Fossils are the remains of plants or animals or their tracks and traces which are preserved in sedimentary rock. They provide scientists with evidence about the history of life on earth. Fossils come in all forms and sizes. They can range in size from the tiniest pollen grain to the largest dinosaur.
Trilobite, Cambrian, northwest Queensland
Fossils are classified in the same way as living animals and plants. Thus there are fossil plants, fossil vertebrates (animals with a backbone), and fossil invertebrates (animals lacking a backbone). In addition there are other forms of life which also have a fossil record which include fossil cyanobacteria (stromatolites), protistans (foraminiferans and radiolarians) and many more.
A special group of fossils are the tracks and traces made by animals and plants which are preserved in rocks. These include footprints, burrows, crawling and feeding traces and rootlet traces. These are referred to as trace fossils.
There are many different ways that a fossil is preserved:
- Original material; original bone, shell or exoskeleton is preserved.
- Replacement; the original shell or skeleton is replaced by some other mineral whilst preserving the fine detail of the fossil. Petrified wood is an example of this where cell walls of wood are replaced by silicon dioxide.
- Compression; fossil remains are flattened by the force of compression of the overlying sediment which buried the organism. Fossil plants are often preserved in this way.
- Impression; Fossil material is compressed and then dissolved leaving only a trace of the organism.
- Natural moulds and casts; skeletal material within the sedimentary rock is dissolved leaving a space. If the space remains empty then both internal and external moulds of the fossil can be formed. If the space is filled by chemicals deposited in the rock a natural cast is formed. Opalised fossils are examples of natural casts.
- Permineralisation; a fossil consisting of porous material has its pores filled up with minerals. This makes the fossil more robust and heavier. Many fossil bones are preserved in this way.
- Special preservation: some fossils are preserved in exceptional circumstances. These include mummification, freezing, preservation in amber and desiccation.
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