Chapter 31: Evolving earth
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History of life on earth
•
Fossils provide information about past life
– preserved remains or indications of organisms
•
Whole organisms or parts of organisms
– shells, bones, leaves, pollen
•
Tracks and traces
– footprints, burrows
•
Chemical fossils
– organic compounds
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Fossils
•
Organisms are preserved when they are protected
from decay
– buried in sediment
– encased in resin
•
Compaction and chemical changes turn sediments
into rock
• Depending on type of rock, fossils may
–
–
–
–
–
–
retain their shape
be flattened
be carbonised (soft parts preserved as a carbon film)
be filled in with minerals
dissolved out leaving an empty mould
replaced with other material, such as silica (opal)
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Time scales
•
Rock layers (strata) contain characteristic sets
(suites) of fossils
– rocks with the same suite of fossils are of the same age
•
Fossils are used to divide the geological time scale
into eras and periods
• Ages of rocks and fossils determined using
radiometric dating
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Fig. 31.5: Geologic time scale
(based on the 2000 edition of the International Stratigraphic chart)
(cont.)
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Fig. 31.5: Geologic time scale (cont.)
(based on the 2000 edition of the International Stratigraphic chart)
Copyright © T Itoh & R M Brown Jr, Planta Journal, vol. 160, pp. 372–81. Springer-Verlag, 1984
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Eras
•
Major divisions of geologic time
–
–
–
–
•
Precambrian ( 4 500–545 m.y.a.)
Palaeozoic (545–250 m.y.a.)
Mesozoic (250–65 m.y.a.)
Cenozoic (65–0 m.y.a.)
Divided into periods
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Precambrian Era
•
Archean eon (4 500–2 500 m.y.a.)
• Proterozoic eon (2 500–545 m.y.a.)
• Oldest fossils 3 500–3 300 m.y.a.
– prokaryotes: cyanobacteria (including stromatolites)
•
Photosynthesis released oxygen into atmosphere
– changed composition of atmosphere
– 2 300–1 800 m.y.a.
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Eukaryotes
•
Prokaryotes only form of life for 2 100 m.y.
• Oldest eukaryote fossils 1 400 m.y.a.
– single-celled protists with chloroplasts
•
Multicelled organisms
– metaphytes (red algae) 1 260–950 m.y.a.
– animals 680–640 m.y.a.
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Ediacaran fauna
•
Fossils of soft-bodied animals
– impressions
– tracks and burrows
– no hard exo- or endoskeletons
•
Difficult to classify some fossils
– do not resemble living phyla
•
Fossils first recorded from Ediacara Hills, South
Australia
– fauna on all continents
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Palaeozoic era
‘Ancient life’ (545–250 m.y.a.)
• Aquatic groups
•
–
–
–
–
trilobites
brachiopods
nautiloids, ammonoids (cephalopod molluscs)
jawless and jawed fish (‘Age of Fish’)
(cont.)
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Palaeozoic era (cont.)
•
Life moves onto land in Silurian period
–
–
–
–
•
arthropods
amphibians
reptiles
land plants
Ended with mass extinction
– Permian period (250 m.y.a.)
– trilobites, many invertebrate groups
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Mesozoic
‘Middle life’ (250—65 m.y.a.)
• Plants and invertebrates
•
–
–
–
–
–
ferns, conifers, cycads
flowering plants
bivalves, predatory gastropods
ammonoids
reef-building corals
(cont.)
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Mesozoic (cont.)
•
Vertebrates
– mammal-like reptiles, dinosaurs, marine reptiles, flying
reptiles (‘Age of Reptiles’)
– birds
– mammals
•
Ended with mass extinction
– K-T boundary
– ammonites (shelled cephalopods), many large reptiles
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Cenozoic
‘Modern life’ (65—0 m.y.a.)
• Major groups
•
– mammals diversify (‘Age of Mammals’)
– hominids
– modern forms of most organisms
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Biogeography
•
Biogeographic regions
– areas of the earth occupied by characteristic biota
Terrestrial
Marine
Palaearctic
Arctic/Antarctic
Oriental
Cool temperate
Ethiopian
Warm temperate
Australian
Tropical
Nearctic
Neotropical
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Fig. 31.20: Biogeographic regions
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‘New World’
•
Nearctic
– North America, Greenland
– examples:

kangaroo rat
 pronghorn
 turkey
•
Neotropical
– South America, Central America
– examples:

New World monkeys
 sloth
 rhea
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‘Old World’
•
Palaearctic
– Europe, Asia
– examples:

desman
 saiga
•
Ethiopian
– Africa, Madagascar, Arabia
– examples:

hippopotamus
 giraffe and okapi
 lemurs (Madagascar)
(cont.)
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‘Old World’ (cont.)
•
Oriental
– India, South-East Asia
– examples:

tree shrew
 gibbon
 leafbird
•
Australian
– Australia and adjacent areas
– examples:

echidna, platypus
 kangaroo
 emu
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Australian region
•
More than Australia
– New Zealand, islands of Western Pacific, New Guinea,
eastern Indonesia archipelago
– biota determined by history and proximity
•
Wallace’s Line
– boundary between Australian and Oriental biogeographic
regions

Oriental biota dominates to west, Australian biota to east
– approximates collision zone between tectonic plates
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Fig. B31.5: Wallace’s Line
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Plate tectonics
•
Earth’s surface formed from mobile plates
– tectonic plates
•
Move relative to one another
– plate tectonics
– continental drift
•
Configuration of land masses have changed over
geologic time
– still changing
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Fig. 31.6: Distribution of plates
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Sea floor spreading
•
Plate movement driven by sea-floor spreading
• Lava rises from mantle to surface at oceanic
ridges
– forms new oceanic crust on either side
•
Continents pushed away from oceanic ridges
– example: Indian–Antarctic ridge in Southern Ocean
separating Australian and Antarctic plates
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Subduction zones
•
Where edges of plates meet
• One plate slides beneath the other into mantle
– subduction zone
– deep water trenches
– examples: Marianas Trench, Java Trench
•
Plates slide past each other
– major fault zones
– example: San Andreas Fault
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Fig. 31.7: Cross-section of lithosphere
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Ancient earth
•
Movement of continents over geologic time
• Land masses accreted into different configurations
at different times
– Gondwana

southern masses
– Laurasia

northern masses
– Pangea

southern + northern land masses
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