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| Fossil Records | Dinosaurs | Fossil Image Gallery |

Fossil Records

Fossils are the remains or traces of prehistoric plants and animals preserved in rocks. They usually comprise the hard parts of the animal or plant, or structures resistant to decay. So any organism which has shells, bones, teeth or wood is likely to be preserved as a fossil, examples include molluscs, mammals and trees. Less common are fossilised hard parts or traces of soft parts of animals or plants, hence worms and amoebae have a much smaller chance of being preserved. Footprints and burrows of animals, along with plant roots can be preserved to become fossils and these are called trace fossils.

The study of fossils is called palaeontology and early cultures such as the Greeks were studying fossils in the fourth and fifth centuries BC. However, the scientific study of fossils was really begun by Carl Linnaeus and George Cuvier in the late eighteenth and early nineteenth centuries.

How fossils are formed

Animals or plants that become fossilised are buried by sediments, for example sharks fall to the ocean floor after they have died and are then covered by sediment. As the sediments pile up on the remains of the animal or plant, the pressure increases, some water may be expelled and chemical changes occur to produce sedimentary rocks such as shale, sandstone and limestone. In harder rocks such as limestone, fossils are more likely to retain their original appearance, while softer rocks may compress or flatten fossils.

Fossil examples, that some say are the world's most significant are those in the Burgess Shale, Canada. The Burgess Shale fossils are special because of their great age, and their exquisite preservation.

There are a variety of ways to form fossils:

  • mummification - complete preservation in a relatively undamaged state of hard and soft body parts;
  • skeletons and shells - these are the most common fossilised hard parts; they may loose their colour but are unchanged in their chemical composition;
  • petrification and replacement - impregnation of fossils by secondary minerals which usually leads to an increase of weight and hardness of the fossil;
  • carbonisation - usually formed from woody and chitinous material, which loses its oxygen and nitrogen through decomposition by anaerobic bacteria;
  • impressions and traces - impressions left by dead animals that decayed away or traces left by moving animals;
  • casts and moulds - where the rock containing the fossils has hardened and the original fossil has been dissolved away, leaving a hole which can often be filled by other sediment.

Once they are imbedded, fossils experience the same geological history as the rocks they are incarcerated in. Rocks may be squeezed and distorted with fossils contained inside. Heat and pressure often puts fossils into a sorry state by the time they are found. The resulting fossils are often found as fragments and piecing them together is just like doing a jigsaw. Some species have been identified by only one fragment of their skeleton. Some fossils have travelled great distances through the forces of plate tectonics and mountain building. Ammonites for example, which have died and fallen down to the ocean floor, can be found in rocks at an altitude of over 5000 metres in the Himalaya Mountains.

Types of fossils

Fossils can be grouped into macrofossils, microfossils and trace fossils. Macrofossils are fossils that can be easily seen with the unaided eye. Animal macrofossils are predominantly shells, bones, teeth, chitinous skeletons, calcified skeletons, fish scales, sometimes eggs or impressions of soft parts. Plant macrofossils are mostly woody branches, trunks, stumps, roots, leaves, seeds, cones or fruit.

Microfossils are fossils that can be only seen in detail with a microscope. They are generally smaller than 1mm. Animal microfossils include small fish teeth, fish earbones, worm jaws, and spines of sea urchins, and internal or external skeletons of small animals like the waterflea (Daphnia). Most common however are the hard parts of marine protozoa (single-celled organisms), called foraminifera and radiolaria. Their minute skeletons sink to the ocean floor when they die and are well preserved as fossils, contributing hundreds of tonnes of sediment to the modern sea floor each year. Plant microfossils include pollen, spores, marine algae and are, together with the protozoa, the most abundant fossils to be found in rocks.

Trace fossils are left by moving animals, and may include the back filled burrows by active sediment eaters such as worms or shelter burrows used for retreat by crabs or shrimps. They also record the impression of organisms which stopped to rest on the sediment.

For pictures of fossils found in New Zealand check out our fossil photo gallery.

The fossil record and evolution

Theoretically it should be possible to collect fossils of plants, animals and other organisms throughout the strata, from the oldest rocks through to the youngest to read and explain the past. Anyone who set out to do that would be disappointed due to the gaps in the sedimentary rocks which contain the fossils. Often fossils don't appear where they are "supposed to" simply because they were not preserved. Occasionally fossils have been found in unexpected places and this requires a re-think of some aspects of the record. Unfortunately it is not possible to find an entirely unbroken succession of fossil record that would span all of Earth's history, although some places like the Grand Canyon in the USA have a vast period of time preserved in successive layers. New Zealand also has several places where rock successions can be studied in sea cliffs or river beds or mountain bluffs. By comparing fossil records from different locations, it is possible to see a succession of fossils, which provide evidence for evolution and form the basis of the geological time scale.

One of the biggest factors that affect the fossil record is the way new species develop, also called speciation. Often, a population that gives rise to a new species exists at the fringe of a larger parent population; it is isolated from the main group and exposed to different environmental conditions and selection pressures. Because low numbers of individuals are involved, and because fossilisation is a rare event anyway, it is not always easy or possible to find fossils of that new species in that new environment, so that the fossil record of a particular lineage may at times resemble a mixed up jigsaw.

Transitional fossils

There is little doubt that all living things on Earth share one common ancestor. This insight was first documented by Charles Darwin in 1859 in "The Origin of Species", and the discovery of new evidence of evolution has repeatedly confirmed his conclusion. One major indicator of a single common ancestor is the fact that all life forms share common mechanisms for transmitting inherited information (the DNA) and using this information to control cellular processes (RNA and ribosomes, whereby genetic instructions carried in the DNA are translated into proteins).

Two major lines of evidence available to Darwin and his contemporaries were transitional fossils - the so-called "missing links" - and the presence of shared derived characteristics, or homologous features (where "derived" means altered from some ancestral form). Darwin himself recognised (when he was preparing the "Origin") that the absence of fossils that were transitional between two other species was a major problem for his theory of evolution. Some people may argue that if all of life is related through a single huge family tree extending from the present day back hundred of millions of years to a single point of origin, we should find fossils that are midway between established groups - so called transitional fossils. They claim that none exist, and that this is conclusive proof that Darwin, and generations of biologists, were wrong.

However, the first transitional fossil, or missing link, was found in 1861, shortly after Darwin's Origin of Species was published. The first specimen of Archaeopteryx was discovered in a limestone quarry in southern Germany. Scientists immediately noticed that Archaeopteryx was an intermediate form with both avian and reptilian characteristics: it had feathers and wings, but also a long bony tail, fingers with claws on the forelimbs, and teeth in a heavy jaw. Seven more skeletons of Archaeopteryx have been found to confirm those first findings. Other species of early bird, discovered in Spain and China, are 30-40 million years younger than Archaeopteryx, suggesting that it was not the" first bird".

Often however, evolutionary changes are best preserved in rather inconspicuous ancient life forms. Foraminifera, for example, are small planktic animals that show a very good record of continuous changes due to a changing environment.


Reference Websites  Reference Websites

A number of sites provide excellent information on fossils and transitional forms:

The "Talk.Origins" site provides excellent information and discussion about transitional fossils and hominid fossils.

On the Discovery of Global Warming website hosted by the Centre for History of Physics of the American Institutes of Physics you can find pictures showing the gradual change of the foraminifera Globigerinoides into Orbulina, a good example of a well documented transition between species.

Other scientists have set up websites with collections of interesting information. For example Andrew MacRae from the Dept of Geology and Geophysics, The University of Calgary, Canada set up a site which includes this information about trace fossils.

The New Zealand GNS website has coverage of fossil research by NZ scientists.

The American Geological Institute has a great website about evolution and the fossil record.

For learning activities and teachers resources check out these two sites. The Berkeley site has learning activities about fossils macro and microfossils and is a resource prepared for teachers.

Also great online teacher resources (ready-to-go-classes) about transitional fossils can be found on the action.bioscience organisation website.

The Royal Ontario Museum has a fossil cyber-display which was created by Irene Chalmers of the Discovery Centre with help from Janet Waddington of the Department of Paleobiology. This site provides many nice images.

A UK orientated website called KidsArk has information about fossils and fossil collecting. This website was constructed by the Wilkinson family and is very kids safe.

Reference Books  Reference Books

Conway, Morris S. (1999). The Crucible of Creation: The Burgess Shale and the Rise of Animals. Oxford University Press.

Erickson, J. (1992). An introduction to fossils and minerals: seeking clues to the Earth's past. Facts on File, New York.

Fortey, R. (2002). Fossils: the key to the past. London: The Natural History Museum

Hayward, B. (1989). Trilobites, Dinosaurs and Moa bones. Auckland: The Bush Press

Monks, N. and Palmer, P. (2002). Ammonites. London: The Natural History Museum

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