What are the different modes of preservation of Fossils?

Various types of fossils are observed to have been preserved within the sedimentary formations.

The type of fossil records depends mostly upon the nature and composition of the original part and different conditions of fossilization.

From the point of view of their mode of preservation, several distinct types of fossils have been recognised. Some of the important modes of preservation of animals and plants are given below.

Preservation of the whole organism, including its soft parts:

Dead bodies with soft parts, though very rarely, found to have preserved under most favorable conditions. The woolly mammoth with unaltered hard and soft parts preserved under continental ice sheets unearthed from Siberia the whole body of rhinoceros preserved in natural asphaltic oil pool in California, various insects preserved in amber (fossilized resin) are some of the examples. Besides, quite a good number of impressions of soft parts are found to have been preserved as fossils.

Preservation of unaltered hard parts:

The hard parts of various organisms are composed of minerals like calcite, aragonite, silica and chitin. The original content of the hard parts are commonly destroyed and replaced by secondary mineral matter dissolved in the ground water. But under certain selective conditions, exoskeletons of arthropods and shells of brachiopod, mollusk and bones and teeth of vertebrates are found preserved unaltered at times in their original color and luster.

Carbonization:

In selective cases, the soft and semi hard parts like leaf and wood of some plants undergo decomposition and lose nitrogen and oxygen while its residual carbon is preserved providing a clear impression of the original plant body. Conversion of vegetable matter into coal takes place in this process.

Different stages of formation of moulds and casts 5.3.6. Other types of fossils: In selective cases, the foot prints and trails of animals, burrows and borings of worms and mollusks are preserved as mould and casts. Gastroliths are smooth rounded pebbles found in the rib cages of dinosaurs and crocodiles which probably aided in their digestion, the fossil excreta providing clue to diet as well as the intestinal structure of the ancient animals are preserved as fossils; these are known as coprolites.

Metrification:

In this process the original organic substances are completely replaced by inorganic substances in such a way that the original internal and external structures are well preserved.

This replacement takes molecule by molecule; one molecule of the organic substance is taken away by the ground water and substituted by an inorganic molecule while keeping the original external and internal structure intact even in the microscopic dimension. This is a very slow process in which large tree trunks are fossilised.

Moulds and casts:

In majority cases, the original substance present in the hard parts buried under the sediments are totally removed in solution. In the process a hallow space is created within the rock layer which is a replica of the original hard part.

The wall of this cavity with the ornamentations of the original hard part is known as external mould. At times the inside space of the hard part is totally filled with sediments before its final dissolution and preserved as internal mould, which may bear its internal ornamentation. When the space between the internal and external mold is filled in with very fine sediments and preserved they are called casts depicts different stages of formation of moulds and casts.