What is a Fossil?
Fossils are remains or traces of ancient organisms. They can provide direct evidence of past life through the preserved or fossilized remains of the actual organism in either an unaltered or altered state. Fossils can also provide indirect evidence of past life through trace fossils. Trace fossils do not include any part of the actual organism, but provide evidence of the organism's existence. Trace fossils include tracks, trails, burrows, worm tubes (filled burrows), coprolites (animal dung which can indicate diet and gut size of an organism), artifacts (tools made by humans), etc.
Preservation of Organic Remains
There are two requirements for the preservation of organic remains. The first is the presence of hard parts like shell, bone, etc., and the second is rapid burial through rapid sedimentation. The optimal environment for preservation is a marine environment, in which there is a lack of oxygen and a low energy environment. This type of environment slows down the decay process and the breakdown of organic remains and is likely to result in the organism being rapidly buried in fine sediments. In a terrestrial environment, most remains are found in ancient wetlands and deep water habitats. The best preservation occurs in rapidly accumulating, fine grained sediments, which form rocks like shale, limestone, and siltstone. Coarse clastics such as sandstone and conglomerates have poor fossil preservation, because these types of rocks are indicative of an environment in which there is a lot of energy, and fossils may be unburied and destroyed. Porosity, the amount of space between the rock grains, and permeability, the ability of liquids or gases to travel through pore spaces in the rock, are high in these environments, which means that there is a lot of air in the larger pore spaces, thus resulting in an oxidizing environment which promotes decay.
Methods of preservation
There are two main types of fossil evidence; direct evidence and indirect evidence. Direct evidence is the preserved remains of all or part of the actual organism. Direct evidence includes unaltered remains. Unaltered remains are preserved without change and include shells, teeth, bones and even some soft parts of the organism. The preservation of soft parts is rare and usually occurs in the Quaternary period, which includes the Pleistocene epoch (about 1.6 million years ago to 10,000 years ago) and the Holocene epoch (10,000 years ago to present). Methods of preservation that result in unaltered remains include mummification or dehydration, frozen remains, remains that are encased in amber or asphalt (tar pits), and remains that have been preserved in brines and swamps.
Altered remains are preserved with structural or chemical change to the original organic remains. Permineralization, replacement, and carbonization are all processes that can preserve organic remains in an altered state. Permineralization occurs in porous remains like wood and bone when minerals from ground water or sediment precipitate into the pores in the remains. An example of permineralization is petrified wood.
Replacement is the very slow molecule by molecule exchange, or replacement, of the original organic material with minerals precipitated from the ground water. Replacement occurs in non-porous materials such as shells. The fossil shape and size are not altered during replacement, because of the slow molecule by molecule exchange. Replacement occurs with calcite, silica, pyrite, limonite, and dolomite.
Carbonization, or distillation, occurs through the compaction of sediments and the resulting reducing (lacking oxygen) environment. Carbohydrates (carbon, oxygen, nitrogen, and hydrogen) are driven out of the remains as gases through distillation. A black carbon film, often accompanied by an impression, is left behind (carbonization). Carbonization results in a lot of minute detail, and often includes plants, fish, insects, and graptolites.
Molds and casts are another form of altered remains. A cast occurs when the preserved remains of the organism have been completely removed, but left behind is a detailed impression of the organism in the rock. Often this detailed impression, or cast, is filled with minerals creating a mold. The mold, while not containing any part of the actual organism, is a detailed replica of how the organism appeared as it left the cast in the rock.
The Fossil Record
The fossil record is incomplete, because of how unlikely it is that an organism will be preserved as a fossil. There is only approximately a 1 in one million chance that an organism will die and remain in just the right conditions under which it will be preserved. There are, however, a number of fossils that are relatively common and therefore indicative, in and of a particular time period on the geologic time scale. These organisms are known as index fossils and are used to correlate geologic periods on earth. Fossils are used to date and correlate rock strata, and can reveal past climate, environment, geologic and tectonic movement, and can even be used in the modern search for natural resources as well as to model future shifts in the earth's climate, environment, and geology.
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