The fossil record is used by paleontologists to give strong evidence for evolution. It does not display a linear progression of evolution, but certain patterns of species and habitat diversity can be seen. In the oldest known sedimentary rocks the only fossils present are those of prokaryotes. These are the simplest forms of life and are the first to appear. Fossils starting in the Paleozoic era progress from life forms completely unlike anything living today to forms which are very similar to life today. Also during the Paleozoic, a change occurs from primarily marine organisms to aquatic and terrestrial forms. When moving upward in the geological column, the variety and size of fossils gradually increase. These increases are marked by new types of organisms which exist for a certain time and are then lost to periodic mass extinctions.