Fossils

Fossils are a historical footprint of past creatures and plants preserved in rock that once populated our world millions of years ago. The four ways in which fossils were created are true form, casts, molds, and traces.

True form  were formed from actual parts of the creature or plant that had been replace by minerals, then hardening to form a petrified rock of that organism.

Casts  were formed from an imprint left by the organism that filled with minerals then hardened to a rock.

Molds  are similar as cast molds, where cast are left by imprints.  Where molds are hollow imprints of organisms that are hollow. Meaning the creature or plant was trapped there and decayed, then the void filled with minerals and formed a reverse image. Some examples would be embryos or leaves.

Trace  also known as ichnofossils which are traces of the presence of creatures like their poop, footprints, tooth marks, and so forth.

The study of these various kinds fossils is known as Paleontology and the people doing the study are called paleontologist.