The Cincinnatian Series of the Upper Ordovician of eastern Indiana hosts the earliest known occurrence of a rare and little studied mode of fossil preservation called bioimmuration (Wilson et al., 1994). This mode of preservation is relatively common in the Mesozoic, but rare in the Paleozoic (Taylor, 1990). One of the few other examples of Paleozoic specimens displaying this kind of preservation was found on an erratic Ordovician boulder from Gotland (Hillmer and Schallreuter, 1987).

Bioimmuration is a mode of preservation which results from organic overgrowth, or immuration, of sessile organisms (Taylor, 1990). It most commonly occurs as the immuration of soft or hard-bodied organisms through encrustation by skeletal organisms such as bryozoans, cornulitids, and various other fauna. Bioimmuration occurs when an encrusting organism overgrows another skeletal or soft-bodied organism, thereby preserving a record in negative relief of the encrusted organism's shell. In the Cincinnatian Series, bioimmuration occurs primarily through the encrustation of aragonitic mollusc shells such as those of clams and gastropods, by bryozoans. Taylor (1990, p.2) calls this specific type of bioimmuration "epibiont bioimmuration", which occurs when the overgrowing and overgrown organisms share the same substrate. Cincinnatian seas were "calcite seas", meaning that the seawater was undersaturated with respect to aragonite (Palmer et al., 1988). When the aragonitic shells of these encrusted molluscs dissolved away, what was left was not just the external mold of the mollusc, preserved by the encrusting bryozoan, but a record in negative relief of the succession of organisms that had encrusted the shell prior to the bioimmuring bryozoan.

Some questions arise when analyzing these bioimmured fossils. What kinds of organisms do they represent? A more specific question might ask what this mode of preservation could say about the morphology of some lesser-known soft-bodied organisms. One of the most common negative relief features seen through bioimmuration, apart from identifiable skeletal organisms, appear to be remains of stolon systems. However, on many specimens, bumps and ridges can be observed for which no explanation or description can be found. With these fossils, there is the potential for describing the morphology of some of the more enigmatic soft-bodied encrusting organisms. Something may also be said about their life modes. Most appear to have preferred an exposed life mode, on the outside of the host's shell. However, there have been specimens described that exhibited more cryptic life habits (Lazzuri et al., 1999).

What can be said about the ecological succession of these various epibiont communities? Lazzuri et al. (1999) suggest that there may be a predictable sequence of encrustation that occurs on these substrates. There are a number of different organisms that can be seen in these fossils. What is the nature of the interaction that these animals display? Is there evidence for competition? Wilson (1985) suggested that there is a relationship between the percentage of the substrate covered by encrusters and diversity of the typical Ordovician encrusting community. Wilson also states that there is a correlation between elapsed ecological time and the diversity of encrusting communities. There is an explosion of encruster diversity at first, but as time goes on, organisms such as trepostome bryozoans tend to cover the entire substrate, thus lowering the diversity of the assemblage. Addressing these questions should lead to useful insights about some of the ecological relationships that existed among these encrusting communities in Late Ordovician seas.

The ability to observe this mode of preservation relies almost entirely on the existence of an aragonitic substrate to encrust. These aragonitic shells may have dissolved rapidly in calcite seas such as those of the Late Ordovician (Palmer et al., 1988). Ordovician bioimmurations may be able to illustrate how quickly this dissolution would have occurred. Lazzuri et al. (1999) suggest that by observing the growth patterns of the encrusting organisms, it should be possible to say something about the relative timing of the aragonite dissolution and its relationship with encruster growth. Perhaps the organism grew around the substrate in such a way that allows some conclusions to be drawn about the style of shell dissolution relative to the growth of encrusting organisms. If encrusting organisms are seen encrusting areas previously occupied by aragonitic shell growth (e.g. on the base of a bioimmuring bryozoan), then it can be said that dissolution was occurring within the ecological life of the community.

References Cited:

Hillmer, G. and Schallreuter, R., 1987, Ordovician bryozoans from erratic boulders of Northern Germany and Sweden. In Ross, J. R. P. (editor), Bryozoa: present and past. Western Washington University, Bellingham, 333 pp.

Lazzuri, J. E., W. W. Fischer, M. A. Wilson and C. M. Tang, 1999, Bioimmuration as a key to paleoecology on shell substrates and early aragonite dissolution in a calcite sea (Upper Ordovician, Cincinnati region, USA). Geological Society of America Abstracts with Programs 31(7): 465.

Palmer, T. J., Hudson, J. D., and M. A. Wilson, 1988, Palaeoecological evidence for early aragonite dissolution in ancient calcite seas. Nature 335: 809-810.

Taylor, P. D., 1990, Preservation of soft-bodied and other organisms by bioimmuration ­ A review. Palaeontology 33(1): 1-17.

Wilson, M. A., 1985, Disturbance and ecological succession in an Upper Ordovician cobble-dwelling hardground fauna. Science 228: 575-7.

Wilson, M. A., T. J. Palmer, and P. D. Taylor, 1994, Earliest preservation of soft-bodied fossils by epibiont bioimmuration: Upper Ordovician of Kentucky. Lethaia 27: 269-270.