PHOSPHATIZATION OF VERMIFORM FOSSILS FROM THE WINNESHIEK LAGERSTÄTTE, WINNESHIEK SHALE, NORTHEAST IOWA

The Winneshiek Lagerstӓtte in the Middle Ordovician Winneshiek Shale of northeast Iowa is preserved in a meteorite impact crater, the Decorah Impact Structure, and hosts a diverse and exceptionally well preserved fauna, including examples of soft tissue preservation. Additionally, structures of unknown biogenic affinity referred to as vermiform fossils make up a significant percentage of the biota. Vermiform specimens exhibit a range of morphologies and have been suggested to represent trace fossils (e.g., coprolites), cololites, or soft tissue preservation of wormlike organisms. Despite their potential importance the taphonomic processes responsible for the preservation of vermiform fossils are poorly understood. We analyzed these vermiform fossils from the Winneshiek Shale using SEM and EDS in order to constrain the taphonomic processes responsible for their preservation. Elemental mapping and point analysis of thin sections of vermiform fossils reveal that they are composed primarily of calcium phosphate with lesser amounts of zinc sulfide. The primarily calcium phosphate composition of vermiform fossils suggests that microbially induced phosphatization during decay was the main taphonomic process involved in vermiform preservation. The wrinkled external morphologies of many of these vermiform fossils and the important role played by microbially induced phosphatization are consistent with a cololite interpretation, although coprolites are also present, and phosphatized organisms may yet be discovered.