Morphological variation in turritellid gastropods from the Pleistocene to Recent of Chile; association with upwelling intensity

Environmental change, such as variation in upwelling intensity and consequent variation in marine primary productivity, may have profound effects on organisms. In the fossil record, morphological variation within lineages may be explained by environmental changes, although it is often difficult to establish a causal relationship between morphological and environmental change. Size and shape change were quantified in suspension-feeding turritellid gastropods from the Pleistocene and Recent of Chile, wherein both temporal and geographic patterns of variation were found. Upwelling and productivity conditions were reconstructed from a variety of published sources, including information on diatom, foraminiferan, and coccolithophore diversity and abundance, organic carbon burial, percent carbonate, and phosphorite deposition. In addition, carbon and oxygen stable isotope ratios from turritellid shell material were analyzed in order to determine temperature and productivity conditions. Comparison of results from the morphological analysis to upwelling/productivity reconstructions indicates that wider whorl shape and larger shell size are associated with more intense upwelling. Increases in the amount of shell material secreted may be explained by increased availability of food associated with stronger upwelling. This interpretation is consistent with observations of increased shell growth recorded for modern, non-turritellid mollusks. This type of analysis may help to establish causation for certain kinds of morphological evolution observed in the fossil record of turritellids and other suspension-feeding gastropods.