FOSSILIZATION OF TERTIARY INSECTS AND PLANTS BY POLYSACCHARIDE FILM

The excellent preservation of fossil insects and plants in the Florissant Formation (Eocene, Florissant Fossil Beds National Monument, Colorado) and the Savage Canyon Formation (Miocene, Stewart Valley, Nevada) is in part due to a slime-like coating on the organisms, which prevented decomposition. The fossils occur in thinly laminated shales deposited in fresh water Tertiary lakes. The laminae are a millimeter (or less) thick and consist of diatomaceous layers alternating with smectite-clay layers. Fossils are mainly concentrated in the millimeter (or less) thick diatom layers. Preservation of details of fossil morphology, such as the fine hairs on insect legs, indicates that conditions existed in the lakes to inhibit decomposition before and during burial. SEM observations reveal a coating on the fossils of a mat of material interpreted to be extracellular polysaccharide secretions (i.e. "slime"). Energy dispersive X-ray analysis (EDX) confirms the organic nature of the mats. Because diatoms are known to secrete slime, particularly when stressed, it is suggested that preservation of the fossils occurred as follows: Periodically (seasonally?), siliceous volanic ash weathering to smectite clay washed into the Tertiary lakes as detritus derived from nearby eruptive volcanic centers. This resulted in high concentrations of silica in the lake water, promoting diatom growth and culminating in a diatom "bloom." During this time, slimy mats formed from the diatom-stressed community. Plants and insects accumulating on the lake surface became entrapped on these mats, which settled to the lake bottom. Bacterial decomposition of the organisms during sedimentation and at the sediment-water interface at the lake bottom was inhibited by the presence of the slimy coating. Upon burial, subsequent decomposition was also minimized allowing the detailed morphology of the organisms to be preserved. The key factor in this fossilization mechanism was the presence of diatom-derived slime, which prevented bacterial decay and promoted fossil preservation. This hypothesis is supported by SEM comparison of the fossil diatoms with modern diatoms cultivated in a controlled laboratory environment.