Bioturbation

Bioturbation is the reworking of soils and sediments by animals or plants. It disturbs and changes sediment. Any animal living in the soil or on the bottom under water may disturb the sediment by moving it around. Burrowing, eating and defecation of sediment grains, building galleries (etc.) all disturb the sediment.
Common bioturbators include annelids ("ringed worms") such as oligochaetes, bivalves like mussels, clams, gastropods, holothurians. 
Fossil sites with bioturbation do not leave traces of soft-bodied animals. In contrast, fossil sites with undisturbed sediment have given rise to important lagerstätten with many impressions of small soft-bodied invertebrates.