Sedimentological analyses of eggshell transport and deposition : implication and application to eggshell taphonomy
The interpretation of fossil eggshell assemblages in the absence of nesting structures is problematic because eggshells can be transported by hydraulic flows in fluvial environments. Failure to recognize transported eggshells may lead to erroneous interpretation of the reproductive behavior and ecology of those animals. An inconsistent array of evidence has been used in past studies to assess eggshell transport. Here, a series of flume studies was conducted to establish analytical techniques for assessing eggshell hydraulic transport in the fossil record. Using modern eggshells in a flume, I investigated preferred eggshell orientation after transport, the relationship of flow competence with eggshell shape and size, and size of clastic sediment expected to be associated with transported eggshells. Emu, goose, and ostrich eggshell fragments were released in a rectangular flume with decelerating flow. The transport of each eggshell was observed five times on each of four substrates (coarse sand, sparse gravel, dense gravel, and polyvinylchloride). At eggshell deposition, eggshell orientation and flow depth were recorded. Critical bed shear stress for eggshell deposition was estimated based on the flow depth at the point of eggshell deposition. The probability of concave-down orientation for deposited eggshells was estimated for each eggshell type transported on each substrate. The relationship of the critical bed shear stress for eggshell deposition with eggshell shape and size was tested. Size of clastic grains deposited under the critical bed shear stress for eggshell deposition was estimated. The probability of concave-down orientation after transport was > 85% regardless of eggshell types and substrates. The bed shear stress at eggshell deposition was most closely related to eggshell height and volume. Estimated size of clastic sediment associated with transported eggshell was coarse sand or coarser sediment. One may consider a high proportion of concave-down oriented eggshells in fossil assemblages as indicative of transport regardless of eggshell types. In addition, eggshells may be sorted according to their height and volume. Coarse sand or larger particles observed in matrix of fossil eggshells may be used as evidence of eggshell transport. Further studies are necessary to test reliability of those techniques and broaden their applicability.