Because most decay and fragmentation occurs at the surface, the main factor that contributes to fossilization is quick burial. Marine animals that die near a river delta may be buried by sediment carried by the river.
A storm at sea may shift sediment on the ocean floor, covering and helping to preserve skeletal remains. On land, burial is rare, so consequently fossils of land animals and plants are less common than marine fossils. Land organisms can be buried by mudslides or ash from a volcanic eruption, or covered by sand in a sandstorm.
Skeletons can be covered by mud in lakes, swamps, or bogs as well. Although the animals trapped in the pits probably suffered a slow, miserable death, their bones were preserved perfectly by the sticky tar. In spite of the difficulties of preservation, billions of fossils have been discovered, examined, and identified by thousands of scientists.
The fossil record is our best clue to the history of life on Earth, and an important indicator of past climates and geological conditions as well. The fossil record also plays a key role in our lives. Fossil fuels such as coal, gas, and oil formed from the decayed remains of plants and animals that lived millions of years ago.
Fossilization can occur in many ways. Most fossils are preserved in one of five processes Figure The rarest form of fossilization is the preservation of original skeletal material and even soft tissue. For example, insects have been preserved perfectly in amber , which is ancient tree sap. Several mammoths and even a Neanderthal hunter have been discovered frozen in glaciers.
These preserved remains allow scientists the rare opportunity to examine the skin, hair, and organs of ancient creatures. The most common method of fossilization is permineralization. After a bone, wood fragment, or shell is buried in sediment, it may be exposed to mineral-rich water that moves through the sediment.
This water will deposit minerals into empty spaces, producing a fossil. Fossil dinosaur bones, petrified wood, and many marine fossils were formed by permineralization. In some cases, the original bone or shell dissolves away, leaving behind an empty space in the shape of the shell or bone.
This depression is called a mold. Later the space may be filled with other sediments to form a matching cast in the shape of the original organism. Many mollusks clams, snails, octopi and squid are commonly found as molds and casts because their shells dissolve easily. In some cases, the original shell or bone dissolves away and is replaced by a different mineral. For example, shells that were originally calcite may be replaced by dolomite, quartz, or pyrite.
If quartz fossils are surrounded by a calcite matrix, the calcite can be dissolved away by acid, leaving behind an exquisitely preserved quartz fossil. Some fossils form when their remains are compressed by high pressure. This can leave behind a dark imprint of the fossil. Compression is most common for fossils of leaves and ferns, but can occur with other organisms, as well. Some rock beds have produced exceptional fossils.
Fossils from these beds may show evidence of soft body parts that are not normally preserved. Two of the most famous examples of soft organism preservation are the Burgess Shale in Canada and the Solnhofen Limestone in Germany. These fossils represent the organisms as they were when living, but these types of fossils are very rare.
The heat and pressure from being buried in sediment can sometimes cause the tissues of organisms — including plant leaves and the soft body parts of fish, reptiles and marine invertebrates — to release hydrogen and oxygen, leaving behind a residue of carbon. This process — which is called carbonization, or distillation — yields a detailed carbon impression of the dead organism in sedimentary rock.
The most common method of fossilization is called permineralization, or petrification. After an organism's soft tissues decay in sediment, the hard parts — particularly the bones — are left behind. Water seeps into the remains, and minerals dissolved in the water seep into the spaces within the remains, where they form crystals. These crystallized minerals cause the remains to harden along with the encasing sedimentary rock. In another fossilization process, called replacement, the minerals in groundwater replace the minerals that make up the bodily remains after the water completely dissolves the original hard parts of the organism.
Fossils also form from molds and casts. This empty space is called a mold. A mold shows the original shape and surface of the organism or part. Sometimes, sand or mud fills a mold and hardens, forming a cast of the original organism or part.
A cast is a replica of the original organism. Petrification happens when mineral solutions remove the original organism or part and replace it with new minerals. The replacement of the original materials is generally a very slow process. The result is a nearly perfect mineral replica of the original organism or part. In this investigation, students simulate how fossil molds, fossil casts, and petrified fossils form. They make a mold by pressing a shell or bone into clay.
A mold is left behind after the shell or bone is removed. Students then fill the mold with plaster of Paris, which dries and forms a cast of the shell or bone. Finally, students place sponges into Epsom salt solutions.
They observe as the solution travels through the sponges. After the sponges have dried, they see salt crystals that formed inside the holes of the sponges, forming a petrified fossil of the sponge. Use disposable craft sticks for mixing the Plaster of Paris.
Be sure to mix the Plaster of Paris yourself. Do not pour unused portions of Plaster of Pairs, or water mixed with plaster of Paris into the sink or drain. Dispose of them in the trash instead. Students should wear safety impact goggles when cracking the hardened Plaster of Paris. Monitor students around the hot water to make sure they do not burn themselves. The hot water should not be boiling. Review the investigation for your specific setting, materials, students, and conventional safety precautions.
Ask students to reflect on the first investigation when they studied fossil specimens. Allow them to re-examine the fossils. Have students think about the following question:. Have your students discuss these questions, first in pairs, then groups and then as a whole class. Record their answers on a flipchart that you can refer to throughout the investigation.
Have your students discuss the question in pairs, then in groups, and then as a whole class. Record their answers on a flipchart. Tell your students that they will be investigating this question and at the end of their study they will be able to provide reliable answers.
In the last investigation, students found out that they could slow down decay after an organism is covered by sediment. They probably do not know what happens to the organism after it is covered and the sediment turns to rock. They may think that the organism remains as it is. By the end of the investigation, some of these questions will probably be answered. Activity 2: Petrified Fossils Expected Time: One half- hour session for initial set up; several days for observations of the sponge; one half- hour session for conclusion and discussion.
Show students the Images of Fossils. Ask them to answer the following questions:. As a result of this investigation, students should be able to state that after an organism, or part of an organism, is buried in mud its hard body parts become a fossil as the sediments around it become a rock. If the organism or part dissolves or decays out of the rock, a cavity, or mold, results.
Minerals may then seep into the mold and fill it, forming a cast, or copy. Petrified fossils form when original materials that made up the organism are replaced with minerals. The following passage provides more detailed information related to this investigation that you may choose to explain to your students. Fossils that are the actual organism or some part of it, or the imprint of the organism or some part of it are known as body fossils. In relatively young sediments and rocks, the actual body parts of organisms are often preserved.
In older rocks, however, the body parts are usually dissolved away, or re-crystallized, or replaced by another kind of mineral. Even so, the imprints of the organisms are still preserved, and they can be studied if the rock splits apart in the right place to reveal the imprint. Paleontologists usually collect large numbers of rock pieces and then open them in the laboratory with special splitting devices to try to find at least a few fossils.
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