Mineralogical and microstructural analyses of Quenstedtoceras shells (Callovian, Poland) show that the outer shell material is preserved as nacreous aragonite, while the septal material shows different degrees of alteration. The septa in part are pyritized or show signs of cementation by equant/acicular calcite cement; paleoenvironmental interpretation of the least-altered outer shell material using the Sr/Na ratio suggests that the shell material was deposited in seawater which had a salinity of about 33 ppt and a range of about 32–35 ppt. Other data, such as Mn and Fe, imply that the oxygen level of the seawater was normal. The paleotemperature, using δ18O values with a salinity correction, was calculated to be about 11.0–14.5°C with an average water temperature of about 12.5°C. These paleoenvironmental parameters, in part, account for the faunal character of the Boreal Realm of the Jurassic.