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Fossils are one of the most important sources of information about the Earth's past. They can tell us the age of the rocks in which they are found, what the environment was like when the fossilised organisms were alive, and even how the organisms functioned. They can also tell us about Earth movements, such as mountain building, about the former positions of continents (ancient geography), and about the evolution of life on Earth. Some of these uses for fossils are of economic importance, assisting in the search for oil and minerals.
Fossils as age indicatorsFossils are the most important means of dating sedimentary rock sequences. However, they do not provide an absolute age measured in years, but rather a relative age expressed in terms of the relative geological time scale. The use of fossils in this way relies on the fact that individual species evolved into others through time, so that if the time range of a species is known in one particular region, the occurrence of the same species in another region indicates that the rocks there are of the same age. This process of establishing the equivalence in age of two rock sequences in different areas is called correlation.
Not all fossils are of equal value in dating rocks; the most useful are called index or zone fossils. Ideally, index fossils should be common, readily preserved and easily recognisable. They should have spread rapidly and widely, and for accuracy of dating, they should have evolved rapidly so that individual species existed during only a short interval of time. Very few index fossils meet all of these criteria. Amongst the most important index fossils are graptolites, ammonites, foraminifera, pollen, conodonts and trilobites.
One of the most important groups of index fossils in the Palaeozoic rocks of Victoria is the graptolites. These were extinct marine animals that formed twig-like colonies composed of one or more branches. The colonies were originally three dimensional but usually became completely flattened during fossilisation, though they are still easily recognisable. Some graptolite colonies may have been attached to the sea floor, but most floated freely in the sea. They are of most use in dating rocks ranging in age from Early Ordovician to Early Devonian.
The Ordovician rocks of central and eastern Victoria have one of the richest and most diverse graptolite faunas in the world. They have been used to subdivide the rock sequences into 30 intervals, and to correlate these intervals accurately with other sequences in New Zealand, Asia, Europe and North America.
Fossils as environmental indicatorsBecause fossils are the remains of once living organisms that were adapted to their environments, they can provide valuable information about what past environments were like. We can predict the environmental requirements of organisms in the past from those of closely related organisms living in the present day. Such predictions will be most reliable in the case of younger rocks which contain fossils having representatives alive today. As we go further back in geological time, the predictions become less reliable because we encounter fossils of extinct groups about whose environmental requirements nothing is directly known.
The environmental information obtained from fossils may be as simple as whether the rocks in which they occur were deposited in the sea, in a brackish water estuary, in fresh water, or on the land. For example, rocks containing fossils of corals, brachiopods, cephalopods or echinoderms must have been deposited in the sea because living representatives of those groups are found only in the sea today; and fossils of land-dwelling animals such as kangaroos indicate deposition on land or in an adjacent body of fresh water.
Fossils of reef-building corals indicate that the rocks in which they occur were deposited in warm, shallow seas because, at the present day, reef-forming corals are found in tropical seas and only at depths of less that 200 m where sunlight can penetrate the water to reach the photosynthesising algae within their cells.
The Koonwarra fossil bed of South Gippsland provides a good example of the use of fossils in reconstructing an ancient environment. This fossil bed contains fossilised fish, plants, insects, crustaceans, spiders, bird feathers and a horseshoe crab. There are also bryozoans and a mussel. These fossils tell us that the deposit was formed in the shallow part of a large freshwater lake because the insects include mayflies that are similar to forms living today in cool mountain streams and lakes in Tasmania. The lake may have been frozen in winter because the mass occurrence of fish fossils show no signs of rotting. This conclusion is supported to some extent by the occurrence of a beetle that is similar to a modern species found only in alpine areas. The occurrence of fleas in the fossil fauna suggests that mammals may have been present on the adjacent land, and the occurrence of feathers shows that birds were also present. The small size of the fish suggests that they were juveniles or small adults, which inhabit shallow areas in modern bodies of fresh water. The insects are well preserved, even those that were not aquatic, suggesting that they were not transported great distances after death, so that the fossil deposits must have been formed close to the edge of the body of water.
Fossils as indicators of Earth movementsThe occurrence of fossils at a particular locality may provide evidence that there has been some movement of the Earth's crust since the fossils were deposited. The movement may have been only slight uplift of the land, as indicated, for example, by the occurrence of fossils of marine shells in cliffs around Port Phillip Bay. Alternatively, the uplift may have been on a much larger scale, as indicated by the occurrence of marine fossils far from present-day oceans and even in the middle of continents, or on high mountains, such as the Himalayas or the European Alps. Movement of the Earth's crust along faults or fractures may be indicated, even if the fracture itself is not evident, by the occurrence of fossils of very different ages at adjacent localities. For example, the Whitelaw Fault on the eastern outskirts of Bendigo is not marked by any obvious landform, but its presence is indicated by the occurrence of graptolites of Middle Ordovician age (about 470 million years old) on one side of the fault, in close proximity to Early Ordovician graptolites (about 493 million years old) on the other side.
Fossils as indicators of ancient geographyAs long ago as the middle of the eighteenth century, it became apparent to some palaeontologists that there were sometimes striking similarities in the assemblages of fossils found in rocks of the same age in widely separated continents. The similarities could not be satisfactorily explained by the migration of organisms across vast expanses of ocean, because the fossils belonged to forms that lived only in shallow marine environments, in fresh water, or even on dry land. A few scientists suggested that these similarities were due to the fact that the continents were once joined together and later split apart, but this suggestion was rejected by most geologists because at that time there was no known mechanism by which the continents could move. The favoured explanation then was that organisms had migrated across 'land bridges', which had connected the continents in ancient times but which had later subsided to form part of the present-day ocean floor. We now know that this could not have occurred, because the Earth's crust on the floor of the oceans differs in composition from that of the continents. With the development of the theory of plate tectonics in the 1960s, leading to the widespread acceptance of continental drift, the similarities in the fossil faunas in different continents could be readily explained by the drifting apart of land masses that formerly lay together.
One example of the fossil evidence that the continents were connected in the past is the distribution of the ancient seed-fern Glossopteris and related plants. The fossils of these plants are associated with coal deposits of Permian age in India, Australia, South Africa, South America and Antarctica. The rock sequences in which these coal deposits occur are remarkably similar on all of these continents. The distribution of these plants cannot be explained by wind dispersal of their seeds, as these are too large to have been carried across the ocean. A further line of evidence is the distribution of the reptile Mesosaurus, which is found in Brazil and South Africa at or near the Carboniferous-Permian boundary. Mesosaurus lived in fresh or perhaps brackish water habitats, so it is difficult to imagine that it could have found its way across an ocean as broad as the present day Atlantic.
Fossils as evidence for the evolution of lifeFossils are the main sources of information on the evolution of life on Earth. Without the information they provide, we would have no knowledge of extinct organisms such as trilobites and dinosaurs, and our knowledge of the history of the development and evolutionary relationships of the modern flora and fauna could be derived only from the living organisms themselves. We would also have no direct knowledge of the timing of critical biological events, such as the origin of life, the development of shells or skeletons, the colonisation of the land, the appearance of mammals and flowering plants, the development of flight, and major episodes of extinction.
The role that fossils have played in deciphering relationships among organisms can be demonstrated by the evolution of horses, the family of mammals with probably the best fossil record. The development of the modern horse from its oldest known ancestors can be traced via a number of morphological changes, including body size, shape of teeth, and the structure of the feet. These morphological changes reflect changes in habitat and feeding, from browsing on soft leaves in forests to grazing on hard grasses on open plains. The oldest known horse, Hyracotherium, lived during the early Eocene (about 50 million years ago). It was a dog-sized creature with short-crowned teeth, and with four toes on the front feet and three toes on the back feet, each toe having a small hoof. In descendants of Hyracotherium, there was a progressive increase in body size, to the size of the modern horse. The teeth developed long crowns with complex enamel ridges for grinding hard grasses, and the number of toes was progressively reduced to one on both front and hind feet.