EESA06H3 Lecture Notes - Lecture 6: Chordate, Anomalocaris, Trilobite

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25 Feb 2011
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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 t ^Zoo on earth_- Rocks was molten, no water no atmosphere
- Diversification t heavier elements sank to the middle
- Magnetic field t to hold on to the gases t 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
o Exception: 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 t rock
- Mineral rich water = changes to the shell
- Uplift t by continental crusts coming together (plate tectonics)
- Erosion t wear away rocks
Ammolite t an organic gemstone
Ammonite fossils t iridescent aragonite with trace elements
(Fe, Mn, Al, Ba)
Trace Fossils
- Preserved tracks, trails, burrows (protection/food), borings
- Ichnology t study of trace fossils
- Bioturbation t process of disturbing sediment
Fossils as clues to ancient environments
- Palaeocology t study of ancient organisms and their environments
o (St.Cuthbert rings, Sea Lily - Crinoide)
o Clues 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?
o to establish relative age of rock units, correlate units
- What information do we need to do this?
o Relative age of rock units
o Fossil species present in each unit
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o Geologic range - Establish time of first and last appearance of each species
William Smith t Principal of Faunal Succession t 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 t shed skin
- Thus very abundant in the fossil record
The evolution of Life on EarthY
- 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 t was wrong.
- Thought to be the oldest fossil Eozoon Canadense (1868) t Zthe dawn of animal of Canada[ now
recognized as mineral not organic t dubiofossil
Step 1: The Earliest bacteria 3.3 Ga (?)
- Prokaryotes t some are photosynthetic (called cyanobacteria: blue/green algae)
o Microscopic, single-celled, lacking a nucleus
- The earliest prokaryotes (the Archaea) lives around superheated waters near submarine
volcanic vents (hyperthermophilic)
o Simple bag-like cells
o Only life forms between 3.8 and about 2 Ga
Where on Earth did early life forms flourish?
- Probably in oceans t salts, solvents, mixing of elements to create organic compounds
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