SHELL DISCOLORATION AS AN INDICATOR OF TAPHONOMIC ALTERATION IN THE DEEP-TIME FOSSIL RECORD

Actualistic studies of shell taphonomy in the marine subfossil record have found alteration in shell color to be a highly useful taphonomic indicator. However, the viability of shell discoloration as an taphonomic indicator in the more profoundly altered material of the deep-time fossil record has been largely unexplored. We investigated the relationship between the shell color of Ordovician brachiopods and their taphonomic state, and, based on this, whether color could provide meaningful information about the taphonomic conditions under which the shells were preserved. We examined 1033 specimens of Platystrophia and Hebertella from type Cincinnatian outcrops in southwestern Ohio and northern Kentucky, using a sampling design that allowed us to test the taphonomic significance of shell color in multiple stratigraphic units and across a wide geographic area. For each specimen, detailed taphonomic data were recorded, including degree of fragmentation and corrasion, articulation, the presence of encrusting organisms or borings, breaching of the valves, and shell color. Numerical scores were assigned to the taphonomic data, allowing for multivariate analyses of the taphonomic attributes of the specimens. Our results demonstrate that fossil shell color is strongly related to overall taphonomic state, with dark gray/black colors associated with evidence of shell degradation, reflecting longer residence time in the taphonomically active zone. Given evidence that shell color is strongly influenced by exposure on the sea floor, shell color may be a useful addition to taphofacies models, with dark shell colors most indicative of environments with slow sedimentation rates or multiple burial and exhumation cycles.