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Chapter 13The Early Paleozoic World
Guiding Questions• What kinds of animal skeletons arose during the
Cambrian period?• How did Ordovician life differ from Cambrian life?• Why did stromatolites decline during Cambrian and
Ordovician time?• What kind of highly successful reef community
developed during the Ordovician time?• What major continental movements took place late
in the Ordovician time?
444 Million years
488 Million years
542 Million years
Cambrian Explosion• Lowermost Cambrian
– Simple skeletal fossils– Teeth
Cambrian Explosion
• Large animals with skeletons– Trilobites– Arthropods with
calcified segmented skeletons
Cambrian Explosion• Bottom-dwelling
forms create scratch marks– Similar to some
Neoproterozoic tracks
Cambrian Explosion• Other abundant
Early Cambrian animal groups– Monoplacophoran
mollusks– Inarticulate
brachiopods– Echinoderms
Cambrian Explosion• Chengjiang fauna
– Soft- bodied creatures including:
• Cnidarians
• Predatory worms
• Anomalocarids– Huge carnivores (2 m)
– Swimmers
– Impaled prey
Cambrian Explosion• Modes of Life• Deposit feeders
– Extract organic matter from sediments
– Trilobites, arthropods
• Suspension feeders– Collect organic matter
from the water– Eocrinoids
• Attach by stalk
Cambrian Explosion• Stromatolites
– Less abundant; more restricted – Weak grazing pressure in inter-tidal zone
Cambrian Explosion
• Reefs– Archeocyathids– Suspension
feeders– Probably
sponges
Cambrian Explosion• Evolutionary
experimentation– Bizarre echinoderm
classes• Few species and
genera– Tried out many
body plans
Cambrian Explosion• Middle and Late
Cambrian– 15 Million year duration– Expansion of many
groups• Trilobites• Echinoderns• Conodonts
– Early fish• Isolated bony external
plates
Cambrian Explosion• Burgess Shale Fauna
– Western No. America
– Deep-water setting (low O2)
• Chordata– Pikaia: Notochord
• Arthropods– Onychophorans
• Intermediate between segmented worms and arthropods
Ordovician Life• Great radiation
– Graptolites– Nautiloids
• Life in sediment– Burrowers expanded
• Pump oxygen-bearing water into sediment
• Diversification of worms and other soft-burrowers
Ordovician Life• Life on the seafloor
– Diversity of benthic organisms increased
– Jawless fishes– Grazing snails– Articulate brachiopods– Crinoids expanded
• Coral-strome reefs– Rugose corals– Tabulate corals– Stromatoporoids
Ordovician Life
• Sediments indicate burrowers flourished
• Extinctions– Large extinction
events limited diversification
– Cambrian mass extinctions
– End of Ordovician mass extinction
Ordovician Life
Ordovician Life• Plants may have
invaded land– Inconclusive
evidence– Probably restricted
to moist habitats
Paleogeography• Cambrian
– Cratons formed supercontinent early in Cambrian– Progressive flooding of continents
• Regression in Middle Cambrian and again in Late Cambrian
Ordovician Life• Transgression
– Yields characteristic sedimentary pattern
• Siliciclastic sediments– Innermost belt
• Carbonate platform– Seaward of
siliciclastics
Cambrian Events• Episodic mass extinctions
– Shallow- water trilobites
Cambrian Events• Took a few thousand years each
• Temporary cooling of the seas
Paleogeography• Early Ordovician
– Baltica began move from South Pole
• End of Ordovician– Baltica moved to tropics
• Gondwanaland nearing south pole– Glacier expanded– Sea-level fell– Mass extinction (2 pulses)
Taconic Orogeny• Ordovician mountain building• Early Ordovician carbonate platform east coast of Laurentia• Mid-Ordovician carbonate deposition stopped; flysch
sedimentation dominated
Taconic Orogeny• Flysch
overlain by molasse
• Clastic wedge tapering towards northwest
Taconic Orogeny• Carbonate platform wedged into subduction zone
• Exotic terrane
• Fossils of different fauna but same age
Taconic Orogeny
Taconic Orogeny
• With continued collision, foreland basin migrated westward
Western Laurentian Margin
• Stable continental shelf
• Steep carbonate platform edge– Accumulated thick
limestone sequences
Western Laurentian Margin• Burgess Shale
– Unusual fauna – Collected by
Walcott
Western Laurentian Margin
• Buried by turbidites– Accumulated in
oxygen-poor environment
Tommotian Fauna
Ordovician Oolites
ReefsColonial reef
building rugose corals
Glaciation and Mass Extinction
Ordovician glaciation
Glaciation and Mass Extinction North Africa tillites
Glaciation and Mass Extinction
North African glaciation