CRYPTIC PRESERVATION: VIEWS FROM A HIDDEN WORLD

Internal bivalve and gastropod casts or steinkerns from the Plio-Pleistocene Caloosahatchee and Pleistocene Bermont Formations from three Florida quarries reveal a view of the cryptic epibiont and endobiont communities of the host shells. The maximum grain size of the sediment infill at the sites varies from 0.125 to 0.50mm, with occasional larger skeletal clasts. The fine-grained siliciclastic and carbonate infill and cements at one of the sites have allowed for the preservation in exquisite detail of the endobiont galleries, their extensions and processes. The endobionts are preserved as secondary steinkerns of traces in relief, visible where the shell material has been dissolved or broken away. Epibionts within the aperture and lumen are preserved as embedded secondary steinkerns, truncated against the plane of the dissolved shell surface and in places, retaining their original material around the infill. The best-preserved gastropod specimens exhibit both the endobiont and epibiont traces. The nature of the sclerobiont communities indicates that some of the host shells were occupied, post-mortem, by pygurid crabs, and that some may have been inhabited as empty shells. Although the epibionts and facets present on the external surface of the shell were not preserved in these specimens, an inspection of associated hermitted shells suggests possible external sclerobionts for a more complete picture of the community structure. The presence of cryptic endobionts and epibionts, particularly large numbers of spirorbid worms deep within the whorls of some of the gastropods raises the question of how these animals dealt with the micro-environment within the shell and the interior lumen. The chamber formed behind the body of the crab may have had restricted nutritional resources and hypoxic conditions. Some of the animals may have used the interior lumen as a refuge and others may have been parasitic. Bivalves at one site also host cryptic epibionts within the post-mortem cavity of the articulated shell, where the gape or other entrance to the interior (e.g. edge break or drill hole) were sufficient for water circulation, but limited enough to retard the infilling of the shell for a period of time. Overall, the examined specimens provide a picture of the distribution of cryptic bionts deep within the molluscan shells.