Fossils and extinction

L.O  To learn how fossils are formed and why extinction occurs.


How are fossils formed

Fossils are the remains of animals and plants from many centuries ago, when animals and plants die, and soil and rocks from on top of them, some of the animal carcasses many leave an indentation on one of the rocks, due to the pressure build up over the years.

Fossils can form on rocks in one of three ways

1 – when an animal dies the carcass which consists of bones, teeth, shells etc, which don’t decay and they are eventually replaced by minerals which eventually form a rock like substance. The surrounding areas will also turn to rock, however the fossil will form a distinct shape within the rock

2 – sometimes fossils are formed when plants and animals are buried in soft substances such as clay which will then form a cast around the organism and therefore preserving the shape

3 – sometimes fossils are formed when the whole animal or plant is preserved due to the environmental conditions, such as the lack of oxygen in tar pits, or due to the temperature, for example in extreme cold where the animal may get frozen in ice.


Why does extinction happen – this can be due a number of reasons

  • The environment changes – the living conditions can change causing the habitat to be destroyed and therefore an organism can die out
  • Predation – the introduction of a new predator can lead to an organism can be becoming extinct
  • Disease – a new disease can wipe out a species, if the species fails to recover quickly
  • Competition – if a species cannot compete then it will soon start dying and become extinct
  • A natural event – events such as volcanic eruptions or earthquakes can wipe out a whole population


Speciation is the development of a new species, which occur due to populations of the new species becoming separated to such an extent that they develop new characteristics until they become totally different species.

The process of speciation

  • Isolation occurs which separates a population, the isolation occurs due to a physical barrier
  • Conditions on either side of the barrier will be different and therefore the organism will develop different characteristics due t natural selection
  • Each population will show variation and therefore will have a wider range of alleles, in each population the ones with the preferred characteristics will survive and pass on those characteristics to the offspring
  • Eventually the two populations will be so different to such an extent that if they were to breed together they would not be able to produce fertile offspring


The image illustrates this process



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