Fossils

Fossils are the remains of animals and plants which lived prior to historical times. Their study is a specialised branch of geology known as palaeontology.

As fossils are normally preserved by burial in mud and silt, it naturally follows that fossils are found almost exclusively in sedimentary rocks. Rare examples are known of animals found in volcanic rocks where they were trapped and overcome by lava flows or volcanic ash.

Flesh is subject to extremely rapid decay, and is rarely preserved in fossils; among the few exceptions are the frozen mammoths of Siberia and Alaska, and a rhinoceros found in oil sands in Poland. Thus, under normal circumstances, only animals possessing hard parts of shell, bone or chitin (the hard matter in insects) are capable of preservation as fossils. Rapid enclosure in a protective medium is necessary to protect hard parts from the destructive action of scavengers and the weather. This protection is usually accomplished by burial in waterborne sediments, thus giving aquatic, particularly marine, forms of life the best chance of fossilisation. Terrestrial animals have comparatively little chance of preservation unless washed into lakes, as the land surface is largely an area of erosion.

How a fossil forms

1. After death the animal is buried and natural openings in the shell allow the space formerly occupied by fleshy parts to be filled with sediment

2. When the enclosing sediment has turned to rock, impressions are made of both the interior and exterior of the shell. Should the shelly matter now be dissolved away internal and external moulds would be formed

3. Mineral solutions percolating through the rock can deposit solid matter in the cavity formerly occupied by the shell. This produces a cast of the original shell rather similar to the process of making a plaster cast of an object from a mould

After burial a fossil can undergo various changes, depending upon its physical and chemical environment after the confining sediments have turned to rock. Such influences can cause the formation of a variety of fossil types.

1. Unaltered hard parts. Under very favourable circumstances some shelly fossils have been preserved with the interior mother-of-pearl layer and the exterior colour patterns intact.

While this degree of preservation is far more typical of the youngest fossils, it has nonetheless been recorded in fossils of Cambrian and Ordovician age, some 500 million years old.

2. Altered hard parts. Apart from the examples cited above almost all of the older fossils have undergone some alteration. In many instances the alteration is discernible only at the microscopic stage. Most molluscs, for example, are composed of calcium carbonate in the form of aragonite. This particular mineral is unstable, and easily reverts to calcite, the stable form of calcium carbonate. Thus, few molluscs older than the Cainozoic Era (65 million years) are still unaltered mineralogically. Brachiopods, another group of bivalved animals, are naturally composed of calcite and so persist for much longer periods in an unaltered state. Heat and pressure acting upon fossils composed of calcium carbonate can cause the minute calcite fibres in the shell to amalgamate and grow into comparatively larger crystals, thus destroying any microscopic structure in the shell.

Apart from these physical changes, alteration of hard parts also can be brought about by chemical means. These changes are dependent upon the composition of the fossil, and the permeability and nature of the surrounding rock. Such forms of alteration can be put into three major groups:

A. Carbonisation is a feature typical of plant fossils and of animal remains which initially contained large quantities of organic matter. Under conditions of very rapid burial, or in deep stagnant water, oxygen being excluded in either case, organic matter is steadily reduced to carbon. Probably the best example of carbonisation is coal, consisting of plant material which has been reduced to almost pure carbon. The more delicate parts of plant fossils, the leaves and thin stems, are often preserved as carbonaceous impressions. Skeletons of fossil fish are commonly associated with carbon residues, the bones sometimes being completely obscured by carbonaceous matter. Graptolites, common fossils in Lower Palaeozoic rocks, are normally preserved as carbonaceous films in shales.

B. Replacement occurs when the fossil reacts with liquids percolating through the surrounding rock, or with fluids present at the time of burial of the organism. In both cases the mineral composition of the hard parts of the fossil differs from that in the living animal or plant. In stagnant or poorly circulated waters large amounts of hydrogen sulphide (rotten egg gas) are created by decaying animal matter. Together with iron salts in the sediment, this gas forms iron sulphide or pyrite, which can replace both calcareous and chitinous fossils. However, the micro-structure of fossils so preserved is usually obliterated. In similar fashion, fossils can be replaced by limonite or other iron oxides, although it is possible that many limonite fossils are only the result of weathered pyrite replacements. Silica solutions percolating through the sediments can also replace fossils, even when the fossil has the same composition as the surrounding sediment (e.g. calcareous shells in limestone, both of which consist of calcium carbonate). Silicified fossils in limestone are easily extracted by dissolving the limestone in acid. Silicified wood is often so perfectly replaced that even microscopic details of the tissue are preserved.

Moulds and casts represent an extreme case of alteration, as the hard parts are removed altogether. After burial, the interior of fossils is usually filled with sediment, which ultimately turns to rock with the confining sediment. Subsequent solutions percolating through the rock can dissolve the fossil, leaving a cavity in its place. The impression left of the fossil's exterior is known as an external mould, while the sediment which previously filled the interior of the fossil is known as an internal mould. Later deposition of another mineral in the cavity produces a cast of the original fossil.

In this category may be included moulds or impressions of animals which do not occur as actual fossils. Such soft bodied organisms as jelly-fish are totally lacking in hard parts and consequently have no chance of preservation under normal circumstances. However, when stranded by tides on a mudbank, the weight of the jelly-fish is often sufficient to cause it to sink into the sediment, leaving an impression when the flesh has decayed. If the deposition immediately afterwards is sufficiently gentle, the impression will be preserved as an external mould of whatever part of the jelly-fish rested upon the mudbank. It was probably in circumstances similar to these that the famous Precambrian fossils found at Ediacara, South Australia, were formed.

Indirect evidence of ancient life is shown by the preservation in sedimentary rocks of animal tracks, trails, and burrows. Except in the more obvious examples, such as footprints made by large reptiles, the animals which made these impressions can only be identified by intelligent guesses, particularly as there are only two authentic finds reported to date of fossils associated with the tracks they have made. It is not even certain if many of the trails observed were made by animals simply crawling from one place to another, or if they were ploughed by scavengers searching for food. Similarly some burrows are regarded as food gathering sites, others as living quarters.

Coprolites are fossilised dung pellets, and like trace fossils can but rarely be attributed to any particular organism.

Many collectors are deceived by certain inorganic structures resembling fossils, commonly termed pseudo-fossils. Dendrites, delicate branching deposits of water deposited minerals, are often mistaken for plant remains Concretions, formed by concentration of minerals in sedimentary rocks, frequently assume a variety of shapes some of which are mistaken for fossils. However, many concretions have fossiliferous cores.

Naming and classifying fossils

Time ranges and relative abundances of common fossil groups

All animals and plants, whether dead or alive, are identified by a dual name system, the first name being that of the genus to which the individual belongs, the second, or specific, name identifies the particular species within a genus.

As an example, the scientific term for man is Homo sapiens. The first part, Homo refers to the genus in which man is included; the second part, sapiens refers to the particular species of man known today. Closely related genera are grouped in families, closely related families grouped in orders, and so on, the highest category normally used being phylum. Man would thus be classified as follows:

Phylum: Chordata
Class: Mammalia
Order: Primates
Family: Hominidae
Genus: Homo
Species: Sapiens

The name of the person who first described the species often follows the scientific name e.g. Conularia chapmani Fletcher 1938. This means that Conularia chapmani was first named and described by Fletcher in 1938. When the author's name is enclosed by brackets, e.g. Denckmannites volborthi (Barrande 1852), it means that a subsequent worker has placed the species originally described by Barrande in a new or different genus.

Fossils when collected should be wrapped or placed in bags as soon as possible after extracting from rock to prevent any damage. In addition, a label or note giving exact details of the locality where the fossil was found should be added immediately. As the chief application of fossils is in dating particular beds of rock, a fossil without a locality is virtually useless.

Identification of Australian fossils is difficult for amateur collectors as most descriptions are scattered through a number of periodicals and monographs published by various learned societies. Some identifications can be made by comparison with museum exhibits, although this is only possible in Sydney.

The Australian Museum in College Street features a wide variety of Australian fossils from NSW and other states, as well as specimens from overseas localities. Fossils not readily identifiable from exhibits are best shown to a palaeontologist.