Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky

Epibiont bioimmuration is the process by which sessile animals or plants are molded within the mineralized skeletons of organisms that overgrew them. Recent work on Mesozoic and Cenozoic faunas in Europe has drawn attention to the value of bioimmured fossils (Vialov 1961; Voigt 1966; Taylor 1990). This unusual mode of preservation allows us to study many organisms, especially soft-bodied forms, that would not ordinarily be found in the fossil record, and it reveals new morphological and paleoecological details of common plants and animals. The most common bioimmuring organisms (cemented bivalves, tubicolous serpulid worms, and cyclostome bryozoans) have preserved an extraordinary array of soft-bodied organisms as impressions on their attachment surfaces, from marine algae to the onlyknown fossil entoproct (Taylor 1990; Todd & Taylor 1992). Morphologic detail down to the micrometer scale is sometimes seen. There is considerable potential for expanding the fossil record of little known groups by exploiting this preservational mode in other regions and in older rock, and we here report the first record of soft-bodied fossils preserved in this way from the Lower Paleozoic of North America. Epibiont bioimmurations are formed when encrusting organisms overgrow others on a shared substratum. Mesozoic and Cenozoic oysters commonly overgrew cheilostome bryozoans, inarticulate brachiopods, hydroids and other smaller encrusters in this manner, preservingtheir outlines with extraordinary accuracy. This is the mode of bioimmuration that contains the most information for paleontologists but has not been previously described from the Paleozoic.