Fossils are used to refer to the traces or remnants of the past life. They are the remains of the animals, plants, organisms, or things that existed on the earth in the past. The activities of such beings can also be categorized as fossils. The term fossils means something which is obtained by the process of digging. Since fossils provide the remnants of the creatures that existed in the past, they are a good reflection of the life and life forms in the past, and much about the conditions and creatures of the past can be determined by examining the fossils. Fossils are important in determining the evolution of life across the geological time frame. Fossils enable paleoanthropologists to reconstruct the animals and plants that existed during a certain period. Using this they can then determine the conditions and climate on the earth during the time the animal or plant lived. Thus, fossils are a great help to paleoanthropologists in tracing the probable progression of evolution and extinction of a variety of species on earth. They also help form linkages amongst the various species and help trace their ancestries.
Fossils are mostly found in sedimentary rocks as it is the most common rock type on earth. The process of formation of sedimentary rocks itself reflects why most fossils are found in sedimentary rocks. Sedimentary rocks are formed when sediments get deposited through wind or water, and are then solidified over the span of time. Fossils typically form when remnants of organisms get trapped in the sediments and then undergo several process along with the rock to finally form a fossil. Sedimentary rocks include sandstone, shale, limestone; these rocks are known to contain fossils. Fossils are also found in resins of trees where small insects or animals get trapped and die. Tar pits are also known to contain fossils as they contain bones and teeth of animals that become trapped in the tar and expire. Ice is another fossil site which preserves intact remains of animals or organisms that die and get buried in the snow.
The formation of fossils is an interesting aspect and involves several complex processes. Formation of fossils through any of the processes is known as fossilization. Majorly two types of fossils exist: body fossils and trace fossils. Body fossils are helpful in exposing the body structure of the organism, whereas trace fossils aid in exposing the activities of organisms. In general, the process of fossilization involves three crucial steps: demise of the organism, processes that occur before the organism is buried, and finally the processes that take place after the organism is buried. It is the third step, the processes that take place after the organism is buried, which determines the various categories of fossils. These processes help determine the fossil type and the various forms of fossilization. Buried organisms may undergo several types of processes depending on the environment, climate as well as body structure of the organism. Various forms of fossilization are discussed in greater detail in the following paragraphs.
Body Fossils – For fossilization to occur, dead animals and plants must get buried rapidly to avoid destruction by biological and physical factors. Process that occur post-burial include chemical changes, substitution, and compression.
Soft Tissue Fossilization – In soft tissue fossilization, soft tissues of organisms are preserved. The preservation of soft tissues depends upon the climatic conditions. Such tissues are sensitive to climatic changes and are less likely to sustain climate changes over large period of time. Such tissues are formed under anaerobic conditions lacking oxygen under lower temperatures. Such conditions reduce the rate of decay and predatory impact, and help in burying and preserving soft tissues. Fossils sites with soft tissue fossils are known as Lagerstätte.
Imprints – Some fossils are also recovered in the form of imprints. Imprints are the inscriptions or markings of skinny organisms such as leaves or bony marine arthropods that get trapped in rocks or volcanic ash. Over time, they get buried by the sediments and as their body dissolves owing to the various processes occurring within the rocks, molds are formed which take the shapes of their body or silhouette. Major site of fossilization include sedimentary rocks like shale and limestone as well as ash of the volcano.
Freezing – Fossils are formed through freezing when an organism collapses into a pit or cavity and gets buried into the snow. Its remains get frozen in the snow and is thus conserved. Fossil formation through freezing is uncommon and occurs only when the remains of the organisms remain frozen until the time they are discovered. Freezing is probably the best form for fossil preservation. Remains of wooly mammoths and rhinoceros have been discovered from Siberia as well as Alaska. These remains include skin, bones, hair, flesh, and even some internal organs.
Drying or desiccation – Fossilization through drying, also known as mummification generally occurred in the arid or semi-arid parts of the world where the bodies of the dead organisms got parched and dried out. Remains of such animals have been discovered in Australia, South America, New Zealand as well as southern parts of USA. The dry and hot conditions cause the dead material to dehydrate, thereby enabling its preservation for thousands of years. Apart from deserts, dry caves are also an ideal site for fossilization through desiccation. Animals getting trapped and dying in the dry caves resulted in their dead material getting preserved in such caves. Desiccation helps preserve the bones and tissues of the dead animals. However, since they are devoid of any moisture and completely dry, they are highly fragile and may break at the slightest touch.
Replacement – It is a fossilization form in which the hard and rigid parts of the dead organisms are softened and gradually dissolved by water, while replacing them with mineral material. Body parts preserved in this manner include bones, tissues, shells, wood, etc. The replacement process involves chemical reactions which occur over long periods of time. The original parts get dissolved while being gradually replaced by minerals like calcite, pyrite, silica as well as hematite. Replacement fossils involve the snails of Green River formation whose bodies have been replaced by silica. Agatized woods are also conserved through replacement. The body parts of dead organism may retain its shape but loses its internal matter.
Permineralization – One of the common methods of fossilization is permineralization. When an organisms dies and gets buried, ground water carrying minerals seeps into the cavities of the shell, wood, bones or any empty spaces that were filled with liquid or gas during the lifetime of the organism. The minerals in the water get precipitated and form deposits resulting into stone type fossils. Such fossils retain their original hard material. Most marine life, teeth, and bones are fossilized through permineralization. Permineralization occurs when the organisms get buried in the sediments early on as this determines the extent to which the original body material of the organism is preserved. With rapid burial and less decay, skin, soft tissues as well as feathers might be preserved. Jurassic dinosaur bones found in Colorado and Utah are examples of permineralized fossils.
Molds and Casts – In mold and cast type fossil formation, the dead organism gets buried in the sediments and gradually the sediments starts thickening and begins to turn to stone. Due to several chemical and biological reactions, the organism starts dissolving and a cavity is formed in the sediment, which has now converted to rock. Such a cavity is called a natural mold. This mold retains the outer structure of the dead organism. Mineral-rich water enters the mold and minerals start depositing forming a cast from the mold. The cast retains the shape of the original organism, while the internal matter is completely different. Procheloniceras ammonite fossils found in Morocco are formed in this manner.
Asphalt or tar – Asphalt normally preserves the hard and solid parts of organisms. These parts include bones, teeth, insect exoskeletons and seeds of plants. The La Brea tar pit in Los Angeles is a heaven for fossil researchers as it contains thousands of fossils in the form of exoskeletons, bones, leaves as well as flowers preserved by tar.
Carbonization or Distillation – Carbonization is a process wherein the more unstable material of the organism deteriorates, while leaving residual carbon. Huge amounts of heat and pressure lead to such type of a process. The carbon leaves an imprint in the rocks or stone delineating the fossil, at times in large details. Flaky woods with deposits of lignite is an example of the carbonization process.
Polymerization – It is a process in which organisms, more commonly insects, get trapped in the sticky tree sap produced by the tree to protect itself from fungus. It finally gets entombed in the tree sap. Over the span of years, the tree sap polymerizes and becomes hardened to form amber, thus forming a natural plastic casing around the insect and preserves its fragile body. Areas of the Baltic Sea are primary sites with amber fossils.
Coprolites – Coprolite literally means “dung-stone”. Coprolites are the fossilization of the remnants of the intestine as well as the excreta of the organism. Coprolites are rare to be found as the feces or waste matter of the organisms is prone to rapid decay. As such, these types of fossils are mostly found in marine organisms. The waste matter of the organisms is most preserved by the process of petrification or cast and mold. Scales of fish, teeth, shells as well as bones of other organisms are mostly found in coprolites. Thus, they contain a vast amounts of phosphorous. Coprolites have been found in the Eocene fish beds in Wyoming.
Gastroliths – Gastroliths are gizzard stones. They are primarily used in the research on reptile fossils. The stones are located in the gizzard of the organism to aid it in grinding the food while the muscles of the stomach move. They have a slick appearance with rounded edges. However, they can be termed as gastroliths only if they are discovered with reptile remains. It is common to confuse gastroliths with pebbles as both have the same appearance. However, gastroliths are usually discovered in the proximity with stomach region of the fossil bones. Gastroliths indicate the eating patterns as well the habitats of the reptiles.
Trace fossils – They record the biological activity of the organisms, mostly in sedimentary rocks. They are useful as they indicate the structure as well as the actions of the organisms. Footprints, trail prints, track marks, borings, burrows, and such are trace fossils. Trace fossils are difficult to determine by the type of organism that produced them, and are rather classified by the behavior of the organism producing them. For instance, walking, crawling, running, sleeping, activities of the organism.
There are many other forms of fossilization which lead to the formation of fossils. The ones discussed above are the primary ones. Fossil study is an important aspect in determining the linkages of evolution as well as probing and reconstructing origin as well as extinction of various species on earth.
Works Cited
"Fossilization." How Do Fossils Form. N.p., n.d. Web. 20 Nov. 2014. <http://www.fossilmuseum.net/fossilrecord/fossilization/fossilization.htm>.
"How Are Fossils Formed?" Fossils Facts and Finds.com. N.p., n.d. Web. 20 Nov. 2014. <http://www.fossils-facts-and-finds.com/how_are_fossils_formed.html>.
"How Do Fossils Form?" K-5 GeoSource Content Science Content Fossils. N.p., n.d. Web. 20 Nov. 2014.
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