Rusophycus - Trilobite Trace Fossil

This is a Rusophycus, a type of trilobite trace fossil. Given the collecting location, size and shape, it almost certainly was left by a Flexicalymene. These trace fossils are uncommon at the locality, much rarer than the trilobites themselves.

Rusophycus were left by trilobites digging straight down into a muddy botton. The holes they left were filled in with sand. So, they are actually the sand casts of tracks. The trilobite dug into the mud layer from side to side with it's legs. This left a set of scratch marks on each side and create a Rusophycus approximately the same size and shape as the body of the trilobite.

Scientists are certain these are trace fossils of trilobites because in several occasions fossilized trilobites have been found still resting right on top.