Taphonomic processes in modern estuarine death assemblages along the Texas coast : rate of taphonomic loss, covariance of species, and size frequency analysis

Abstract

The formation of the death assemblage from the living benthic community was studied at two estuarine sites along the South Texas coast. Three basic topics were investigated: (1) Rates of taphonomic loss; (2) covariance of species in the death assemblage; and (3) effects of taphonomic processes on size frequency distributions. (1) The post-mortem fate of shells added by natural mortality to modern death assemblages was studied to determine the importance of taphonomic loss during the initial stages of the formation of a fossil assemblage. Very little addition to the death assemblage could be detected in spite of the transitory addition of hundreds of shells. (2) The paleoecologist must be able to distinguish a transported assemblage from an in situ one. A method is proposed to assess whether the post-mortem transport of individuals affected their observed spatial (horizontal) distribution. The degree of covariance between individuals of several species of similar hydrodynamic propensity in the death assemblage is dependent on the amount and intensity of post-mortem movement. The more that species covary, and the larger the size classes covary, the more likely that transportation plays an important role in the species' distribution patterns. Conversely, the absence of covariance suggests that, for at least some species, biological factors determined the species' spatial distributions. (3) Taphonomic processes result in the formation of death assemblages which bear little resemblance to the pre-taphonomic size frequency distribution. Circumstantial evidence suggests that differential preservation has taken place. Visibly distinct multi-modal size frequency distributions have the potential of being a valuable tool for paleontologists in the recognition of unique ecologic events in the fossil record. Biomass appears to be a viable tool in paleontologic reconstruction. Despite fundamental ecologic differences between the two study sites and major differences in size frequency distributions of the species components, biomass estimates in the death assemblage for both sites were quite similar.