Lumbricaria

This group (ichnogenus) is currently defined as calcitic coprolites that are entangled/intertwined strings having a round cross-section. Constrictions are often present at irregular intervals. The surface is rough. 

Source: The ichnogenus Lumbricaria Münster from the Upper Jurassic of Germany interpreted as faecal strings of ammonites by Dirk Knaust and René Hoffmann (2020).