflecting that thermal buffering is much less in the 968 H. O. Po¨rtner terrestrial than in the aquatic environment. As a consequence, temperature changes occur on shorter timescales and at larger amplitudes in terrestrial than in aquatic environments. Therefore, terrestrial organisms at the same latitude are likely to experience much larger temperature fluctuations than aquatic organisms. Opposite to the marine realm, where temperature oscillations are less in polar and tropical than in temperate areas, those in air increase at high, especially Northern, latitudes, as estimated for mainland locations in the Americas (Gaston and Chown 1999). According to these considerations, extreme stenotherms comparable to Antarctic marine species are highly unlikely to exist in (cold) terrestrial environments. Eurytherms will predominate instead.

Pörtner, H. O. (2004). Climate Variability and the Energetic Pathways of Evolution: The Origin of Endothermy in Mammals and Birds. Physiological and Biochemical Zoology, 77(6), 959–981. https://doi.org/10.1086/423742

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A mammal would have to breathe 800 times more volume to throughput the same weight of medium, so, in a given time, the fish puts through more oxygen than the land animal.

Considering salmon and tuna and sea iguanas and turtles, reptiles aren't more active than fish, and air is inefficient to lift against, so sharks and mammals move at around the same speed. Great white sharks and humans have the same hemoglobin concentrations, 14g/l.

We can also compare a great white shark and a killer whale, a tuna and a wolf.

Yes the maximum available oxygen is higher on land, although the limiting factor for energy throughput can be measured through food consumption and metabolism temperature:

Dolphins consume 4-10% of their bodyweight per day Tuna consume 5-15% of their bodyweight per day.

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