Fig. 2.

Modern and fossil nacre from 3 molluscan classes, exhibiting irregularly shaped nanoparticles. Arrows indicate fractured tablets. (A) Cryofractured modern nacre from the gastropod Haliotis rufescens. (B) Nacre as in A, but bleached and etched to better reveal its nanoparticulate texture. (C) Nacre from the modern cephalopod Nautilus pompilius, cryofractured, bleached, etched. (D) Nacre from the Miocene (∼13 Ma) bivalve Atrina harrisii nacre, which is still 100% aragonite (28). (E) Nacre from a Cretaceous (∼100 Ma) ammonite Desmoceras sp. (F) Nacre from the Upper Ordovician (∼450 Ma) cephalopod Isorthoceras sociale nacre, which is secondarily phosphatized. Lower-magnification, cocentered images from the same locations as these are presented in SI Appendix, Fig. S1, where they are correspondingly labeled. Insets show photographs of similar samples.