ENCRUSTATION AND TAPHONOMIC HISTORY OF CEPHALOPODS FROM THE MIDDLE DEVONIAN MILWAUKEE FORMATION

The Middle Devonian Milwaukee Formation of the western Michigan Basin contains a diverse marine fauna that includes cephalopods, brachiopods, bivalves, echinoderms, corals, trilobites, bryozoans, ostracods, tentaculitids, and fish. While outcrops of the Milwaukee Formation are rare and poorly exposed today, abundant fossil material was collected in the past from exposures at the quarries of the Milwaukee Cement Company. This material, which primarily came from the lowest portion of the Milwaukee Formation (Berthelet Member), is currently housed at the Milwaukee Public Museum and the Greene Collection at UW-Milwaukee. The most common fossils found in these collections are cephalopods, which possess a variety of encrusting organisms and a complex taphonomic history that may provide new insights into the paleoecology and depositional environment of the Milwaukee Formation. Encrusting organisms observed comprise auloporid corals, microconchids, and encrusting bryozoans including hederellids. The taphonomic history of these cephalopods is somewhat unclear. The cephalopods are preserved three-dimensionally as internal molds, with no sign of original aragonite. Encrusting organisms can be found completely around most specimens, indicating encrustation occurred: 1) during the life of the cephalopod host and was then superimposed onto the internal mold during aragonite dissolution; and/or 2) the internal molds were exhumed and subsequently encrusted. Additionally, pyrite crusts and a dark film (possibly phosphate) on some of the cephalopods and the occurrence of associated concretionary dolostones suggest that both hardground formation and early diagenetic carbonate precipitation played a role in cephalopod preservation. Furthermore, the majority of these cephalopods appear to be covered by worm burrows. It is not clear if these are superimposed on the internal molds, or possibly represent traces of scavengers after burial; regardless, this is a seemingly unique association. Ongoing work with these collections will result in a better understanding of encrusting organism paleoecology and the apparently distinctive taphonomic history of these cephalopods.