TRILOBITE TAPHONOMY OF THE SILURIAN WAUKESHA BIOTA OF SOUTHEASTERN WISCONSIN

The Silurian Waukesha Biota exhibits an unusual trilobite assemblage that differs from other trilobite associations found in the Silurian rocks of the American Midwest. This highly localized Lagerstätte within the Silurian (Telychian) Brandon Bridge Formation at Waukesha, Wisconsin is known for the exceptional preservation of a variety of soft-bodied and lightly sclerotized organisms while lacking more typical attached shelly benthos. Arthropods dominate this biota and trilobites are the most diverse and common members of this group. Numerically, specimens of a new genus and species of a dalmanitid dominate the trilobite fauna which includes rare specimens of Meroperix, Ekwanoscutellum, Stenopareia, and Arctinurus, and other taxa. Regionally, most of these trilobites occur in other exposures of the Brandon Bridge with specimens of Stenopareia most common but articulated specimens of any taxa are rare. However, the dalmanitid is absent, along with soft-bodied taxa from other Brandon Bridge localities in Wisconsin and Illinois. At Waukesha, hundreds of specimens of the dalmanitid occur convex up on bedding planes of the “soft-bodied zone” but rarely on the same surfaces as soft-bodied taxa. While commonly articulated, these specimens lack a hypostome and in contrast with associated non-trilobite arthropods, exhibit no evidence of appendages. Based on fossil preservation and depositional features, it appears that trilobite and possibly other arthropods fossils represent molts that were washed into a sediment trap and were not living individuals buried in debris flows. In contrast, the preservation of the fossils of the Gravicalymene celebra Association, (Silurian, Wenlock) indicate they are undisturbed examples of molting. Articulated specimens of Gravicalymene celebra and Cerauromeros hydei are upright, have split facial sutures, are flexed, with their hypostome “in place”. The distribution of these specimens is more random than the localized abundance and occasional directional orientation of the Waukesha dalmanitid although rare examples of molting-created “peletons” are present. In conjunction with depositional features, the characteristics of these two trilobite assemblages demonstrate how similar appearing patterns of fossilization can result from different taphonomic histories.