EXCEPTIONAL PRESERVATION: PHYSICAL, CHEMICAL, AND BIOLOGICAL FACTORS

Exceptional preservation offers unusual insight into the structure and taxonomic composition of paleocommunities. Special attention has been focused on Cambrian deposits because of the seemingly large number of deposits of exceptional preservation, especially Burgess Shale-type deposits, and the close correspondence in time to the early diversification of metazoans. In Cambrian and other deposits, exceptional preservation involves a complex of physical, chemical, and biological factors. Among them, sufficiently rapid (usually episodic) deposition, reduction of biodegradation and burrowing activity, and rapid diagenesis seem to have been preeminant factors.

Deposits of exceptional preservation usually occur in transgressive systems tracts or highstand systems tracts, and in shelf and shelf-edge paleonvironments dominated by thinly laminated siliciclastic or carbonate mudrocks. Locally dysoxic or anoxic environments within the sediment are commonly inferred, even where overlying water columns were oxic. Clay minerals, organic carbon, phosphate, carbonate, and iron sulfide comprise the major mineralogic bases for preservation. Fossils are commonly surrounded by diagenetic aureoles (iron sulfide or other halos, or concretions). Fossil microbes present in some auroles and associated with macroscopic body fossils may have both played a role in decay and mediated early diagenetic mineralization. Predation and scavenging evidence, including macerated bits of exceptionally preserved, non-biomineralized organisms (inferred to be incompletely digested skeletal material), is common in Cambrian deposits. Temporary reduction of the activity of predators, scavengers, and burrowers may have been due largely to chemical or other changes in seawater (reduced oxygen content, fluctuating salinity, etc.).

This work was supported by the National Science Foundation.