Most fishes are able to regulate their buoyancy by inflating a gas bladder next to their spine. However, some fishes, like sharks, compensate for the fact that they are slightly more dense than their surrounding fluid by swimming. By constantly moving through the water, sharks stay afloat.
The now extinct Ammodnoidea and armored fishes like the Dunkleosteus almost certainly used the strategy of motion to compensate for their lack of buoyancy in water. They were perpetually swimming to keep afloat, and this may be the reason they went extinct some 300 million years ago.
When their shells grew relatively more massive than the water they were immersed in, they were not able to compensate by swimming faster. Unable to adapt fast enough to their surroundings, they eventually sank to the bottom and died, or they starved to death from exhaustion. Their armor, once a big advantage had turned into a huge disadvantage.
The reason the armor grew relatively more massive than the surrounding water, is that mass condenses onto matter in direct proportion to the mass already accumulated. Heavy atoms accumulate more mass than light atoms.
A hydrogen atom has only one proton to grow. A helium atom has two protons and two neutrons. Helium is almost exactly four times as massive as hydrogen. For this reason, helium grows four times as fast in mass as hydrogen. This maintains the relative difference in mass between hydrogen and helium. Helium remains four times more massive than hydrogen. However, the absolute difference grows.
Likewise, the relative difference in mass between water and calcium remains unchanged when mass condensation occurs. But the absolute difference in terms of weight grows.
If a hundred kilogram fish had ten kilogram of mass that it had to compensate through motion, and its mass suddenly doubled, it would at once be a two hundred kilogram fish with twenty kilogram of mass that it needed to compensate for by swimming. However, its muscles would not have grown any stronger in the process. It would still be the same fish in terms of size and muscle strength. The only difference would be that it was twice as heavy.
A dramatic change in inertial mass due to mass condensation would kill off all sorts of animals relying on buoyancy for their survival. This includes armored fishes and swimming shell fish, and also flying insects.
To fly, insects take advantage of the fact that they are small. They float in the air a little like fishes float in water.
Interestingly enough, it was above all sea animals with heavy armor, and large flying insects that died off in the extinction event that took place 300 million years ago. The increase in inertial mass that happened around that time changed the buoyancy of water and air in a dramatic way. Carbon and calcium increased in mass much more rapidly than air and water.
Fishes adapted by becoming more like water. They dropped their armor in favor of relatively thin scales. They developed better and more efficient gas bladders. Some swimming shell fish became bottom dwellers. Others dropped their shells, and others still developed efficient gas bladders.
However, flying insects could only adapt by becoming smaller. The buoyancy of air relative to carbon and calcium became much too small to overcome in any other way.
Large flying insects like the Meganeura went extinct due to a dramatic fall in buoyancy of air relative to carbon and calcium. An increasing tendency to crash into things as they flew was probably not their main problem as their mass increased. The loss of buoyancy must have been a much bigger problem.
Meganeura, lifesize model (from Land of the dead blog)
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