THE ROLE OF BIOFILM IN FOSSIL PRESERVATION IN THE LATE EOCENE FLORISSANT FORMATION, COLORADO

Scanning electron microscopy (SEM) of the extraordinarily well-preserved fossil insects and plants in the lacustrine shale of the late Eocene Florissant Formation reveals the role of biofilms in the process of fossilization. Seen with SEM, the fossils appear to be coated with a biofilm (i.e., mucilaginous extracellular polysaccaride secretions, or “slime”). This biofilm originated during frequent intervals of diatom growth (blooms) in the late Eocene lacustrine environment. Fossils are found entombed in the diatomite layers of the shale unit, which contains numerous 0.1 to 1.0 mm thick couplets of alternating laminae of diatomite and a fine-grained volcanic ash weathering to smectite clay. The morphology of the fossil diatom biofilm is similar to that produced by modern freshwater diatoms (e.g. Nitzschia, Navicula, and Synedra) that were grown in test tubes under conditions of nutrient stressing suspensions with sodium silicate solution. A diatom mat formed during blooming in the tubes. It is suggested that during periodic intervals the water of Lake Florissant also became enriched in dissolved silicon due to heavy runoff into Lake Florissant from the nearby volcanic source area. This chemical change triggered diatom blooming and produced a biofilm consisting of a mucus mat which sank to the lake bottom with embedded remains of insects and leaves. There is also evidence of minor decomposition of the biofilm mat by bacteria. It is suggested that bacteria initially began to break down the biofilm (a polysaccaride) in preference to the chitin of the insects and cellulose of the plants. The rapid sedimentation and subsequent burial of the mucus-coated organisms by the successive lamina of smectite is suggested as a mechanism promoting minimal chemical or bacterial decomposition and thus promoting the exquisite preservation of fossils.