XCT-BASED COMPARATIVE TAPHONOMY OF EXCEPTIONALLY PRESERVED FOSSILS FROM THE ORDOVICIAN AND DEVONIAN OF THE MIDWESTERN UNITED STATES

X-ray Computed Tomography (XCT) has emerged as a powerful, non-destructive tool in paleontology. In this study, we have used XCT to search fossils from two “ordinary” sedimentary deposits of the Midwest, the Cincinnatian (Upper Ordovician) and the Silica Shale (Middle Devonian), for exceptionally preserved remains. XCT permits rapid identification of pyritized nonbiomineralized tissues, as pyrite has a strong density contrast with calcite and clay minerals forming the matrix in both of these deposits. Neither the Cincinnatian nor the Silica Shale fits the prevailing paradigm of a classical Konservat-lagerstätte; both deposits represent generally aerobic, tropical, shelf environments.

A total of 247 fossil specimens from the Silica Shale and 102 specimens from the Cincinnatian were scanned using XCT. They represent a variety of taxa, including brachiopods, trilobites, corals, cephalopods, gastropods, bivalves, and echinoderms. Among studied specimens, 63% from the Silica Shale and 21% from the Cincinnatian contain pyritized nonbiomineralized tissues. In both the Silica Shale and the Cincinnatian, exceptional preservation of nonbiomineralized tissues occurred in what were otherwise normally oxygenated environments. In both settings, rapid burial during storm events likely created localized, anoxic, chemical microenvironments in which microbial action served to both decompose organic tissues and mediate rapid diagenetic mineralization. The results of this study open the possibility that other “ordinary” deposits also have been overlooked for exceptionally preserved fossils, and such deposits merit reexamination through this new lens.