fossil

These unique deposits are formed by the growth of bacteria around old tree stumps. The bacteria have become fossilized, and now form a fossil forest in Dorset, England. The fossil forest is even more spectacular because of the concentration of so many examples in a relatively small area.

One way in which fossils were formed. When a marine animal dies it sinks to the sea floor (if it is not eaten), where it is eventually buried by sediment. The soft body parts decay and the hard parts may be preserved. Over time more and more sediment accumulates forming layers of rock.

Cast, impression, or the actual remains of an animal or plant preserved in rock. Dead animals and plant remains that fell to the bottom of the sea bed or an inland lake were gradually buried under the accumulation of layers of sediment. Over millions of years, the sediment became sedimentary rock and the remains preserved within the rock became fossilized. Fossils may include footprints, an internal cast, or external impression. A few fossils are preserved intact, as with mammoths fossilized in Siberian ice, or insects trapped in tree resin that is today amber. The study of fossils is called palaeontology. Palaeontologists are able to deduce much of the geological history of a region from fossil remains. The existence of fossils is key evidence that organisms have changed with time, that is, evolved (see evolution).

About 250,000 fossil species have been discovered - a figure that is believed to represent less than 1 in 20,000 of the species that ever lived. Microfossils are so small they can only be seen with a microscope. They include the fossils of pollen, bone fragments, bacteria, and the remains of microscopic marine animals and plants, such as foraminifera and diatoms.

Fossils may be formed in various ways. They may be formed from the hard parts of animals which do not decay easily, or from parts of animals or plants which have not decayed because one or more of the conditions needed for decay are absent. Fossils may also be formed when parts of the plant or animal are replaced by other materials as they decay, as preserved traces of animals or plants, for example, footprints, burrows, or rootlet traces. These conditions are rare and so the number of fossils is tiny in comparison to the number of organisms that once lived. Knowledge of the stages of evolution of most organisms is often incomplete because of this. Now and again, a fossil is found that appears to show a link between two different groups. One example is the fossil Archeopteryx. This organism is part reptile and part bird in its features and could be the ancestor of modern birds, which it is believed have evolved from reptiles.