Fossils: Window to the past

Compactions

What are compactions?

Compactions are fossils that have undergone some degree of flattening to their three-dimensional structure. However, these fossils do not expereince the extreme degree of flattening found in compressions. Compactions often contain a larger percentage of the original three-dimensional structure with more or less correct proportions.

This fossil occurs more often with plants as plant 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 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, the 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. In this case the surrounding shell is hard tissue, but there is also soft tissue inside. Again, the hard shell would not be as resistant to flattening because it would have little support from the soft interior of the organism. All the cells of plants have stiff cell walls. 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 more commonly found amongst recently fossilized plants.

The presence of organic material found in compactions also distinguishes them from compressions. Compactions, being more recent in origin, are able to preserve organic material before chemical reactions with the environment cause more decay. However, cellular detail is often lost from some of the flattening done to the fossil sample. The cells begin to press into each other during flattening. In more extreme cases, the cells become forced into each other and the cell walls begin to break down. The chemical components of the cell walls the may 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 much about the internal structure of the organism. They also allow for some examination of the organism's organic material preserved in the fossil. This is especially true of pollen and plant spores. Pollen and spores are covered in an outer shell made up of sporopollenin, an especially rigid and water-proof material that is very resistant to flattening. Thus, the internal structures of pollen and spores can be examined closely.

What are the best conditions for compactions? Typically, the organism can not withstand the pressures of layers of sediment that bury it forever. Recent fossils work best. The tissues should also be preserved fairly quickly to minimize mineralizations of the cells themselves. To better retain cellular structure, an increase in surrounding temperature and pressure should exist. This would allow more carbon, the basic organic element, to be retained in the cells and thus also the organic matter.