GRAVICALYMENE CELEBRA (RAYMOND) FROM THE SILURIAN LAUREL DOLOSTONE: WHERE ARE THE PIECES?

The fossil record of trilobites is dominated by disarticulated carapace elements, and whole trilobites (carcasses) and intact molt ensembles frequently are segregated in rock layers in which disarticulated carapace elements are scarce. Gravicalymene celebra (Raymond) in dolostones of the Laurel Member of the Salomanie Formation (Silurian, Wenlockian) in SE Indiana and NC Kentucky, appears to occur predominantly as carcasses. Systematic prospecting of outcrop faces, split slabs and streambed talus near Madison, IN yielded 20 carcasses of this trilobite, disseminated throughout the unit, but not a single disarticulated carapace element. By contrast, specimens of Calymene niagarensis Hall collected from the Liston Creek Member of the Wabash Formation (Silurian, Ludlovian) near Peru, Indiana consist almost exclusively of disarticulated pieces. Additionally, the Laurel Member carapaces are either flexed or (less frequently) partially enrolled and exhibit a variety of orientations relative to bedding. Associated normal marine fauna consist mostly of abundant disarticulated crinoid ossicles but also include rhynchonelliform brachiopods (in many cases articulated), solitary rugose corals, fenestrate and stick-like bryozoans, gastropods, nautiloids, receptaculitid chlorophytes, and extremely rare pygidia and cranidia of Dalmanites, Encrinurus and Sphaerexochus. Neither these fossils nor G. celebra form shell hash layers, which host the majority of disarticulated trilobite remains in shaley Upper Ordovician rocks underlying the Laurel Member. The apparent absence of calymenid pieces in the Silurian rock unit probably is not an artifact of misidentification, non-recognition or diagenesis. If G. celebra carcasses represent allochthonous remains, which seems unlikely given the (originally) fine texture of the host carbonate sediments, then they were deposited separately from carapace elements. If on the other hand G. celebra carcasses are authochthonous or parauthochthonous remains, then the apparent absence of associated carapace elements may have resulted from peculiar characteristics of the life history of the trilobites (e.g., extremely infrequent molting in the Laurel Member paleo-environment) and/or unusual characteristics of local sedimentation and/or rock preservation.