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

Abstract

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.

Keywords::

• taphonomy
• transport
• eggshell
• abrasion

Acknowledgements

We thank D. Mogk and the staff at the Montana State University Image and Chemical Analysis Laboratory for the use of the SEM; J. Horner for access to the Gabriel Laboratory for Molecular and Cellular Biology at the Museum of the Rockies; and D. Brinkman at the Royal Tyrrell Museum and J. Simon at Montana State University for kindly providing fossil eggshell specimens. We also thank D. Varricchio, T. Imai, J. Simon and D. Barta for edits and valuable discussion. Special thanks to T. Evans for advice, critiques and supplying chicken eggshell and tumbler time. We are indebted to D. Varricchio and the National Science Foundation Division of Earth Sciences for providing funding for field work (Grant Number 0847777) and the 2011 and 2012 Egg Mountain field crews for excavation assistance.

Notes

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