Microbial mats play a major role in the formation of exceptionally preserved fossil deposits by overgrowing and binding organic remains and sedimentary particles. This minimizes hydrodynamic and biological disruption of dead organisms and sedimentary laminae, but published works all implicate prokaryotic cyanobacteria as the microbial agent. However, exceptionally well preserved macrofossils of the Oligocene Florissant lake beds (Colorado, United States) are enveloped in matted aggregations of mucous-secreting, pennate diatom frustules. It is suggested that the macrobiota became entrapped in mucous-secreting mats of surface water blooms of planktonic diatoms in lake Florissant. As the mats and the incorporated macrobiota were sedimented out of the water column, the mucosic mats and their associated bacterial communities arrested decay and promoted preservation of refractory tissues. Thus, by a completely different mechanism, the diatom mats fulfilled the same preservational role previously suggested for cyanobacterial mats. This hitherto unrecognized mode of preservation may be an important causative factor in the formation of exceptionally preserved lacustrine fossil biotas.