PRESERVATION OF FOSSILS
The majority of fossils are found in marine sedimentary rocks. These are rocks that were formed when salt-water sediments, such as limy muds, sands, or shell beds, were compressed and cemented together to form rocks. Only rarely do fossils occur in igneous and metamorphic rocks. The igneous rocks were once hot and molten and had no life in them, and metamorphic rocks have been so greatly changed or distorted that any fossils that were present in the original [rock] have usually been destroyed or so altered as to be of little use to the paleontologist.
But even in the sedimentary rocks only a minute fraction of prehistoric plants and animals have left any record of their existence. This is not difficult to understand in view of the rather rigorous requirements of fossilization.