Fossils – Lesson Summary

This is a lesson summary. The full lesson can be viewed by purchasing an online course subscription.

Learning Objective

In this lesson we will learn about different types of fossils and how they are formed.

Learning Outcomes

By the end of this lesson you will be able to:

  • Describe what fossils are and distinguish between body fossils and trace fossils.
  • Explain why fossilisation is a rare event and why the fossil record is incomplete.
  • Describe the processes involved in the formation of fossils.
  • Distinguish between altered and unaltered organic remains.
  • Describe different fossilisation processes, such as permineralisation, replacement, casts and moulds, and compression.

 
fossils online science lesson contents

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Lesson Topics

1 | Fossils and the Fossil Record

2 | Types of Fossils

3 | Formation of Fossils

4 | Altered and Unaltered Organic Material

5 | Fossilisation Processes

6 | Summary

Lesson Summary

  • A fossil is the preserved remains or traces of an organism from a previous geological age.
  • Fossils are mostly found in sedimentary rock and they are usually mineralised.
  • The collection of all known fossils is known as the fossil record and the study of fossils is known as palaeontology.
  • The two main types of fossils are body fossils and trace fossils.
  • A body fossil is preserved remains of an organism.
  • A trace fossil is preserved evidence of an organism’s presence or activity.
  • Fossilisation is a rare event that only occurs under specific conditions.
  • It mostly occurs when a dead organism is quickly covered by sediment which slowly transforms into sedimentary rock over long periods of time.
  • Tectonic uplift can push buried fossils to Earth’s surface, where they can become exposed when surrounding rock is weathered and eroded away.
  • Since soft tissues decay quickly, fossilisation mostly preserves hard structures, such as bones and shells.
  • The degree to which organic remains are preserved in fossils varies.
  • Unaltered remains contain preserved organic materials, including frozen remains and unaltered mineralised remains.
  • Altered remains have undergone partial or complete mineralisation.
  • There are several fossilisation processes, including:
  • Permineralisation – the filling of porous materials with minerals that have precipitated out of solution.
  • Replacement – the substitution of organic materials with minerals.
  • Petrification – a combination of permineralisation and replacement.
  • External moulds, internal moulds and casts – impressions that preserve the three-dimensional shape of an organism.
  • An external mould is an impression of an organism’s external structure.
  • An internal mould is an impression of an organism’s internal structure.
  • A cast is an external mould that has been filled with minerals.
  • Compression (carbonisation) – the formation of darkened imprints of organisms that contain a thin layer of organic material that has been converted to carbon.
  • Preservation in amber – the preservation of whole insects or other small organisms in fossilised tree resin.

 
internal external mould fossil

(Image: James St. John, Wikimedia Commons)

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