In examining these layers of rocks, the oldest layer to show any fossil marks of life is the Precambrian layer. Mostly what is found is Algae and some microscopic creatures known as Radiolaria. The next layer of rock is associated with the Paleozoic period, and the fossils in the rock reflect an increasing diversity of life forms, mostly small sea creatures and things that crawl. It is in the next layer, the Mesozoic, that we finally start seeing not only a record of fossils, but reptilian bones as well. Also known as The Age of Reptiles, the Mesozoic Period is best known for its dinosaurs.