PALAEONTOLOGY 157 articles on the various groups of fossils, and this article is designed simply so that the enquiring reader may find his or her own way through the ensuing contributions The Preservation and Condition of Fossils Very few fossils are found that show no changes from the original organism There is a very approximate correlation in many cases between the age of the fossil and the degree of change, but age is in itself no guide For example, a fossil may be distorted by tectonic activity within a few hundred thousand years of its deposition, whilst other rocks and their contained fossils undergo little tectonic change over long periods For example, the Lower Cambrian shale in the St Petersburg area of Russia, which is some 430 Ma old, is so soft that it can be dug with a spade, and the marine shells preserved within it can be washed out and look not dissimilar in preservation from those found on a beach today The term diagenesis (see Diagenesis, Overview) describes the changes within both rocks and fossils as the sediments become dewatered and chemical transformations occur Voids, from microscopic size upwards, present both in the original organism and in the surrounding sediment, are usually filled during diagenesis as a result of mineral-charged fluids circulating through the rock This may occur at any time from soon after the fossil was deposited to many millions of years later, when the entombing rock is subjected to sedimentary pressure or tectonic events Replacement of the chemicals in the fossils themselves is widespread; for example, the calcium carbonate (CaCO3) of a brachiopod shell is often replaced by silica (SiO2) during diagenesis Fossils may also be hugely distorted from the original shape of the organisms by tectonic processes In addition the fossils may dissolve, and their chemical contents may form other substances; for example all ‘fossil fuels’, such as crude oil (see Petroleum Geology: The Petroleum System) and natural gas, are made up of the concentrated remains of fossil plankton, and coal is formed from the now usually unrecognizable remains of fossil plants Another process is termed disarticulation Obviously, if an animal or plant has only a single hard part, such as the shell of a snail, then that may become weathered or broken, but there is still only the one shell However, if an organism has more than one hard part (ranging from the two shells of a bivalved mollusc or brachiopod to the several hundred bones found in a mammal skeleton), then the process of transport from its place of death to its final burial place, where it will eventually form a fossil, will often cause the organism to break up or disarticulate The degree of disarticulation often depends on how strong or weak the original articulation was; for example, some brachiopods have weak hinge structures, and others, such as the common Silurian genus Atrypa, have complex interlocking mechanisms between the two valves and are thus very commonly found fossilized with their two valves still closed together Classification of Organisms Fossils and organisms living today are all included within the same system of classification: whether an organism is living or dead, or the species extinct, is not relevant to its position in the classification Of course, all the various systems of classification, whilst attempting to reflect truly natural relationships and groupings, are actually man made and in the last analysis subjective, although objective measurements and methods of analysis have in many cases assisted the systematists in their analyses and helped them to reach their taxonomic conclusions The term ‘systematics’ is used in a specialized sense by biologists and palaeontologists to describe the study and arrangement of organisms into classifications and hierarchies All organisms are classified within the binomial method; in other words, each organism or taxon is defined as a species within a genus The binomial system was invented by the eighteenth-century Swedish naturalist Carl Linnaeus (or Linne´ ) Linnaeus published many classificatory books and papers, and subsequent systematists have agreed that the 12th edition of his book Systema Naturae, published in 1758, should be the technical starting date for animal classification; however, there are several different starting dates for plant names All generic names start with capital letters, and all species names are written in lower case: for example, our own genus is named Homo and our species name is sapiens Although the generic and specific names are the formal minimum for all distinctively named organisms, some genera are divided into subgenera (which are therefore above the species level), and some species are divided into subspecies For example, the English song thrush is the species Turdus philomelos, and those birds in my garden in London are the main subspecies Turdus philomelos philomelos, whilst the subspecies occurring only in northwest Scotland is Turdus philomelos hebridensis Generic, subgeneric, specific, and subspecific names are normally printed in italic Above the genus level, organisms are grouped within families, which may again be divided into subfamilies or grouped together as superfamilies Above the family are the order, the class, the