Fossilization is a rare event
of a particular living organism is sedimentary deposits is a rare
event. Yet, past life is so boundless in number that there exists,
in fact, an enormous quantity of fossils buried in the earth. Finding
and accessing those fossils, however, is not so easy. Normally, a
fossiliferous layer of sedimentary rock will be sandwiched between
layers that contain no fossils. The layers without fossils may be
widely separated from those with fossils.
Animals or plants become fossilized after burial by sediments. Sediments
are particulate matter usually transported by water that eventually
is deposited as a layer of solid particles on the bed or bottom of
a body of water or other liquid. Sedimentation is the deposition by
settling of a suspended material. For example, seas, oceans, and lakes
over time accumulate sediment that originates from both the land and
marine environments. Deposited sediments are the source of sedimentary
rocks that can contain fossils of the inhabitants of the body of water
that were, upon death, covered by accumulating sediment.
sediments accumulate on the remains of the organism, the pressure
increases, some water may be expelled and chemical changes occur to
produce sedimentary rocks such as shale, sandstone and limestone.
In harder rocks such as limestone, fossils may be more likely to retain
their original appearance, while softer rocks may compress or flatten
fossils the fossils (show Utah versus Oklahoma trilobites).
a branch of geology, is the study of rock layers and layering (stratification).
It is primarily used in the study of sedimentary and layered volcanic
rocks. It is useful to review the basic principles
of stratigraphy as set forth some three and one half centuries
ago by Nicolaus Steno.
they are imbedded, fossils experience the same geological history
as the rocks they in which they are contained (show distorted Housia).
Rocks may be mashed, twisted and distorted with the contained fossils
undergoing the same stresses and strains. Over geological time, temperature
and pressure may leave fossils into a morphological state markedly
altered from the initial organism, if not fragmentized. Fossils may
also have traveled great horizontal distances owing to plate tectonics
and great vertical distances owing to mountain building events (e.g.,
ammonites can be found in rocks at an altitude of over 5000 meters
in the Himalaya Mountains).
examples, that some say are the world's most significant are those
in the Burgess Shale, Canada. The Burgess Shale fossils are special
because of their great age, and their exquisite preservation.
are ways that fossils form:
mummification - complete preservation in a relatively undamaged
state of hard and soft body parts;
skeletons and shells - these are the most common fossilized hard
parts; they may loose their color but are unchanged in their chemical
petrification and replacement - impregnation of fossils by secondary
minerals which usually leads to an increase of weight and hardness
of the fossil;
carbonization - usually formed from woody and chitinous material,
which loses its oxygen and nitrogen through decomposition by anaerobic
impressions and traces - impressions left by dead animals that decayed
away or traces left by moving animals;
casts and moulds - where the rock containing the fossils has hardened
and the original fossil has been dissolved away, leaving a hole
which can often be filled by other sediment.
Principles of Stratigraphy