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The Fossil Record

Biology (Year 12) - Evidence for Evolution

Ben Whitten

Fossils are preserved remains and traces of organisms. They may include hard parts of organisms, including:

  • Bones

  • Teeth

  • Shells

  • Footprints

  • Faeces

Fossils are one of the many things which provide evidence for evolution, alongside branches of science such as palaeontology, biogeography, developmental biology, comparative morphological anatomy, genetics and comparative molecular biology.

Process of Fossilisation

The process of fossilisation is the way in which an organism becomes stone, which had existed in some earlier age. The majority of long-extinct organisms may never be found, and therefore the fossil record is incomplete and biased towards those organisms more likely to fossilise well. To become a fossil, the organic remains need to be deposited and covered in sediments that exist in a low oxygen environment which is nearly always aqueous mud, sand or clay. In specific conditions, minerals from the sediments can replace the organic matter (bones) and turn them to stone.

1. Sedimentary fossils: Organism is covered by fine sands or clay which is usually carried by rivers and lakes/ocean currents. The sediments protect the organism from scavengers/predation. Low oxygen environments assist this process, and harder parts are usually fossilised but occasionally, structures like feathers may be preserved. Sedimentary fossils are not found in volcanic rock but can be in eroding volcanic ash layers. 2. Mineralisation fossils: Previously preserved sedimentary fossils can be mineralised over time. Minerals leach into the organic remains from the surrounding sediments and replace the organic structures in concentrated crystalline patterns, and this turns the fossil very hard compared to surrounding sediments. 3. Freezing and dehydration: This can result in fossilisation. Plants and tree petrification result from a freeze-drying process, where much of the plant matter becomes dissolved and is replaced with mineralised salts.

Transitional Forms

Transitional forms are the immediate states which occur between an organism's ancestral form and that of its descendants. For example, a bird to a dinosaur. The intermediate state between an organism's ancestral forms and that of its descents is hard to identify due to the bias of the fossil record, as the specific requirements for fossilisation mean only certain remains are fossilised; there is information missing.

Theories of Evolution in Fossils

There are two theories; gradualism and punctuated equilibrium.

  • Gradualism is a theory of evolution that assumes evolution occurs at a steady, slow divergence of lineages at an even pace; sudden bursts of evolution are not a real record of evolution, but an illusion of the fossil record due to the absence of fossils.

  • Punctuated equilibrium states that apparent bursts of evolution are real. Species remain stable for a period of time but change quickly into a new species in response to an environmental change. This theory accepts the existence of transitional forms, but they were so brief that they were not preserved.

Index Fossils

An index fossil is any plant or animal preserved in the rock record of the Earth that is characteristic to a particular span of geologic time or a particular environment. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata. Trilobite fossils are found worldwide, with many thousands of known species existing. As they evolved rapidly, trilobites served as excellent index fossils, enabling geologists to date the age of rocks in which they are found.

Limitations of the Fossil Record

Fossilised remains prove how plants and animals adapted to their changing environments, how some did not survive and how others thrived. However, the fossil record suffers from 3 biases.

  1. Temporal bias: Some periods in geological history are better represented in the fossil record due to more ideal conditions favouring fossilisation. Newer fossils are also easier to locate relative to older fossils.

  2. Geographic bias: Some locations in the world are more favours for fossilisation processes and some locations are easier to access, for example, lowland areas.

  3. Taxonomic bias: Some groups are better represented in the fossil record in comparison to others, for example, fish are very well represented in the fossil record due to their aqueous environment.

Many processes such as tectonic plate movement, changing sea levels, erosion and human activity can also destroy fossils or place them out of reach.

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