Sediment and eggshell interactions: using abrasion to assess transport in fossil eggshell accumulations

Recent excavation at the Late Cretaceous Egg Mountain locality in Montana revealed abundant Spheroolithus eggshell fragments. The depositional history of the fragments is problematic due to the vertical dispersal of eggshell in homogeneous mudstone. The results of previous studies assessing eggshell transport are based on thin, modern eggshell occurring on well-defined horizons. Therefore, the assessment of depositional history required a different method. Chicken (Gallus gallus domesticus) and ostrich (Struthio camelus) eggshells were placed in a tumbler with water and quartz sand for 168–504 h to simulate transport. The resulting wear on these fragments, revealed under scanning electron microscope, was compared to unabraded eggshell. In addition, the modern eggshell was compared to fossil eggshells from a nesting site, crevasse splay and channel deposits, and Egg Mountain. Fossil eggshells from high-energy paleoenvironments display similar edge rounding to modern eggshell placed in the tumbler. In contrast, eggshell from Egg Mountain lacked edge rounding and resembled fossil eggshell from a nesting site and unabraded modern eggshell, suggesting that the Egg Mountain locality represents a parautochthonous assemblage. This study indicates that sediment interaction leaves distinct patterns of abrasion on eggshell, which may be useful for assessing transport of eggshell at fossil localities.