Long-term reconstructions of the seasonality of the West African Monsoon are limited by the lack of seasonally resolved paleoclimate archives in this region. We present an application based on the growth patterns and monthly resolved δ 18O profiles of Anadara senilis bivalve shells to quantitatively estimate the duration of the rainy and dry seasons in the hypersaline Saloum Delta, Senegal. Fortnightly growth patterns are used to estimate changes of the growth rate and to place the shell isotope record into a temporal context. Based on fortnightly growth patterns and sharply declining δ 18O values, the duration of the rainy season was estimated. As an illustrative example, we quantitatively estimated the duration of two rainy seasons and two dry seasons that occurred about 1500years ago from a fossil shell of the Dioron Boumak archaeological site to the nearest month. This precision can be statistically improved by the analysis of a larger shell number or by higher resolution isotope sampling. Our results open the possibility of using large fossil shell accumulations in the Saloum Delta for quantitative studies of the long-term variability of the West African Monsoon seasonality. © 2012 Elsevier B.V.