Actualistic Taphonomy of Cyanobacteria: Implications for the Precambrian Fossil Record

Abstract

Actualistic taphonomy can elucidate aspects of the fossil record that would otherwise be uninterpretable. Investigation of taphonomic alteration patterns in modern cyanobacteria permits evaluation of preservation biases and decomposition timescales in the Precambrian rock record. In this study, a method for assessing alteration of filamentous cyanobacteria was developed, and cyanobacterial decomposition was evaluated during short-term (days to months) experiments. Results indicate that extracellular sheaths are more likely to be preserved than are trichomes, and that rates of decomposition are variable among cyanobacterial taxa. The effects of oxygen, temperature, and desiccation on taphonomic alteration were also investigated. Desiccation of cyanobacteria produces cell shriveling, a feature that may be diagnostic of evaporitic environments. Short-term decomposition of cyanobacteria by heterotrophic bacteria under anaerobic and aerobic conditions did not produce systematically different degradation patterns; thus, alteration patterns are of limited utility for constraining the oxygen content of depositional environments. Rapid rates of decomposition suggest that in order to achieve excellent morphological preservation, heterotrophic activity must have ceased within days to weeks of deposition. These results provide constraints on the timing of lithification for comparable Precambrian lagerstdtten.