Preservation Of Fossils

Fossils are the remains of plants and animals found buried in sedimentary rocks. To be preserved, an organism must leave some permanent record in the rock such as a shell, a burrow or some other impression. But it is also necessary for the sediment in which fossils occur to be preserved; this can only happen by subsidence of the area where the sediment is deposited.

The process of subsidence does not affect all habitats equally. In upland areas the remains of the inhabitants tend to have been eroded or to have disintegrated before burial. Most of the rocks in which fossils are found were originally laid down in seas or in low-lying land areas. There is thus a strong bias in the geological record against the preservation of terrestrial organisms, especially those that lived in upland areas.

Examination of a physical map of Britain today shows that south-east England between the Wash and the Thames estuary is low-lying; here fluviatile sediments (interrupted by the occasional marine incursion like the 1951 surge) are accumulating in areas of subsidence like the Fens. Further east, sediments have been accumulating in the North Sea since the Carboniferous. Though neither the Fens nor the North Sea have had a continuous record of subsidence, both areas contrast with the greater part of the British Isles which have been generally characterized by uplift, especially in Wales, western Ireland and western Scotland, during most of the Tertiary. These areas are being eroded by rain and rivers. Any small areas of sedimentation are likely to be very temporary; all the sediments resulting from the erosion of upland Britain are carried out into the surrounding marine or fenland areas. The past sedimentary record is very similar. The majority of sediments are marine or deposited in deltas, estuaries or the lower reaches of rivers.

Most fossils are the hard parts of animals. Many soft-bodied animals are only known from rare impressions (often preserved in unusual conditions) or from burrows or other traces, which can seldom be assigned with confidence to any particular species. So a second bias in the fossil record is that towards animals with shells or bones. The fossils illustrated in this book are those that are most easily found; both types of bias apply to the examples given.

Some readers may think that there are far too many fossil names presented, but this has been done so that individual fossils can be looked up in other reference books if the reader wishes to find out more about them. Names should be used as reference indices, and not as long lists to learn by heart. Although many names are given, it must be stressed that the fossils in this book are only a very small sample of the more common genera. Rare fossils, many microfossils, and those that occur in unusual environments with a restricted geographic range are all omitted.

The major divisions of the animal kingdom are called phyla (singular: phylum). Most phyla have some members which secreted calcareous skeletons (Table II), and all phyla, except some groups of worm-like organisms, have some members preserved as fossils.

Calcium carbonate comes in two distinct crystalline forms: calcite and aragonite. Both crystal types occur in fossils, and in addition organically formed calcite may have a low or a high magnesium content. Aragonite and high-magnesium calcite are both more unstable than low-magnesium calcite if exposed to air or to fresh water (as will happen when rocks are uplifted to form land). This means that many benthic foraminifera, scleractinian corals, and various molluscs are often only preserved as moulds, or, if the mould is filled in, replaced by another mineral. Some sand formations may even have all their calcareous fossils removed by solution.

The hard parts are primarily for support in the Protista, the Porifera, the Coelenterata, the Hemichordata and Chordata: they also serve for protection in the Bryozoa, the Brachiopoda, the Mollusca, the Arthropoda and in some Echinodermata; but in many animals they perform several additional functions, notably in brachiopods, molluscs, arthropods and chordates where they often serve as muscle attachments. In these latter groups a careful study of the hard parts can give the palaeontologist a lot of useful information about soft parts of the animal that are not preserved fossil and on how extinct animals lived (functional morphology).

Burrowing animals frequently have thinner shells than have epifauna living exposed on the sea floor. Burrowers are, however, less likely to be destroyed by currents or free-swimming predators and scavengers. Mechanical abrasion can seriously affect the preservation of some epifauna. Experiments have been conducted by tumbling shells in a barrel with pebbles and noting how soon they were ground down; bryozoans and some calcareous algae did not last long compared with gastropods, bivalves or corals (Chave,

Table II

Composition of the hard parts of animals.

1964). Fossils preserved in coarse sediment may have been treated in a similar manner, with selective mechanical destruction of the less robust material.

Was this article helpful?

0 0

Post a comment