Introduction to Fossils

Fossils are the remains of plants and animals as well as traces of their activity found in the rock record. Fossils can tell us much about life of the past, about their external size and shape as well as, in some cases, about their internal structure and chemical composition. Sometimes we can find evidence of their diet, habitat and life cycles.

Plants and animals are preserved rarely, however, and this may depend on special circumstances. The basic principal is that the animal or plant must be buried quickly to exclude oxygen and prevent decay. Most fossils are preserved, therefore, in sedimentary rocks laid down in the sea or in fresh water and are not generally found in igneous or metamorphic rocks where they may have been destroyed. However some fossils are found in volcanic ashes and lavas.

Many invertebrate animals have readily preservable hard parts, such as shells made up of Calcite (calcium carbonate). Sometimes these accumulate in large numbers to form limestones. In other cases the animals hard parts are made of other materials such as silica or phosphate. Some minerals are unstable, such as the mineral aragonite (also a form of calcium carbonate) and these may be dissolved and be replaced by other minerals or sediment giving a mould. Iron pyrite is a common mineral replacing some shells and also forming moulds. Most often the soft tissues of the animals decay rapidly but under exceptional circumstances such tissues may be preserved. Such exceptionally preserved fossils are widely studied (such as insects in amber, or vertebrates from ice or tar pits). In some cases the tissues also yield chemical data although claims for very ancient DNA must be treated with caution. In some fossil invertebrates soft tissues may be preserved as bacterial films or as clay mineral replacements.

Vertebrates are often found as fossil, but most commonly as isolated bones and teeth. Because many vertebrates live on land they are less likely to be found whole as they are often scavenged for food. The teeth and bones material may be recrystallized or replace by the growth of new minerals. In some exceptional cases mass accumulations of whole animals are found indicating mass death such as by a volcanic eruption of because of a sudden pollution event.

Plants may be preserved in a variety of ways. Some microscopic plants, such as diatoms and coccoliths are rock formers. Coccoliths have ‘skeletons’ made of calcium carbonate and form chalk, whereas diatoms are made of silica and form diatomaceous earths. Most plants are made of organic tissues. The plants may be buried and the organic matter preserved and this results in the formation of compression/impression fossils. The plant-bearing sediment may be buried and subjected to heat and pressure and the organic material will be coalified. It is possible that original organic chemical imformation of the plant tissues is preserved but often there are alterations which take place during burial. In some cases minerals will precipitate (such as calcite, silica, iron pyrite) within spaces within the organic matrix and give a permineralization where the three dimensional anatomy is still preserved. Subsequently the orgainic material may decay and another phase of mineralization takes place to give a petrifaction. Here no original organic material is preserved. Organic material may decay after the plant is buried in sediment and the space left is filled with sediement giving rise to a cast and mould. Finally plants may be burnt and not completely destroyed. Plant tissues may be charcoalified (turned to almost pure carbon). In these cases the charcoal may preserve exquisite anatomical detail and this a common, yet understudied, from of plant preservation.

Finally activities of living organisms may be preserved as trace fossils. These include evidence of movement (such as trails or footprints), resting and feeding. Feeding traces may be preserved in sediment in marine settings for example. Such traces may be common churning up the sediment giving rise to ‘bioturbation’. In other cases, such as on leaves and in woods traces may show the feeding activity of insects, or holes in marine shells may show predation by snails. Studies of such trace fossils are very valuable in helping us understand the lifestyle of ancient organisms. This can be combined, as in some vertebrates, with a study of teeth morphology and gut contents to provide a more dynamic reconstruction of many extinct animals.