EESA06H3 Lecture Notes - Lecture 6: Subduction, Bivalvia, Anomalocaris

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8 Aug 2016
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EESA06 Lecture 6 Page 1
EESA06 Lecture 6
Fossils
- Step 1: The earliest Bacteria: 3.8 Ga(?)
- Step 2: The Cambrian Explosion: 542 Ma (multicellular animals with shells, backbones etc)
- Step 3: Diversification During the Paleozoic: c.400-250 Ma
- Step 4: Role of Meteorites
- Nova DVD: Becoming Human
Evolution of Earth:
- Hadean Period – “hell on earth” - Rocks was molten, no water no atmosphere
- Diversification – heavier elements sank to the middle
- Magnetic field – to hold on to the gases – which build up atmosphere + oceans
How are fossils formed?
- Shells buried and preserved unaltered (<100 million years)
- Mineralization - Cavities are filled with silica, calcium carbonate, iron (ex: tree stumps)
- Most fossils are found in sedimentary rock
oException: Pompeii people covered by pyroclastic flow and fossilized in ash
Per-mineralization
- Die in or near water
- Soft parts consumed by bacteria
- Sedimentation (fine grained =more detail, chemical makeup of rock = color)
- More sediments pile on = pressure – rock
- Mineral rich water = changes to the shell
- Uplift – by continental crusts coming together (plate tectonics)
- Erosion – wear away rocks
Ammolite – an organic gemstone
Ammonite fossils – iridescent aragonite with trace elements
(Fe, Mn, Al, Ba)
Trace Fossils
- Preserved tracks, trails, burrows (protection/food), borings
- Ichnology – study of trace fossils
- Bioturbation – process of disturbing sediment
Fossils as clues to ancient environments
- Palaeocology – study of ancient organisms and their environments
o(St.Cuthbert rings, Sea Lily - Crinoide)
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EESA06 Lecture 6 Page 2
oClues from: fossil types, assemblages (group of fossils tend
to be found together), fossil morphology, trace fossils
Fossils and stratigraphy
- How do we use fossils in stratigraphy?
oto establish relative age of rock units, correlate units
- What information do we need to do this?
oRelative age of rock units
oFossil species present in each unit
oGeologic range - Establish time of first and last appearance of each species
William Smith – Principal of Faunal Succession – fossils appear in layers
What kinds of Fossils are most useful for stratigraphic work?
- Index Fossils (ex: Ammonites) - Are Short-lived and widespread
- Assemblages (groups) of species
Trilobite Growth
- Trilobites molted regularly – shed skin
- Thus very abundant in the fossil record
The evolution of Life on Earth…
- Intense bombardment from 4.5 to 4 Ga prevented life becoming established
- No magnetic field to prevent oceans and atmosphere being stripped off the planet’s surface by
the solar wind?
Sir James Dawson – was wrong.
- Thought to be the oldest fossil Eozoon Canadense (1868) – ‘the dawn of animal of Canada’ now
recognized as mineral not organic – dubiofossil
Step 1: The Earliest bacteria 3.3 Ga (?)
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