Earth Science: Evolution – Petrifactions
On Earth, organic material decomposes and is returned to its basic inorganic elements in a continuous cycle.
Occasionally, however, the remains of plants and animals from past geological eras survive through fossilization.
These fossils can provide important clues to scientists about evolutionary processes and the history of the Earth.
Earth Science: Biology – Evolution – Fossils and Fossilization
Only a tiny fraction of all plants and animals that used to inhabit Earth were preserved as fossils. The oldest fossils are microorganisms that used to live on Earth more than three billion years ago.
Fossilization is an extraordinarily lengthy process. First of all, an organism’s body needs to be quickly covered by sediment. This prevents decay, decomposition, or disintegration by scavengers and carrion feeders. Aquatic environments allow sediments to deposit more rapidly and continuously. Therefore, most fossils are former marine organisms. The remains of terrestrial organisms are much less likely to be covered by sediment in the fashion that would allow preservation.
During fossilization, many substances that previously made up the organism are converted into stable materials. Relatively fragile materials such as bones or calcareous shells become hard and durable minerals. When wood petrifies, silicic acid enters its structure, often preserving even details such as tree rings. Fossils embedded in shale or clay often get broken during petrifaction unless they are enclosed in a mineral shell.
These kinds of fossils, however, are especially valuable because the fine grain sediment is able to preserve even very fragile and soft-bodied organisms. A classic location for finding fossils of this type is the Burgess Shale located in the Canadian Rocky Mountains.
Body fossils, steinkerns, and trace fossils
Entire bodies or parts of organisms-whether hard parts or rarer soft tissuesare referred to as body fossils. Internal casts (steinkerns) are created when organisms (or parts of them) leave hollow spaces in the sediment, to be filled later by other materials that harden.
Occasionally, entire organisms or tissue parts are Dreserved, such as insects trapped in resin (amber) or mammoths found in the permafrost soils of Siberia. The traces that an organism left behind while still alive can also be preserved and are known as trace fossils.
These are often footprints left in the mud or traces of motion such as digging. This type of fossil requires that the sediment containing the traces retains its structure and does not mix with the sediment deposited on top.
NICOLAUSSTENO AND THE DISCOVERY OF FOSSILS
The Danish naturalist and later cleric Nicolaus Steno was the first to recognize that fossils were the remains of once living organisms. He noticed that a stone he had found in the mountains very closely resembled a shark’s tooth.
From then on, he dedicated all his efforts to the study of fossils. During his studies, he developed the first theory about the formation of sediment rocks and concluded that the Earth must be older than 6,000 years, which was the accepted age at the time.