This picture shows a trace fossil or ichnofossil I have not found before. It is a trilobite burrow also referred to as a Rusophycus. It was found in the Kope Formation (Eden Group) of Carroll County, Kentucky, USA. The trace fossil dates to the Ordovician Period.
Maybe it was made by a Flexicalymene trilobite. The three main trilobite remains found in that area are Cryptolithus, Flexicalymene, and Isotelus. I originally picked up this rock because there was a branching bryozoan on the other side that was partially covered by matrix. I thought if it could be prepped out it would make a nice display specimen. Thanks to my cousin Kenny, he recognized the trace fossil on the back which was even cooler!
