Flexible nacre in the nautiloid Isorthoceras , with remarks on the evolution of cephalopod nacre

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Abstract
Secondarily phosphatized shells of I. sociale (Hall) from the Upper Ordovician Maquoketa shale, Iowa, USA, often have their original ultrastructure preserved. Scanning electron microscope study reveals that the nacreous layer is similar to that in the shells of Recent Nautilus, except that the nacreous tablets usually have a much larger diameter, and that numerous vertical interspaces are left between adjacent stacks of tablets. Because of the latter condition, the nacre in I. sociale is porous; it is compact in Nautilus. The pores in the nacre were probably filled with a non-mineralized organic matrix. The high porosity and high content of organic matrix made the nacreous layer of I. sociale much more flexible and considerably less strong than that in Nautilus. As a consequence, the shells of I. sociale, and probably also of other orthoconic cephalopods, were more flexible and could not withstand such high hydrostatic pressures as the shells of Nautilus. During evolution, the porosity of the nacreous layer became completely reduced. The mechanical properties of the nacre changed towards a lower flexibility and a higher tensile strength.

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