Concretions are an important source of soft-bodied fossils. In order to determine the controls on exceptional fossilization within concretions, we scored 88 concretion-bearing sites for 11 environmental and compositional variables together with presence/absence of soft-tissue preservation and analyzed their relationships using multiple correspondence analysis (MCA) and qualitative logistic regression. Sites yielding exceptional fossils were distributed randomly through the 88 examples considered, suggesting that exceptional preservation can occur in almost any environment where concretions form. The patterns of interaction of the variables in the MCA revealed that the most important factors controlling exceptional preservation in concretions are related to (1) the potential for fossil preservation in the broader depositional setting (i.e., how likely fossils are to be preserved without concretions but in the same depositional environment) and (2) the rate of concretion growth. In an analysis of the contribution of individual variables, logistic regression showed that two features correlate with the presence of soft-tissue preservation in concretions: (1) fine-grained host lithology, and (2) relatively constant δ13C values. The role of the first of these can be tested experimentally. Decay experiments on fish tissue in glass beads of three different sizes and therefore of different permeability showed that decay is inhibited, and mineral precipitation enhanced, in low-permeability sediments. Thus a process of positive feedback promotes exceptional preservation where early cementation results in a rapid decrease in permeability during concretion formation. The second feature, a relatively constant δ13C trend, suggests that certain patterns of concretion growth, pervasive growth or concentric growth with one growth layer, are more conducive to fossilization than others.

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