Preservation of soft-bodied and other organisms by bioimmuration - a review

Bioimmuration is a poorly-known mode of preservation which results from organic overgrowth of sessile organisms. Soft-bodied organisms (and organisms with lightly mineralized skeletons) can be preserved if overgrown by other organisms possessing mineralized skeletons. These bioimmured fossils are visible on the attachment areas of the overgrowing organisms as natural moulds which sometimes become filled by diagenetic mineral growth to give a natural cast. Three types of bioimmuration are distinguished: substratum bioimmuration, epibiont bioimmuration and bioclaustration. Common bioimmuring organisms include oysters and oyster-like cemented bivalves, serpulid worms and cyclostome bryozoans. Among organisms found preserved by bioimmuration are algae, marine angiosperms, hydroids and ctenostome bryozoans. Most documented finds of bioimmured fossils are from the post-Palaeozoic, especially the late Cretaceous, reflecting both the stratigraphical distribution of potential bioimmuring organisms and concentration of research effort. Future finds of bioimmured fossils offer considerable scope for adding to our knowledge of the fossil history and ecological contributions of soft-bodied organisms living on firm or hard substrata.