fossils and fossilization

fossils and fossilization Fossils (Latin fodere, to dig up) are the remains of once-living organisms preserved in rocks or sediments. The term originally referred to anything dug out of the ground, whether animal or mineral and, therefore, included gems and minerals, archaeological remains, and even strange-shaped nodules; however, the measuring of the word has gradually become more restricted. We now generally divide fossils into two main types: body fossils, which are the remains of hard parts or more rarely, soft parts of organisms, and trace fossils, which are the traces of biological activity, such as burrows and tracks and trails. The record of fossils goes back 3400 million years to preserved single-celled organisms (bacteria and blue-green algae). Complex organisms or metazoans do not appear in the fossil record until about 700 million years ago when the Ediacara fauna has an almost worldwide distribution. Hard parts were not commonly present in organisms until about 544 million years ago at the beginning of the Cambrian Period, when there was a tremendous development of invertebrate organisms often called ‘the Cambrian Explosion’. Since then a diverse assemblage of invertebrates and vertebrates is available throughout Phanerozoic time for study by palaeontologists.

Fossilization is a rare occurrence, and it has been estimated that of the more than one million living species only 10 per cent are likely to be preserved as fossils. It must be appreciated, therefore, that the fossil record is exceedingly incomplete and biased. The natural preservation of an organism, when it does occur, is dependent on a number of factors, of which the main ones are: the composition and structure of the organism; its abundance; the sedimentary environment in which it lives; and what post-depositional changes take place.

All organisms are composed of delicate tissues known as soft parts, but many also have more resistant tissues referred to as hard parts. The hard parts may be mineralized, as in the shells of bivalves, or composed of organic material, such as the chitin that makes up the exoskeleton of arthropods. Although in general the possession of mineralized hard parts is a prerequisite for preservation, soft-part preservation can occur and some entire fossil groups, such as the graptolites, have only organic hard parts. Another important fea-ture is the density of the hard parts, particularly from the point of view of the ratio of organic to inorganic material; in general a low proportion of organic material favours fossilization.

The numerical abundance of organisms is generally considered to be important, since, all other things being equal, abundant organisms would seem to be more likely to be preserved than rare ones. It has, however, been shown in a number of instances that the relative abundance of an organism in the fossil record does not accurately reflect its abundance in the living population. This differential preservation is frequently related to the preservability of the hard tissues.

The most common sedimentary rocks are those deposited in shallow-marine environments, and because of this the majority of fossils are of organisms that lived in those environments. Deep-water sediments are less commonly exposed, and in consequence fossils of organisms that lived in those environments are rarely found. Terrestrial organisms generally lived in areas of non-sedimentation and thus stood little chance of preservation. In addition, the type of sediment will have an effect on preservation: coarse sediments, indicating high-energy conditions, are unlikely to contain fossils, whereas fine-grained sediments are more conducive to preservation. Sediments, while directly preserving organisms by enclosing them, can also produce indirect fossils by creating impressions of surfaces that are termed moulds. These may be impressions of the outer surface, or of the inner in which case they are often called steinkerns (Fig. 1).

Post-depositional changes are important because both biochemical and chemical processes act upon the remains of the organism to preserve or destroy it. Groundwater percolating through sediments can dissolve enclosed hard parts, removing all trace of the incipient fossil, though if an external mould is left this might be filled later by other minerals to form a cast (Fig. 1). Minerals can also be deposited in the pore spaces of hard parts such as bones in a process called permineralization, thus strengthening them and improving the chances of preservation. A mineral may also be completely replaced by another mineral; for example, calcium carbonate shells can be replaced by silica or pyrite (Fig. 1). Another process that destroys details in a fossil, although it will improve the preservation potential, is recrystallization. In this process less stable minerals forming hard parts are transformed through time to more stable forms, as when gastropod shells formed of aragonite are transformed into calcite.

Fossils are the main phenomena studied by palaeontologists to gain a better understanding of evolution, extinctions, palaeoecology, and many other topics. Preservation is of vital importance to the acquisition of this information and it is important that it should be understood.

David K. Elliott

Bibliography

Briggs, D. E. G. and Crowther, P. R. (eds) (1990) Palaeobiology: a synthesis. Blackwell Scientific Publications, Oxford.
Clarkson, E. N. K. (1993) Invertebrate palaeontology and evolution. Chapman and Hall, London.