Nacre, or mother-of-pearl, the tough, iridescent biomineral lining the inner side of some mollusk shells, has alternating biogenic aragonite (calcium carbonate, CaCO3) tablet layers and organic sheets. Nacre has been common in the shells of mollusks since the Ordovician (4.50 million years ago) and is abundant and well-preserved in the fossil record, e.g., in ammonites. Therefore, if any measurable physical aspect of the nacre structure was correlated with environmental temperatures, one could obtain a structural paleothermometer of ancient climates. Using X-ray absorption near-edge structure (XANES) spectroscopy, Photoelectron emission spectromicroscopy (PEEM), and X-ray linear dichroism we acquired polarization-dependent imaging contrast (PIC) maps of pristine nacre in cross-section. The new PIC-map data reveal that the nacre ultrastructure (nacre tablet width, thickness, and angle spread) is species specific in at least eight mollusk species from completely different environments: Nautilus pompilius, Haliotis iris, Haliotis rufescens, Bathymodiolus azoricus, Atrina rigida, Lasmigona complanata, Pinctada margaritifera, and Mytilus californianus. Nacre species-specificity is interpreted as a result of adaptation to diverging environments. We found strong correlation between nacre crystal misorientations and environmental temperature, further supported by secondary ion mass spectrometry measurements of in situ delta O-18 in the nacre of one shell. This has far-reaching implications: nacre texture may be used as a paleothermometer of ancient climate, spanning 450 million years of Earth's history.