EXPERIMENTAL SILICIFICATION OF BIVALVES: UNDERSTANDING TAPHONOMIC BIAS

Silicification, the early replacement of skeletal material with silica, results in fossils that can be extracted and imaged in three dimensions, with consequent advantages for systematic description. Differential silicification, related to the composition of organisms and the availability of silica through time, potentially creates a taphonomic bias. Silicification of skeletal material has been a subject of recent consideration with the demonstration that it has a significant effect on the fossil record of some groups, particularly bivalve molluscs and brachiopods, and that it occurs far more frequently in Paleozoic than in post-Paleozoic sediments (thus introducing a potential megabias). But the processes and controls on silicification are poorly understood.

Experiments allow us to understand the process from a different perspective. We can address the relative importance of shell microstructure, mineralogy, silica availability, and Ca-saturation state. Bivalves of the genus Mytilus were placed in solutions of sodium metasilicate and kept at a constant pH of 1, 2, and 4 in replicate sets. All specimens were gathered live, stripped of flesh, and frozen prior to use to ensure the preservation of intra- and intercrystalline organic material. The periostracum was either conserved or removed with bleach.

Silicification appears to be controlled by the availability of organics in the shell and occurs preferentially in high-organic microstructure. Under SEM, specimens in solution for 60 days showed silica (confirmed by EDS) either adsorbed to, or replacing the organic sheaths surrounding the crystallites. This occurred in both the fibrous calcitic and nacreous shell layers. The initial stages of our experiments resulted in weight loss of 5-10% in shells and generated an open, honeycomb-like structure. This initial texture has not yet been identified in silicified fossils. It is possible that silicification of shells includes several generations of silica precipitation – very early silicification of organic matrix concurrent with dissolution of crystallites, followed by permineralization of the crystallite cavity. Both of these stages may be necessary for preservation to occur and silicification may be biased by the presence, location, and abundance of organic material.