Life on Earth - Fossils
Fossils are the remains of plants and animals found in rock. A fossil may be the remains of the organism, or be a cast, impression or mould of an organism.  It can even be a track or footprints that an animal has left behind in mud or clay. Sometimes, when organisms die and fall in to mud and clay they are then covered and flattened by layers of sediment, which harden to form sedimentary rock. Usually it is only the hard shell or skeleton that is preserved as the softer parts often decay. But soft organisms such as jellyfish have also been known to leave an imprint in rock.

Fossil from Streedagh Strand, Sligo

Fossils can be formed in different ways:

  • Some fossils are found in ice and are preserved so well that the skeleton, flesh and skin are still present. An impression or mould is left in rock when sediment covers an organism or its tracks. The organism can decay over time leaving the impression that can then be filled to make a cast of the original organism or its tracks.
  • Many organisms were preserved in the thick resin called amber that is given off by pine trees.
  • Some animals such as the sabre-tooth tiger were found preserved in pits of tar or asphalt. Asphalt is a black or brown mineral found in the earth in liquid or solid form.
  • In some cases minerals in the water and ground replaced the minerals in the bodies of animals and plants, which had decayed. These new minerals hardened forming the structure of the organism.

Paleontologist at work. Image from http://www.abc.net.au/science/ausbeasts/img/alcoota01.jpgPaleontology is the study of fossils and the geological processes that formed them. A palaeontologist is a person who studies fossils. Fossils are very important because not only do they help us understand animal and plant life that existed millions of years ago but they help show what the climate was like at the time and how organisms have changed since then. Scientists also use fossils to age the earth and rocks, as certain fossils are characteristic of a certain rock of a particular age and time. Many fossils are of aquatic animals that lived in water and were covered by mud and clay, which then hardened on top of them. When these rocks eventually rose above sea level the fossils were exposed by erosion from the wind and rain.

Fossils in Ireland can be found in the GSI in Dublin, the Geological Museum of Trinity College, National Museum of Ireland in Dublin, Ulster Museum in Belfast, National University of Ireland in Galway and the Department of Geology in University College Cork.

Dating - Relative dating
Dating in geology may be relative or absolute. Relative dating is done by observing fossils, as described above, and recording which fossil is younger and which is older. The discovery of means for absolute dating in the early 1900s was a huge advance. The methods are all based on radioactive decay:

  • Certain naturally occurring elements are radioactive, and they decay, or break down, at predictable rates.
  • Chemists measure the half-life of such elements, i.e., the time it takes for half of the radioactive parent element to break down to the stable daughter element. Sometimes, one isotope, or naturally occurring form, of an element decays into another, more stable form of the same element.
  • By comparing the proportions of parent to daughter element in a rock sample, and knowing the half-life, the age can be calculated.


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