Taphonomy of tidal flat moluscs in the Northern Gulf of California: Paleoenvironmental analysis despite the perils of preservation.

Taphocoenoses can be powerful indicators of ancient environments, even in depostional settings in which rates of sedimentation are low and tidal currents are strong. The rich subtropical molluscan fauna of the N Gulf of California exhibits a distinct zonation across an extensive tidal flat at Bahia la Choya (Sonora, Mexico). The distribution of live faunas is largely controlled by substrate, energy level, availability of food, and period of submergence. Despite strong tidal currents and the effects of time-averaging, the distribution of taphocoenoses closely reflects the distribution of the live communities. Gastropods dominate on hard and firm substrates while bivalves prevail on sandy substrates. Herbivores are most common on rocky and shelly substrates; detritus feeders occur on the firm substrates of the inner flat and marsh; suspension-feeders predominate in sandy and rocky bottoms. Shell abrasion, encrustation, and boring decrease landward. We tested an intergrated approach to paleoenvironmental analysis by examining a Pleistocene shell bed from Bahia la Choya. Taxonomic, ecologic, and taphonomic criteria suggest deposition by a storm event in a very shallow subtidal to lowest intertidal environment.-from Authors