Most organisms do not fossilize and those that do are usually destroyed by geological processes or they never surface for examination. Fossils are usually formed when an organism is covered by sediments that then harden into sandstone, slate, mudstone or flint. Organisms also fossilize when they are buried in volcanic ash or entombed in tar or tree sap. A mold fossil forms when the material surrounding the organism hardens followed by removal of the organic matter. Permineralized fossils are created when ground water percolates through the remains of the organism and leaves behind minerals in the cellular spaces.
Petrified wood is an example of a permineralized fossil.
Replacement fossils are formed when ground water first dissolves out the tissue and then leaves minerals in their place. Both types of petrification fossils are generally composed of either SiO 2 or CaCO 3 . The discipline of comparative anatomy is important in interpreting relationships among organisms. Structures are said to be homologous if they have similar embryonic origins and analogous if they are similar only in function. The wings of birds and flies are examples of analogous structures.