BRYOZOANS AS TAPHONOMIC ENGINEERS, WITH EXAMPLES FROM THE UPPER ORDOVICIAN (KATIAN) OF MIDWESTERN NORTH AMERICA

A combination of encrusting calcitic bryozoans and early seafloor dissolution of aragonitic shells recorded in the Cincinnatian Series of the upper Midwest of North America allowed the preservation of abundant molds of mollusk fossils bioimmured beneath the attachment surfaces of the bryozoans. We here call this preservational process bryoimmuration, defined as a bryozoan-mediated subset of bioimmuration. The bryozoans molded very fine details of the mollusk shells, usually with more accuracy than inorganic sediment molds. Most of the bryozoans are heterotrypid trepostomes with robust low-Mg calcite skeletons. The mollusks are primarily bivalves, gastropods, nautiloids and monoplacophorans with their originally aragonitic shells now dissolved. Many of the encrusting bryozoans are so thin and broad that they give the illusion of a calcitic mollusk shells clinging to the molds. Some mollusks in the Cincinnatian, especially monoplacophorans and epifaunal bivalves, would be poorly known if they had not been bryoimmured. Unlike internal and external molds in sediment, bryoimmured fossils could be transported and thus record aragonitic faunas in taphonomic assemblages (like storm beds) in which they would otherwise be rare or absent. In addition, bryoimmurations of aragonitic shells often reveal the order of encrustation on the shells by exposing the earliest encrusters and borings that were later overgrown. Bryoimmuration was common during the Late Ordovician because the calcite sea at the time was quickly dissolving aragonitic shells on the seafloor before final burial, and large calcitic bryozoans were very common encrusters. Bryoimmuration is an important taphonomic process for preserving aragonitic faunas, and it reveals critical information about sclerobiont paleoecology. Several Cincinnatian mollusk holotypes are bryoimmured specimens because of the detailed nature of this preservation. These include monoplacophorans such as Archinacella richmondensis and Salpingostoma richmondensis. Bryozoans involved in bryoimmuration enhance the preservation of aragonitic fauna and thus act as taphonomic engineers.