Detailed study of gastropods recovered from the Lake Theo Site, Briscoe County, Texas, has revealed a progressive decline in species diversity from early Holocene to the present. Knowledge of the microhabitats preferred by the recovered species allows a partial reconstruction of environments required to produce such communities. Since the early Holocene, the Lake Theo area has experienced a progressive loss of deciduous riparian gallery woodland habitat. Alteration in plant communities was caused by a dramatic decrease in effective precipitation. Analysis of the sequence of local extirpation of the various snail species indicates that the decrease in precipitation and increase in temperature probably were not concurrent.