Why do traces of trilobites, shells and fish appear on rocks?

Indicates that the place was originally an ocean, trilobites first appeared in the Cambrian period, peaked in the early Paleozoic era, became extinct and disappeared at the end of the Permian period 250 million years ago, and were deposited on the seabed to form rocks. With the changes of the earth, geological movement, the seabed elevation, the pale sea into a mulberry field, these traces are left in the rock.

In the following years, the organic matter in the remains of these organisms decomposed, and the hard parts such as shells, bones, branches and leaves, etc., together with the surrounding sediments went through petrification and turned into rocks, but their original form and structure are still preserved; similarly, the traces left behind by those organisms when they were living can be preserved in this way. We call these petrified remains of living things fossils.

Simply put

Fossils are the remains of creatures that lived in the distant past, or the remains of creatures turned into "stone". Many were buried in the mud and sand of the time. In the years that followed, the organic material in the remains of these creatures decomposed; in the same way, traces of the life of those creatures can be preserved.

These have been found on land, in lakes, in rivers, and in strata within the offshore. Pollen, spores, and algae (Flagellates and Skeptophytes) are used to demarcate the order of stratigraphic rocks. Residual plant fossils are not as common as animal fossils, but plant fossils are found in abundance in many parts of the world.