Nacre, the biomineral lining that gives certain mollusk shells an iridescent gleam, can also indicate the temperature and pressure under which the mother-of-pearl material was formed (J. Am. Chem. Soc., DOI: 10.1021/ja210808s). Because there are many examples of ancient mollusk fossils, researchers could use nacre as a paleothermometer to figure out how hot it was for these creatures in ancient oceans. A University of Wisconsin, Madison, team led by Pupa Gilbert collected polarization-dependent imaging contrast maps of nacre cross sections from eight mollusk species, all of which came from different environments. They found that the nacre’s ultrastructure—the width and the thickness of its aragonite crystals, as well as the disorder between crystals—is specific to each species. They noted a correlation between the maximum temperature of the mollusk’s environment and the degree of crystalline disorder. Also, the thickness of the crystals relates to the depth at which the mollusk lived. “This combination of temperature and distance from the water surface may prove valuable to reconstruct ancient climate on a timescale potentially spanning 450 million years,” the researchers note.