Fossils are remains or imprints of organisms that lived in the past. They can form in five ways: by being buried and preserved in sedimentary rock, trapped in amber, frozen in ice, replaced by minerals through petrification, or trapped in tar or asphalt. Fossils provide information to scientists about past organisms, environments, and how organisms have evolved over time. Certain index fossils are especially useful for determining the age of rock layers based on the period when that type of organism lived.

1. FOSSILS Chapter 9, Section 1