Lecture 8

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Published on 30 Oct 2010
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A very short history of life on earth
How are fossils formed?
- Shells buried and preserved unaltered (<100 million years)
- Cavities filled with silica, calcium carbonate, iron t a process called mineralization
o Minerals come from water that flows through the rocks and gets trapped in the cavity
Methods: fossilization
- Freezing
o Not common at all but is the best methods
- Drying
- Asphalt
o tarpits
- Amber
o E.g. tree saps that preserves and insect in it
- Carbonization
o When only the carbon is left from the decay
- Permineralization
o Most common
- Peat bogs
o E.g peat swamp
- Paraffin
- Volcanic ash
Permineralization
- Die in or near water
- Soft parts consumed by bacteria
- Sedimentation (fine grained = more detail; chemical makeup of rock = colour)
o E.g. clay preserves more details that sand
o E.g. if a lot of iron, fossil may be red
- More sediments pile on = pressure = rock
- Mineral rich water = changes to the shell
- Uplift
- Erosion
o When eroded, fossils will be seen
- E.g. ammolite
o An organic gemstone
o Iridescent aragonite with trace elements (Fe, Mn, Al, Ba)
o Very colourful
- Trace fossils
o Preserved tracks, trails, burrows, borings, footprints
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o When cavity is not filled
o Study of trace fossils: ichnology
o Process of disturbing sediment: bioturbation
Mixing sediments together
- Fossils as clues to ancient environments
o Palaeoecology: study of ancient organisms and their environments
o Clues from:
Fossil types
Assemblages
Fossil morphology (body types)
Trace fossils
Fossils and stratigraphy
- Used to establish relative age of rock units, correlate units
Each strata/layer represents a different time
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v[PvÆu
o What information do we need to do this
Relative age of rock units
Fossil species present in each unit
Establish time of first and last appev}(Z][P}o}P]vP
o What kinds of fossils are most useful for stratigraphic work?
Short-lived, widespread
x KPv]uZo]À}µvZÁ}ovÁv[}µv(}Zo}vP
Assemblages (groups) of species
x Fossils that live around the same time period
o Trilobite growth
Trilobites molted regularly
Therefore very abundant in fossil records
The evolution of life on earth
- The earliest bacteria appered 3.8 Ga
- Hadean
o Intense meteorite bombardment from 4.5 to 4 Ga prevented life becoming established
o Oceans of magma
o E}uPv](]o}Àv}vvu}Z]vP]}((Zov[
surface by the solar wind
The earth had not differentiated yet and the magma in the core had not started
moving around yet
- The oldest fossils?
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