DECAY OF THE SEA ANEMONE METRIDIUM (ACTINIARIA): IMPLICATIONS FOR THE PRESERVATION OF CNIDARIAN POLYPS AND OTHER SOFT-BODIED DIPLOBLAST-GRADE ANIMALS

Fossils preserving the anatomy of soft tissues provide rare but essential evidence for the reconstruction of metazoan evolutionary history. Decay is inherent to the fossilization process and features may be distorted, displaced, or missing even in exceptionally preserved fossils, and non-anatomical artifacts may be introduced. Here we describe the results of experimental decay of the epibenthic actinian (sea anemone) Metridium senile and document serial changes in its morphology. Decay proceeded rapidly and followed a consistent, reproducible trajectory, which we divide into six stages; in the final stage, no anatomically illuminating information remained. The column, one of the most salient anatomical features of actinians, contracted near the time of death and changed shape dramatically thereafter. The tentacles decayed from their distal ends. Fibrous bundles representing the interior musculature were among the morphological features most resistant to degradation, and taphonomically induced transverse bands were also long lasting. These experimental results provide a semi-quantitative and predictive framework which can be applied to the interpretation of putative polypoid cnidarian organisms. Furthermore, this decay series shows that diploblast-and triploblast-grade fossils are unlikely to be confused and provides the means to evaluate the taxonomic validity of the long-standing assumption that many enigmatic soft-bodied Ediacaran and lower Cambrian taxa are of actinian grade.