A frequent and often difficult problem in paleobiology is assigning probable causes to morphological change in fossil lineages. Though such causes can never be determined with total confidence, a multi-pronged approach combining several independent lines of evidence is most likely to approximate actual processes. Turritellid gastropods from the Pleistocene of southern California form an ideal system for such investigation, which includes four components: morphometric analysis, stable isotope analysis, survey of modern processes, and an analysis of the cost of calcification. The first three of these components have provided evidence that the change in shell morphology in this lineage may be related to changing paleoproductivity conditions. While each component strengthens the argument for this view, only the fourth component can determine whether the difference in paleoproductivity (and by implication, food supply) observed over time is sufficient to produce the changes in shell growth that account for much of the morphological change. Here we summarize the evidence gathered from the first three parts of our approach for an association between productivity and shell morphology, and then present a model for determining the level of primary productivity required to allow different rates of shell growth. This model includes quantification of the intrinsic and extrinsic factors affecting shell mass and rate of formation. The results of such analysis provide support for the idea that morphological change in this lineage has been strongly influenced by productivity change. This model may also be useful for similar purposes in other lineages with well-preserved calcium carbonate remains. In addition, knowledge about the degree to which primary productivity affects marine gastropods is an invaluable predictive tool for both past and future environmental change in the marine realm.