University of Bristol EARTH SCIENCES

A standardised experimental procedure was used, and samples of the freshly killed polychaete worms were taken at various stages in order to identify 1) the stage of decay reached and 2) the onset of mineralisation in order to identify decay rate.

The results of the experiments showed that five discrete stages of decay could be identified;

Stage 1. Whole and shrivelled	Stage 2. Flaccid	Stage 3. Unsupported gut
Stage 4.cuticle sac	Stage 5. Jaws and setae

Although these are seen as a crude measure, they are represented throughout the fossil record and are identifiable.

The discussion highlights the crucial findings of the paper. The onset and rate of decay is cited as the essential prerequisite for mineralisation, and therefore fossilisation through the mobilisation of molecules. Organisms that decay rapidly will potentially have no organic material left to mineralise and will therefore not be represented in fossil assemblages. Those organisms that decay more slowly have a higher potential for preservation and hence are recorded in the fossil record.

Briggs and Kear's findings conclude that nothing will be seen of earlier decay stages if only later stages of decay are present in an assemblage. Their results have implications for biases observed in the fossil record. Further work needs to be carried out in order to identify taphonomic biases observed in normal states of preservation which do not include exceptional fossils such as the Ediacaran.

The top picture is a reconstruction of the Burgess Shale community. The majority of genera lacked a biomineralised skeleton. The bottom diagram shows how we normally find Cambrian localities without the "soft tissues" where normal decay processes have operated.

Reproduced from The Fossils of the Burgess Shale, Briggs et al 1994.