EMPIRICAL OBSERVATIONS OF FLUVIAL EGGSHELL TRANSPORT AND DEPOSITION

Taphonomic histories of fossil eggshell accumulations are frequently interpreted based on the ratio of concave-up to concave-down shells, which are used to infer the processes that operated during assemblage formation. To develop robust inferences, a causal link must exist between taphonomic processes and their identifiable consequences in the fossil record. This empirical trial was initiated to determine if a consistent concavity ratio is causally linked with fluvial transport. Nine and a half boxes (44x28.5x23 cm) of dried chicken (Gallus gallus domesticus) eggshells were released in the East Fork Sevier River, Utah, and their transport and deposition monitored. Half-shells moved by floatation both concave up (displacement), and concave down (trapping air at the top of the shell). Half-shells most frequently moved by rolling or skipping along the bed, or moved as fish swam by. Small shell pieces rarely moved, though many rode in concave up floating eggshells. Half-shells frequently deposited in pools or depressions in the bed, while eggshell pieces were deposited in nearly all parts of the river. Half-shells most frequently oriented concave up in pools or depressions, often appearing much like an in situ hatched clutch of eggs, shell fragments were deposited almost exclusively concave down. Concavity ratios were a function of both shell shape (half or fragmentary) and the facies of deposition (shoot, pool, thalweg, etc.). Variable concavity ratios were observed across facies, demonstrating that the concavity ratio method for identifying fluvially transported shells is inconsistent between fluvial microenvironments. Further evaluation is needed to determine the method’s reliability.