Fossils communicate what organisms were alive during which period of time, as well as their shape and size. Fossils are well-preserved remains of animals and organisms where the bone has been transformed into a rock-like structure. Fossils can be examined, and by using dating techniques, paleontologists can determine the time period in which the organism was alive.
After a creature dies, most of its organs and living flesh decays. However, it is possible for its bones to settle in the ground or sea floor, which can then be buried. Over millions of years, minerals from the dirt and surrounding rocks cause the bone to turn into a shell-like structure known as a fossil.