either trace fossils or mold fossils or both since both would fit in your description but its probably mold fossils if you dont think so here is a list from enchanted learning
mold fossils (a fossilized impression made in the substrate - a negative image of the organism)
cast fossils (formed when a mold is filled in)
trace fossils = ichnofossils (fossilized nests, gastroliths, burrows, footprints, etc.)
true form fossils (fossils of the actual animal or animal part).