|Themes > Science > Paleontology / Paleozoology > Fossils And Fossilisation > Fossilation: the process and its limitations|
Nearly everything we know about life in the past, and about the evolution of organisms is derived from the study of fossils. The word 'fossil' literally means 'something dug up', but it is now used in the technical sense of 'remains or traces of animals and plants, which lived under conditions different from those of the present'. The science of burial is called Taphonomy .
This definition needs careful consideration. The remains of plants, as in coal or lignite, are usually found in forms of carbon. The remains of animals usually consist of the hard parts of bones, teeth or shells. Traces are such things as footprints, coprolites (i.e. fossil dung or faeces), dwellings, burrows and eggs. Conditions different from those of the present normally excludes an organism which has lived in the last 1000 years or so. The mummy of an ancient Egyptian or Inca, or the skeleton of a bronze age donkey would not be regarded as a fossil.
Fossils are found in sedimentary rocks -
mainly limestones, siltstones and mudstones - that were deposited in
former lakes, rivers, estuaries, seas and caves. Fossils are never
found in igneous rocks nor in highly metamorphosed rocks. The only organic
remains which are available to us are those which have been preserved in
rocks. There are some rare cases of preservation of soft tissue, as for
Fossil photographs by Conway Morris and Whittington,Courtesy of the Geological Survey of Canada
Preservation and Fossilisation
In vertebrates, the hard parts such as bones and teeth are composed mainly of calcium phosphate, Ca5(PO4)3, with combined fluorine, chlorine or hydroxyl. This composition is related to, or identical with, that of the mineral apatite and all such phosphates are relatively insoluble. Most invertebrates (e.g. molluscs, echinoderms, crustaceans and coral) construct their shells out of calcium carbonate, although many arthropods such as smaller crustaceans, insects and arachnids construct their hard parts of chitin.
Fossil bones, and especially the enamel of teeth, are likely to be stained by chemicals in the sediment which buries them; many bones are black or dark brown when discovered. Apart from staining, the minerals of bones are not altered, although calcite (CaCO3) or silica (SiO2) may be deposited in cavities that were empty in the living animal or were occupied by soft tissue. Bone protein may be replaced by minerals which strengthen the fossil.
In some fossils the form of the hard parts may be preserved but the mineral matter may be replaced. Soluble calcite of a shell may, for example, be replaced by relatively insoluble silica. This process is properly known as 'petrification' (literally, 'turning to stone'). Petrification takes place by detailed molecule by molecule replacement in which the full detail of the original structure is preserved. Petrified plant remains have provided important details of early plants. Animals and plants which are composed entirely of soft parts may be preserved, under some circumstances, by carbonisation, that is a slow distilling process by which oxygen, hydrogen and nitrogen are driven off, leaving behind a concentration of carbon. Compressed (squashed flat) carbonised fossils are the normal form of preservation of plant remains in coal and lignite.
A cast (or mould)
of soft tissue may be created if sediment or mineral crystals form in a
cavity within a skeleton, or as replacements for material which has
decayed or been lost in solution. The bodies of the inhabitants of Pompeii
who were buried by volcanic ash when Vesuvius erupted in A.D 79 became
encased in a strong material as the ash hardened. Most of the bodies
decayed, leaving cavities which preserved the exact shape of each body.
When Pompeii was excavated, plaster casts were made from these cavities,
thus giving an impression of the original people. In natural conditions,
sediments are often carried into the brain cavity of a skull
Burial is an essential part of the process of fossilization. Preservation of a skeleton is most likely to occur if burial occurs soon after death. Such an event is very rare. Terrestrial organisms usually fall to the ground at death. There they may be subject to a range of processes:
Decay within the soil is largely controlled
by soil acidity: in wet acid soils containing air, bone dissolves
completely; in wet alkaline soils bone is preserved; in dry alkaline
soils, bone protein is lost but a weakened fossil is preserved; in wet,
airless, acid soil (such as peat) soft tissue may be preserved with bone. A
result of such processes is that the great majority of animals which fall
on land are not preserved in the fossil record. The best chances
of preservation occur when an animal falls into a fissure or cave, is
drowned and sinks to the bed of a lake, sinks into a swamp, is swept by a
flood into a swamp or lake, is buried in a cool volcanic ash shower, or is
overtaken by some other rapid process which preserves the body intact and
buries it quickly. The great majority of hominid and early human remains
have been found in cave deposits, river terrace deposits, lake beds and in
down-faulted troughs (such as East
A rare but important natural trap for animals is a tar pit. Petroleum and natural gas migrate upwards in sediments because of their low density. Evaporation of the light elements leaves a layer of tar which may be covered with water in a wet season. Animals coming to drink become trapped in the tar and their bodies attract carnivores, carrion eaters and other scavengers. Over a long time the tar helps to preserve their skeletons. Perhaps the best example of such a swamp is the asphalt tar pit at Rancho La Brea in central Los Angeles. There, preserved remains of animals which lived during the last glacial have been found including elephants, camels, early horses and sabre-tooth cats. Tar pits in the Ukraine have yielded trapped woolly rhinoceroses and many birds.
A consequence of the emplacement and burial processes, is that only a highly selected and incomplete set of samples of organisms is likely to be preserved in the fossil record: generally, aquatic organisms stand the best chance of being fossilised. Of early human traces, it is likely that stone tools will be preserved to a vastly greater extent than human skeletons. Furthermore, the skeletal parts which do survive are likely to be scattered and to be of the most durable parts - jaws, skulls, teeth and to a much lesser extent parts of the pelvis, limbs and spine. The skull is likely to be squashed or broken and often separated from the jaw.
The fossil record is further limited by erosion of sediment bodies and rocks by streams, waves, wind and ice. Whole river floodplains and terraces may be removed by the river that originally deposited them. Bodies of sediment may be tilted up and exposed to erosion as a result of ground movements.
The record is further limited to exposure of the sediments. Most fossils are found on hillslopes, valley walls and cliffs where weathering of the sediment may leave the fossil protruding and obvious. Such finds may encourage scientists to excavate in search of further fossils, but it is rare for such exploration for fossils to commence with a digging or drilling programme which will study a large rock mass.