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Powerpoint Presentation: Palaeontology

Evolution Index

The origins of life
Evolution and Fixity
Natural selection
Lamarkian Evolution
Antibiotic Resistance
Industrial melanism
The C-14 Decay Curve
In Search of Deep Time
Evolution of the Horse
Punctuated Equilibrium
The classification of living organisms : Taxonomy
Kingdoms
Humans: Neotonous, bipedal African apes
The Hominids
The Changing Trees of Human Evolution
Genetic verses Cultural Evolution
Phenylketonuria (PKU) Fact Sheet
Cystic Fibrocis (CF) Fact Sheet

Topic Chapters Index

 

Artefacts are artificial remains, made by organisms. They include human tools. Human tools are used to divide the recent past into cultural divisions e.g. Palaeolithic = the Old Stone Age, which is further subdivided into industries of which the Oldovan is the oldest.

 

Palaeoanthropology is the study of the fossils and artefacts of hominids.

 

Archaeology is the study of the prehistory of Homo sapiens.

 

 

EVOLUTION

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Palaeontology : The Study of Fossils

Introduction

  • Fossils are generally of rock that, under suitable conditions, had replaced the preserved organism or its traces.

  • It usually occurs when the organism is covered quickly, e.g. by silt, and so it is preserved.

  • Sediment, forming sedimentary rock, is then laid down. Petrifaction follows.

  • Not all fossils are petrified some are preserved by dehydration (mummified), in ice, in peat bogs, in tar beds or trapped in amber. NB The current limit for fossil DNA appears to be about 100 000 years old. Oxygen and water damage the molecule with time. So DNA from dinosaurs is not an ongoing possibility but it has permitted scientists to compare our own DNA with our closest relative Homo neanderthalensis who disappeared about 28 000 years ago.

  • The discovery of fossils is greatly assisted where there has been natural erosion, which exposes the deeper, older layers containing the fossils. Useful sites include gorges (e.g. Grand Canyon, areas such as the River Omo and the Olduvai Gorge of the Rift Valley in Africa, the Neander Valley in Germany) and caves (e.g. in the Transvaal and Shanidar in Iraq) and desert areas (e.g. the Fayum in Egypt). Quarries are also useful. The best-preserved remains of fossil birds come from the Solenhofen Limestone in Germany (a stone quarried for lithography).

  • Therefore, fossils are only formed under certain conditions and then have to be uncovered. The chance that a body will be fossilised is rare and the chance that it will be discovered is even rarer. Therefore the fossil record is far from complete.

  • But palaeontologists can improve their chances by searching systematically in places where fossils are likely to be found.

  • This may account for the 'missing links' and for apparently restricted distribution of many species.

 

Dating of Sedimentary Rock

Relative dating

  • Such rock is laid down in layers or strata the deepest usually being the oldest. This sequence forms the stratigraphy of the rock and together with the fossils and artifacts which are present, give a relative dating.

  • However, due to earth movements in the past and to the great time spans and migrations of some organisms, this method is not very accurate.

Absolute dating

Accurate dating can be obtained using radiometric dating.

  • This uses the phenomenon of radioactive decay of isotopes. When sedimentation occurs radioactive isotopes are incorporated, and these decay to form other atoms at a known rate.

  • This rate is measured as the half-life of the isotope, defined as the time taken for half the parent atoms to decay to the daughter atoms.

Potassium-Argon Method

Potassium-40 (40-K) decays to form Argon-40 (40-Ar), which is trapped in the rocks.

  • The amount of argon is measured, so that this is known as an accumulation method. The half-life of 40-K is 1.3 x106 years, so it is useful for dating very old rock (as old as the Earth), the minimum age being 100 000 years. The limitation is the degree of precision of the measuring devices. As these improve more recent events may be dated.

  • Volcanic rock is particularly useful for this technique. When it melts the rock releases any 40-Ar it has in it, setting the clock to zero. Then when the molten rock crystallises it becomes impermeable which traps 40-Ar gas so it cannot escape. This sets the clock to zero. With time the 40-Ar builds up and the 40-K diminishes.

  • Volcanic rock, however, does not contain fossils. So when fossils are dated using this method their association with the lava flow or ash fall needs to be established.

Carbon-14 Method

Carbon-14 (14-C) decays to form nitrogen-14.

  • C-14 is formed in the upper atmosphere by the action of cosmic rays on Nitrogen-14. It is oxidised to 14-CO2 that gets taken up by plants in photosynthesis. The 14-C becomes incorporated in living tissue and travels up the food chain like other isotopes of Carbon (e.g. 12-C). Whilst an organism is living it incorporates a known amount of carbon-14.

  • At death, no more is taken in, and so the amount declines as the 14-C decays back to 14-N.

Death starts the clock

  • The ratio of 14-C to 12-C is measured. 12-C is a stable isotope, which does not decay. So as time goes by the ratio of 14-C/12-C gets smaller.

  • The half-life of 14-C is 5730 years, so it is used to date very recent remains, the maximum age being 50 000 years (there is not much 14-C left after 9 half-lives).

  • The amount of 14-C in the atmosphere varies with the amount of bombardment of the atmosphere by cosmic rays. Therefore, correction factors are used which have been calculated using other methods (e.g. dendrochronology - tree ring dating)

These are the two main radiometric methods there is a gap in absolute dating between 100000 and 50000 years - a critical time in the evolution of Homo sapiens.

 

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