Exceptionally preserved fossils provide major insights into the evolutionary

history of life. Microbial activity is thought to play a pivotal role in both the

decay of organisms and the preservation of soft tissue in the fossil record,

though this has been the subject of very little experimental investigation.

To remedy this, we undertook an experimental study of the decay of the

brine shrimp Artemia, examining the roles of autolysis, microbial activity,

oxygen diffusion and reducing conditions. Our findings indicate that

endogenous gut bacteria are the main factor controlling decay. Following

gut wall rupture, but prior to cuticle failure, gut-derived microbes spread

into the body cavity, consuming tissues and forming biofilms capable of

mediating authigenic mineralization, that pseudomorph tissues and structures

such as limbs and the haemocoel. These observations explain patterns

observed in exceptionally preserved fossil arthropods. For example, guts

are preserved relatively frequently, while preservation of other internal anatomy

is rare. They also suggest that gut-derived microbes play a key role in the

preservation of internal anatomy and that differential preservation between

exceptional deposits might be because of factors that control autolysis and

microbial activity. The findings also suggest that the evolution of a through

gut and its bacterial microflora increased the potential for exceptional fossil

preservation in bilaterians, providing one explanation for the extreme rarity

of internal preservation in those animals that lack a through gut.