Paleontology, pay lee on TOL uh jee, is the study of animals, plants, and other organisms that lived in prehistoric times (more than 5,500 years ago). Fossil remains of organisms occur in layers of sedimentary rocks (rocks formed when mineral matter settled out of air, ice, or water). The organisms that are now fossils were alive when the rocks were being formed. They were buried and preserved as the layers of rock piled up.