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In the vast majority of fossils, the organic matter is degraded with only an impression or cast of the organism remaining. In rare cases, ideal burial conditions result in a rapid fossilisation with an exceptional preservation of soft tissues and occasionally organic matter. Such deposits are known as Lagerstätten and have been found throughout the geological record. Exceptional preservation is often associated with finely crystalline quartz (e.g., cherts), fine sediments (e.g., muds) or volcanic ashes. Other mechanisms include burial in anoxic/euxinic sediments and in the absence of turbidity or scavenging. Exceptional preservation can also occur when an organism is encapsulated in carbonate cement, forming a concretion. This mechanism involves complex microbial processes, resulting in a supersaturation in carbonate, with microbial sulfate reduction and methane cycling the most commonly suggested processes. In addition, conditions of photic zone euxinia are often found to occur during concretion formation in marine environments. Concretions are ideal for the study of ancient and long-extinct organisms, through both imaging techniques and biomolecular approaches. These studies have provided valuable insights into the evolution of organisms and their environments through the Phanerozoic and have contributed to increasing interest in fields including chemotaxonomy, palaeobiology, palaeoecology and palaeophysiology.

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