|Name: Florissant Formation
Age: Eocene/ Oligocene (33-34 Mya)
The Florissant fossils are located in a lacustrine shale with alternating layers of diatomite (the fossil containing layer) and volcanic debris. Microbial preservation may be very important in this lagerstatte. Insect and plant fossils are covered in a film of mucus supposedly produced by diatoms. This mucus formed because diatom abundance was very high at certain times of the year, times of 'bloom', high enough to leave evidence in the fossil record. A mat of this mucus was formed every bloom and dead organisms were caught up in it. The mucus slowed decomposition until burial on the lake bottom, which may be the main reason for the exceptional preservation found here. Both hydrodynamic and biological disruption would have been kept to a minimum. Relatively quick covering by an ash-clay layer probably also helped.
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