A fossil refers to the remains of (prehistoric) organisms that are preserved in rocks. These remains may include bones, shells, exoskeletons, stone imprints of animals or microbes and DNA remnants.
There are various types of fossils: carbon fossils, petrified fossils, mold fossils, cast fossils, trace fossils and true form fossils.
Among these types, the cast and mold fossils can be used to provide the indirect evidence of organisms as they lack a lot of detail.
This happens when the organisms which have exoskeleton or shells are trapped in a porous sedimentary rocks, where the soft tissue of the body can be dissolved by the flowing water over time. This will leave a hollow cavity where the organisms once was.
We say that the fossil leaves an indirect evidence, because in these cases, it is the paleontologists who create a synthetic cast to mimic the organism that was once in the shells.
