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
discussed in many future chapters) was first evolved as a respiratory organ in response to the low-oxygen content of the Cambrian oceans—and still functions like this today in what is the last remaining externally shelled genus.
All of this wonderful majesty of design came about because a late Cambrian mollusk discovered a new way of beating the problem of low oxygen: it evolved a new kind of molluscan respiratory pump, so that muscular action, instead of cilia, created a high-pressure volume of water that passed over the gills. After being mined for its oxygen and laden with carbon dioxide from respiration, all that water had to be expelled from the animal with as much force as it was brought in, if the same water was not to be recirculated. What better way than to jet it out through a tube! What a surprise it must have been when each jet of water jerked the entire animal backward. When predators came, an even more vigorous jet of water would jerk the earliest cephalopod (or was it still a monoplacophoran?) out of harm’s way.
Natural selection then honed the system. But the shell was a heavy burden to bear, and thus a solution for neutral buoyancy was selected. When all was combined, in the late Cambrian, a new kind of animal was set loose. This was a hugely important episode in the history of life, as the cephalopods remained the dominant carnivores in the sea from that point until 65 million years ago when the ammonites, descendants of the cephalopods, were killed off by the Chicxulub asteroid strike, a time interval of about 450 million years. And as we now know, extinction of the ammonites was not the end of the cephalopods, for in large areas of the ocean, such as the pelagic or midwater regions, cephalopods remain the dominant carnivores, outcompeting fish in these dim regions of the sea. As we will see, cephalopods will be obvious and common players in the local marine ecosystems.
Hypothesis 3.3: The molluscan class Cephalopoda, today comprised of squids, octopi, and the two externally shelled genera,Nautilus and Allonautilus (which still maintain the originalcephalopods’ body plan), produced its original distinctiveshelled body plan as a way to build a highly efficient pump gill.