A compilation of over 2000 measurements of d18O and d13O on Phanerozoic low-Mg calcite shells, such as brachiopods, belemnites and oysters, delineates secular 18O/16O and 13C/12C variations that are similar to those previously described for whole rocks. The trend for the d18O suggests about ~5 ± 2 enrichment from Cambrian to today. In contrast, the d13C rise during the Paleozoic is followed by its decline in the Mesozoic and Cenozoic. Optical (textural) and chemical criteria suggest that the interior "secondary" layer of the brachiopod shells, the material that carried these signals, is well preserved in many samples and the extracted secular isotopic trends are therefore a primary feature of the geologic record. The similarity of
the d18O / d13O isotope patterns in ancient and modern brachiopods also supports such an interpretation. In our view, the 18O enrichment in progressively younger samples is principally, although not exclusively, a reflection of the evolving 18O/16O composition of sea water. If so, a delineation of this trend may ultimately result in development of a valuable paleoclimatic and paleoceanographic tracer for the Phanerozoic.