Ward, Peter. "3 Evolving Respiratory Systems as a Cause of the Cambrian Explosion." Out of Thin Air: Dinosaurs, Birds, and Earth's Ancient Atmosphere. Washington, DC: The National Academies Press, 2006.
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Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere
its heavy shell. It was long thought that, under pressure, the animal secreted the gas found in the chambers but this is not the case. Chamber pressure is never more than one atmosphere, regardless of the depth of the nautilus (and these animals have been caught at a depth of 600 meters, where the ambient pressure pushing against the shell is 60 atmospheres). This gas enters the shell by simple diffusion in response to the near vacuum conditions caused by liquid emptying (nature does indeed abhor a vacuum), but the gas plays no functional role. If liquid removal can balance the density increases caused by shell formation at the aperture and by growth of the soft parts (also denser than surrounding seawater), neutral buoyancy can be maintained.
So this is the major adaptation first seen in the Cambrian: A mollusk must have somehow built a calcareous wall at the back of its shell but left a strand of tissue within, which then had to evolve into an epithelial pump. So (the story goes) some poor late Cambrian monoplacophorans find themselves getting lighter and lighter and at some point float off the surface of the ocean bottom, off for an uncontrolled ride. Eventually, evolutionary forces shape the soft parts into tentacles and more importantly, shape a propulsion system created by the evolution of a jet of water forced through a tube-like funnel. Since the entire shell and animal is of neutral buoyancy, even a feeble squirt through the funnel would cause the animal to move off the bottom, presumably out of harm’s way.
At any rate this is how the story has been understood. But in recent years many have become increasingly skeptical of this hypothesis, and eventually three lines of evidence have changed our views of the reasonableness of this scenario. First was the realization that the Nautilus does not use its buoyancy system for propulsion. For years, paleontologists thought that nautiluses, and by extension the many fossil ammonoids and Nautilus species, undertook nightly vertical migrations from deep to shallow water and did this by changes in buoyancy. Here a balloon analogy colored our view of things. A hot air balloon rises when new, heated air is vented into the balloon or if ballast is thrown out. In either case the density of the balloon is lessened, and the balloon rises in the sky. The balloon is brought back to Earth by venting the hot air. But much research showed that a powered dirigible is a