Decay and mineralization of soft tissue in Early Devonian boring ctenostome bryozoans

Abstract

The Early Devonian of Podolia, Ukraine, has yielded phosphatized colonies of the boring ctenostome bryozoan Podoliapora doroshivi with 3-D preservation of soft tissues. However, the feeding zooids are not anatomically complete, their preserved soft tissues comprising decay-resistant structures such as the protective cuticular polypide sacs with presumed parietal muscles inside the wall of the sacs, the setigerous collars, the membranous orificial walls and remains of the muscle tissues. Early diagenetic apatite mineralization occured in numerous feeding zooids of Podoliapora at different stages of decay and may be important for the interpretation of decay processes in these colonial soft-bodied fossil organisms. A setigerous collar, which is a characteristic of extant ctenostomes, occurs in P. doroshivi in several stages of decay showing progressive collapse and eventual complete loss. This study indicates that the morphological changes of collars induced by decay often resulted in connection with the membranous orificial wall, producing false anatomical structures, unrelated to structures observed in the earlier stages of decay or to the anatomical structures of extant ctenostomes. The most decay-resistant cuticular polypide sacs mineralized as cryptocrystalline apatite in early stage of decay became degraded in later stages of decay. These data provide evidence that the anatomical interpretation of soft-bodied fossils preserved only in the later stages of decay may have led to imprecise morphological interpretations.