Fossilization: How Fossils Become Fossils

Fossilization is a complicated and intricate process that is cognizant of several biological, physical, chemical, and ecological factors. The science of fossilization is called taphonomy and is consisted of three stages: death, pre-burial, and post-burial.

Death

The first stage, death, can be due to old age, infection, predators, parasites, infection, environmental change, and other reasons. Usually, paleontologists only have a little bit of evidence to figure out the cause of death, so it is difficult for paleontologists to discern the causes of death usually. There are times when paleontologists do find the causes of death, such as in dinosaurs which commonly show indications of medical conditions through their bone structures, showing arthritis for example. In other instances, paleontologists can look at the insides of a predator and learn more about its diet to figure out the cause of death. However, this is not a common occurrence. The physical and chemical causes of death can serve as useful clues for paleontologists to understand the nature of the ancient environments that the organism may have lived in. Organisms trapped in amber or tar pits are the most prominent examples of physical deaths and the most famous tar pit is Rancho La Brea in Los Angeles, California. A variety of vertebrates once drank from a watering hole and became trapped, sucked in, and drowned in the asphalt under the surface.

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Pre-Burial

After death, the organism begins to break down. Chemical and physical breakdown is halted by rapid burial. Rapid burial is instantaneous with death and preserves the organism from scavengers and other factors that would hinder its preservation. An environment with low oxygen levels also has an effect in aiding preservation as seen with the excellent preservation of the Cambrian fossils found in the Burgess Shale. Typically, soft tissues are the first parts of the body to go due to chemical breakdown and the scavengers that feed on the organism. Usually these scavengers are most often found in marine environments with higher oxygen levels. In an environment with high oxygen levels, the tissue is often decomposed into water and carbon dioxide. If the environment does not have oxygen, the degradation process is not fully stopped, for biochemical reactions still break down the tissues into hydrocarbon molecules. After the soft parts of a fossils are dealt with, the hard parts are threatened by biological and physical processes. Tissue often keeps the skeleton together, so if the tissue were to disintegrate, the skeleton would also be in danger of not being preserved. Scavengers may eat the dead remains of the organism as well. Finally, wind and current may scatter the remains, accelerating the disintegration process. Dead organisms that lived in the water may drift for quite a while before sinking and becoming part of the seabed below. Displaced fossils are called allochtonous and may be identified through the physical wear of having been transported or the accumulation of some orientation due to the wind and current direction. On the other hand, fossils that are found in the same place that they died are called autochthonous. This is not common and only certain for certain fossils, like tree stumps and corals. Most of the time,fossils are described as semi-autochthonous, having been through some transport, but showing a stable position later.  From all of this, we can gain a better appreciation for how difficult it is for a fossil to be preserved.

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By Chris Light (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons Fossilized Redwood stump serves as an example of an autochtonous fossil.

Post-Burial

After burial, fossils can still be affected by a multitude of chemical and physical factors. Most of the time, soft parts are preserved under low-oxygen conditions and rapid burial. As mentioned before, the lack of oxygen slows down the degradation process, but even then, anaerobic bacteria can break down the soft parts. Usually, a replica or outline is made through minerals, such as pyrite, carbonates, phosphates, ad silicates. The chemical environment determines what mineral is used. The most common type of preservation is mineral coating which is when mineral growth happens on the surface of a soft part and leaves an outline. The Burgess Shale hosts a large number of fossils that are preserved by mineral coating. Other times, organisms leave an impression in sediments. Usually, burial stops biochemical degradation with the exclusion of oxygen. Despite this, the fossils can undergo chemical attacks from pore fluids in the sediments and physical disturbances from tectonic activity. These changes that a fossil may undergo after burial are called diagenesis.

File:Pleuroceras solare, Little Switzerland, Bavaria, Germany.jpg

By Llez (H. Zell). (Own work.) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons A replica of pleuroceras solare from pyrite

Diagenesis

Diagenesis consists of six components:preservation of original material, recrystallisation, impregnation, encrustation, solution, and compaction. The preservation of original material means restricting the pore fluids to assist in preservation. Recrystallisation is one of the most common effects of diagenesis and is achieved without change in chemical composition sometimes. In invertebrate shells, they often contain calcium carbonate (CaCO3), aragonite, which is quickly recrystallised to a more stable calcite phase. Other times, recrystallisation can lead to replacement. Silica (SiO2) and pyrite (FeS2) are two of the most common replacement minerals. Impregnation happens usually where fossils are porous. Mineralised pore fluids can lead to precipitation inside the pores of the fossils. Impregnation is sometimes referred to as petrification as a result. Encrustation occurs when the skeletal or plant material is surrounded by a crust. This happens near areas of hot springs, where mineral salts supersaturate the warm waters, leading to precipitation. Solution is the most common component of diagenesis. This part of diagenesis happens when the skeleton dissolves and leaves a cavity. This cavity may not be filled in which case the external features are replicated; this is called an external mold. On the contrary, a filled cavity will lead to a replication of the inner features and is called an internal mold. Many times, the cavity is filled by sediment and minerals that are precipitated after the original shell is disintegrated; when this happens, it is called a cast. The cast doesn’t have the actual remains of the fossil, but shows the external and internal aspects of the fossil. Compaction, the last component, happens after the fossil is in the sediment. Often times, fossils become crushed and deformed by pressure generated in the sediment and tectonic activity.

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External mold of Aviculopecten subcardiformis

Trace Fossils

Trace fossils are a type of fossils that consist of tracks, trails, burrows, feces, and other traces of fossils. Coprolites, the feces of earlier organisms, go through the same preservation process as body fossils, but other traces are either formed within the sediment (endogenic) or on the surface of the sediment (exogenic). Endogenic traces are more likely to be preserved than exogenic, but organisms can burrow into the ground, disturbing the fossil record.

Here is an infographic that summarizes the fossilization process that was discussed above, specifically for what happens with recrystallisation. Minerals often replace the bone structure of the fossil, preserving the fossil’s integrity.

fossilization

 

Works Cited
Doyle, Peter, and Florence M. D. Lowry. Understanding Fossils: An Introduction to Invertebrate Palaeontology. Chichester: Wiley, 1996. Print.

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