Презентация, доклад по билингвальной биологии на тему Палеонтология

the preserved organism or its traces
It usually occurs when the organism is covered quickly so it is preserved
Sediment, forming sedimentary rock, is then laid down
Petrifaction follows

© 2016 Paul Billiet ODWS

by dehydration (mummified), in ice, in peat bogs, in tar beds or trapped in amber

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100 000 years old
Oxygen and water damage the molecule with time.

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© 2016 Paul Billiet ODWS

been natural erosion, which exposes the deeper, older layers containing the fossils
Useful sites include gorges, quarries, caves and desert areas.
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
The fossil record is far from complete
This may account for the ‘missing links’ and for apparently restricted distribution of many species
But palaeontologists can improve their chances by searching systematically in places where fossils are likely to be found

© 2016 Paul Billiet ODWS

deepest usually being the oldest
This sequence forms the stratigraphy of the rock and together with the fossils and artefacts 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.

© 2016 Paul Billiet ODWS

phenomenon of radioactive decay of isotopes
When sedimentation occurs radioactive isotopes are incorporated
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

© 2016 Paul Billiet ODWS

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

© 2016 Paul Billiet ODWS

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
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

© 2016 Paul Billiet ODWS

the upper atmosphere by the action of cosmic rays on Nitrogen-14
14-C 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!

© 2016 Paul Billiet ODWS

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 5 730 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).

© 2016 Paul Billiet ODWS