Taphonomy of Tidal Flat Molluscs in the Northern Gulf of California: Paleoenvironmental Analysis despite the Perils of Preservation

Abstract

Taphocoenoses can be powerful indicators of ancient environments, even in depositional settings in which rates of sedimentation are low and tidal currents are strong.The rich subtropical mollusc fauna of the northern 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. Indeed, the zonation of individual species is often reflected in the distribution of their shelly remains.The outer flat, middle flat, inner flat, tidal channel, and salt marsh subenvironments within the tidal flat complex can be defined by the taxonomic composition, trophic/life habit groups and taphonomic attributes of their taphocoenoses. Gastropods dominate on hard and firm substrates (rocky flats, channel and marsh) 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 integrated 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.