Supplementary Tables 1–3 contain raw data. Additional data plots are given in Supplementary Figs 4–6. ( a, b) Data from Mongolian samples. ( b) Illustration of the large offset between the calculated water δ 18O for three apparently well-preserved eggshells from Ukhaa Tolgod, and nodular and spar carbonates recovered from the same strata. Two other eggshells from Ukhaa Tolgod do not show this offset and so were considered of poor preservation. Detailed description of the other Mongolian eggshell types and their preservation can be found in the main text and Supplementary Notes 1 and 3, but all were considered of uncertain preservation due to either petrographic analysis or the fact that they were from a different site and so the comparison between them and other carbonate phases was not possible. ( c, d) Data from Auca Mahuevo samples. Δ 47 temperatures are noticeably higher for layer-2 specimens, compared with layer-4 specimens; layer-2 specimens are also higher in contents of elements such as lithium and magnesium which, in addition to petrographic and scanning electron microscopy analysis, lead to us considering layer-4 specimens to be better preserved. ( e, f) Data from Provence Basin samples. Eggshells from the Rousset locality, stratigraphic layer A, are notably distinct in both carbonate δ 18O (not shown) and calculated δ 18O of mineral formation water. Rousset A eggshells are also comparatively enriched in strontium, but lower in lithium compared with eggshells from other stratigraphic layers and eggshells and soil carbonate nodules from the nearby Roques Hautes site. Rousset A are distinct geochemically and eggshells were found to be most altered by EBSD analysis ( Fig. 5; Supplementary Fig. 7).