Finally, in the 1990s it became clear that the Nautilus, Allonautilus, and other cephalopods as well are very efficient at respiration, even in seawater that has oxygen levels lower than that of normal seawater. New research now shows that the Nautilus commonly visits low-oxygen environments. A nautilus can be removed from water for a half-hour with little ill effect. Squid have recently been observed to enter low-oxygen water masses in the Gulf of Mexico with impunity, places where fish cannot go. The system that allows movement has made these animals champion respirers. These three challenges to the earlier view of Nautilus’s evolution being driven by the need for neutral buoyancy has led me to a new model for Nautilus’s Cambrian evolution.

Cephalopods thus came about following the evolution of a marvelous and efficient gill, one that allows them even today to visit anoxic water masses and still harvest whatever oxygen that can be found there. This was the secret of their success, and later in time, when oxygen levels rose, they became even more efficient. They always had a better respiratory system than their prey and competitors, and the doubling up of the respiratory system with locomotion sealed their success. Eventually most lost their shells, but the respiratory system of the cephalopods remains supreme and will ensure their existence far into the future.

WHY CHORDATES EVOLVED AS THEY DID

Having looked at the respiratory structures of the most populous Cambrian animals, it is time to take a look at a small and insignificant group of fossils found at Chengjiang and later at the Burgess Shale—small, fish-shaped animals that ultimately gave rise to us. The most famous of these was named Pikaia, an animal featured at length in Steven Jay Gould’s book about the Burgess Shale animals called Wonderful Life. The origin of our own phylum is, of course, of intense interest and has been the subject of acrimonious debate for decades, caused in no small way by the dearth of fossils from the time of presumed chordate, or vertebrate, ancestry. Data pertaining to the origins of our phylum come from comparative anatomy and development of living representatives of various phyla seemingly related to us chordates, from DNA studies,



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