10.4 4.2.5 fossils

Introduction

Fossils are traces of organisms which used to be alive. Fossils can tell us something about the creatures they came from. Hard tissue, such as bone orshell, is fossilized better than softer body parts. This is to be expected, since soft tissue is eaten or it decays soon after death, leaving the bones andshells to petrify (turn to stone).However, examples of perfectly fossilized soft creatures, like jellyfish, do exist. In this section, you learn abouttheories of how fossils form, types of fossils, how fossils are dated, and fossil tourism. People who study fossils are called paleontologists.

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The way fossils form: Interactive site: http://www.flashyourbrain.com/pieces/fossils/index.php

Fossil formation

Different kinds of fossils are formed in different ways. Many form by petrifaction, i.e. turning into stone. The following may petrify:

• Parts of the body
• Sediments surrounding the body. Sediments are layers of material (e.g. sand) deposited (dropped) by wind / water / ice.
• Cavities (spaces) left by the body.

Scientists have observed petrifaction occurring both in nature and in laboratory experiments. Minerals(inorganic compounds) enter the body, hardening it andturning it to stone. Fossilization has been observed to happen very quickly when high concentrations of suitable minerals were present, and when theconditions were favourable for certain bacteria which aid the petrifaction process.

Fossil types

There are several kinds of fossils. These include footprints, dung, moulds, casts, permineralised, and trace fossils.

Footprints, made when creatures walked in soft sand or mud, can be preserved (kept safe) when the sand or mud dries, hardens and then petrifies. From thefootprints we can deduce the shape and size of the feet and how the creature which made them walked. A fossilized dinosaur footprint is shown in Figure 1.

Dung can also become petrified. Petrified dung is called a coprolite. From this we can deduce what kind of food a creature ate.

Mould fossils form around a cavity which the organism used to fill. A dead creature can be buried in sediments. These may petrify, forming a mould aroundthe body. The soft body of the creature decays (rots), leaving the mould empty. The shape of the body can be seen from the mould.

Cast fossils look the same as the original body, except that they are made of minerals. Minerals (e.g. calcite) fill the cavity inside a mould, making a castfossil. A cast fossil of a shelled sea animal is shown in Figure 2.

Permineralized fossils form when the body of an organism becomes petrified. Water moves through the tissues of the dead creature and leaves mineralsbehind. These minerals replace the living tissue. These fossils can give a very good idea of the original structure of a dead creature. A permineralised fossilof a tree stump is shown in Figure3.