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*Represents the number of distinct extractions of CO2 from a sample that is then purified and analysed. In each case just one eggshell specimen was analysed. s.d. is reported if only two analyses were made.
†Values given on the ‘absolute reference frame’ or ARF20.
§Calculated using a compilation of published biogenic calibration (Equation (3), Supplementary Note 2) data collected in the Caltech laboratories and synthesized by Eagle et al.26. See Supplementary Note 2 for more information on how this calibration was derived.
¶δ18O of body waters calculated using published mineral-specific equations and using the Δ47 (Biogenic) temperature. All eggshells are calcite, except for turtle and tortoise eggshells that are aragonite.