Window to the past
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.
Amber || Casts & Molds
|| Compactions || Compressions
|| Coprolites & Gastroliths
Drying & Dessication || Freezing
|| Impressions || Molecular
Fossils || Permineralization
Reference || Trace
Fossils || Wax & Asphalt
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