Flexicalymene Trilobite Mystery Piece

Here are some pictures of fossilized Flexicalymene trilobite remains from the Ordovician Period.

What I am trying to figure out is what is the blackish boom-a-rang shape in between the thorax segment and cephalon pieces? Maybe it is not even part of this trilobite since it is a different color.

It was found in Carroll County, Kentucky in the Kope Formation.