Fossils formed from impressions made in soft mud are called trace fossils. They are not the actual remains of the organism, but rather evidence of its activity.

Here are some examples of trace fossils:

* Footprints: The impressions of an animal's feet.

* Trackways: A series of footprints that show the animal's movement.

* Burrows: Holes dug by animals in the ground.

* Coprolites: Fossilized dung.

* Gastroliths: Stones swallowed by some animals to aid digestion.

* Nests: Fossilized nests built by animals.

These trace fossils provide valuable information about the behavior, environment, and even the movement patterns of past organisms.