Key Knowledge:
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Fossils are the preserved remains of any organism from the remote past (this can involve either physical or trace evidence)
- Physical fossils represent actual remnants of life and include body fossils (preserved remains) or petrified fossils (mineralised hard parts)
- Trace fossils represent indirect evidence of life and include moulds (hollow outlines), casts (filled cavities), tracks, burrows and faeces
- Biosignatures represent the chemical evidence of past life and include the products of cell biosynthesis (amino acids, oxygen, etc.)
The study of the different types of fossils and what they reveal about the history of life on Earth is called palaeontology
- Fossils provides evidence for evolution by revealing the features of an ancestor for comparison against their more recent descendants
Fossilisation
Fossilisation is a rare occurrence requiring an unusual set of circumstances:
- The preservation of remains (the fossil must be protected against scavenging, erosion and environmental damage)
- The presence of hard body parts (bones, shells) – soft body parts will not fossilise, but may leave behind trace evidence (e.g. imprints)
- Anoxic (low oxygen) conditions are needed to protect against oxygen damage and prevent decomposition by saprotrophs
- Either cold temperatures to prevent eventual decay (for body fossils) or high pressure to promote permineralisation (for petrified fossils)
One circumstance that would meet all of these conditions is rapid burial of the organism (this requires subsequent exposure of the fossil)
- Because fossilisation occurs rarely, palaeontology remains an inductive (data-poor) science that is subject to frequent reinterpretations
Fossil Record
The totality of fossils, both discovered and undiscovered, is referred to as the fossil record
- The fossil record reveals that, over time, changes have occurred in features of organisms living on the planet (evolution)
- Different kinds of organisms do not occur randomly in the fossil record, but are found in a consistent chronological order
- This ordered succession suggests a sequence of evolutionary development, which is called the law of fossil succession
According to the law of fossil succession, newer species likely evolved as a result of changes to ancestral species:
- Prokaryotes appear in the fossil record before eukaryotes
- Ferns appear in the fossil record before flowering plants
- Invertebrates appear in the fossil record before vertebrate species
⇒ Click on the diagram to compare the evolutionary history of animals and plants
Index Fossils
The chronological ordering of the fossil record requires the identification of fossils that represent defined geological periods
- These fossils are called index fossils and can be used to date other fossils found in same rock layer (strata)
To be an index fossil, the organism must have existed over a wide geographical area but have been relatively short lived
- Fossils that are found in many rock layers (and therefore existed over a long time period) do not qualify as index fossils
Index fossils can also be used to synchronise rock strata at two different locations, allowing for chronological comparisons
- Different geographic regions may have different strata compositions due to environmental exposure (erosion, flooding, etc.)
- As index fossils can be used to identify the geological period of a particular strata, they are a useful calibration tool
Transitional Fossils
While fossils may provide clues as to evolutionary relationships, it is important to realise that the fossil record is incomplete
- Fossilisation requires an unusual set of specific circumstances in order to occur, meaning very few organisms become fossils
- Only the hard parts of an organism are typically preserved, meaning usually only fragments of remains are discovered
- With limited fossil data, it can be difficult to discern the evolutionary patterns that result from ancestral forms (‘missing links’)
Transitional fossils demonstrate the intermediary forms that occurred over the evolutionary pathway taken by a single genus
- They establish the links between species by exhibiting traits common to both an ancestor and its predicted descendents
- An example of a transitional fossil is archaeopteryx, which links the evolution of dinosaurs (jaws, claws) to birds (feathers)
- Another example is Ida (Darwinius masillae), which has features common to both modern prosimians and anthropoids
The identification of transitional fossils can be hotly contested topic of conversation amongst palaeontologists
- As new fossils are discovered, new evolutionary patterns are emerging and old assumptions are continually challenged
