Palaeontology

Study of fossils is called palaenotolgy. Fossils are defined as preserved remains of a living organism that existed on earth a long time ago. Fossils are formed by the preservation of the remains of the organisms that existed in the earlier days. The dead bodies of the organisms are immediately covered by materials which do not allow decomposition at a fast rate. This preserves the dead bodies which form fossils. For example, we know of the existence of dinosaurs only through fossils.

In addition to knowledge about the extinct forms, fossils also provide links between two groups indicating that perhaps one species evolved from the other. For example, a fossil called archaeopteryx show features of both reptiles (teeth) and birds (wings).

The Fossil of the ArchaeopteryxArchaeopteryx the Connecting Link between Reptiles and Birds

Thus it provides a link between birds and reptiles.

The Different Types of Fossils

Entire Organisms

In rare cases, the entire organism may be preserved. This may happen if the dead bodies of the organisms have been frozen into ice during glaciation, encased in hardened resin or tar or trapped in acidic bogs (bogs are marshy areas where acidic conditions prevent any bacterial or fungal decomposition of the bodies).

Hard Skeletal Remains

The skeleton or its parts get preserved with the living matter completely disintegrating.

Moulds and Casts

The skeleton remains for sometime and the sediments harden around it. The soft parts disintegrate and the gaps are filled by fine materials. The skeletal parts also dissolves leaving behind only the moulds and casts. Many details are preserved.

Petrifactions

They are similar to casts in formation but are formed by mineral deposits such as silica, calcium carbonate, etc. Finer details are also preserved.

Impressions

They are only impresssions of the remains of the organisms on the fine-grained sediments.

Imprints

They are footprints, trails, tracks and tunnels made by various organisms that are rapidly baked and filled with deposits.

Coprolites

They are faecal pellets that contain evidences of the food eaten. For example, teeth and scales.

The oldest fossil-bearing rocks contain fossils of few organisms that were simple. However, the younger rocks contain more fossils and these fossils show more complex structure. This indicates that the complex organisms originated from simpler organisms.

One of the best examples of evolutionary record (phylogeny) is of the horse. Almost the complete fossil record was found in North America. The phylogeny of the modern horse can be depicted as follows:

Stages in the Evolution of Horse