<p>Geochemical information from well-preserved carbonate fossils is frequently used as a proxy for paleotemperature and past ocean chemistry. For this study, we analyzed 71 <i>Gryphaea</i> (Ostreoida, Bivalvia) individuals from the upper Sinemurian (Oxynotum Chronozone, upper part of Simpsoni and Oxynotum subchronozones) black shales, organic-rich marls, and limestones cropping out at S. Pedro de Moel (Lusitanian Basin, Portugal).</p><p>Of the 71 initial specimens, 62 passed common geochemical screening for alteration. From the analysis of the 62 specimens, we observe a good agreement between the obtained geochemical data and previous studies. Intra-bed geochemical variability aligns with research in modern oysters, indicating that the variations observed in element and stable isotope ratios among specimens within the same bed fall within expected natural variability among coeval individuals. <i>Gryphaea</i> δ¹³C values from the Lusitanian Basin are comparable with oyster data from the UK basins. Overall, there is a relatively good agreement in δ¹⁸O values from single-sourced carbonate, i.e., brachiopods and bivalves, although the former appear to show more negative values. The largest discrepancy in δ¹⁸O is related to bulk-rock carbonate analysis, likely due to low-temperature burial diagenesis (including meteoric effects) and its effect on bulk carbonate δ¹⁸O. Mean Sr content (519 ± 11&#xa0;mg/kg, 2 SEMean, <i>n</i> = 62) is, within uncertainty, the same as the previously reported 482 ± 68&#xa0;mg/kg (2 SEMean, <i>n</i> = 7) for the Oxynotum Chronozone. The calculated average Sr/Ca_seawater of 6.06 ± 0.13 mmol/mol (2 SEMean, <i>n</i> = 62) for the Oxynotum time interval, based on data from the Lusitanian Basin, overlaps with previous studies. This further confirms the robustness of Sr/Ca from oyster carbonate in reconstructing Sr/Ca_seawater.</p>