What are compactions ?

Compactions are fossils that have undergone some degree of flattening of their three- dimensional structure. However, these fossils are distinguished from compressions in their degree of flattening in that they are not as extreme. Compactions often contain a larger percentage of the original three-dimensional structure of the organism in their more or less correct proportions. This fossil, though, occurs more often with plants since their cells all have either cellulosic or lignified cell walls. This allows for a more uniform degree of resistance against compression within most of the plant. Animals, for instance, often contain both hard (shell or skeletal) and soft tissues, creating varying degrees of resistance to flattening from the sediment that buries it. For instance, in a dinosaur, its skeleton would be highly resistant to flattening (compaction) because of its rigidity, but the soft tissue surrounding those bones would not provide it with any type of support. Another example could be an arthropod, such as a crab or lobster. This time, the surrounding shell is the hard tissue, but inside is the soft tissue. Again, the hard shell would not be able to be as resistant to flattening because it would have little support from the soft insides of the organism. However, all the cells of plants have stiff cell walls, so the cells can support one another when sediments bury the plant and attempt to flatten it through pressure. However, even plants can not resist the compressive forces of layers and layers of sediment as they build up over time. Thus, compactions are also more commonly found amongst the more recently fossilized plants.

This leads to another difference that distinguishes compactions from compressions, which is the presence of organic material found in compactions. Compactions, being more recent in origin, are thus able to preserve that organic material before time has taken its toll and caused it to decay through chemical reactions with the environment. However, cellular detail is often lost from some of that flattening done to the fossil sample. The cells begin to press into each other during flattening, and, in more extreme cases, the cells become forced into each other such that the cell walls begin to break down and the chemical components of the cell walls "run" into each other.

What do compactions tell us ?

Compactions, because they retain much of the original three-dimensional shape of the organism, tell us most about the internal structure of the organism. They also allow for some examination of the organic material of the organism that was preserved in the fossil. This is especially true of pollen and spores of plants. Pollen and spores are covered in an outer shell made up of sporopollenin, an especially rigid and water-proof material that is also very resistant to flattening. Thus, the internal structures of pollen and spores can be examined closely as well.

What are the best conditions for compactions? Typically, the organism should have as much a uniform resistance to flattening as possible. Compactions, however, can not withstand the pressures of layers of sediment that bury it forever, so they are also usually rather recent fossils as well. The tissues would also have to be preserved fairly quickly to minimize mineralizations of the cells themselves. To better retain cellular structure, however, an increase in surrounding temperature and pressure would have to exist. This would allow more carbon, the basic organic element, to be retained in the cells and thus also the organic matter.

Index

Amber || Casts & Molds || Compactions || Compressions || Coprolites & Gastroliths

Drying & Dessication || Freezing || Impressions || Molecular Fossils || Permineralization

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