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CHAPTER 13
The 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
Ordovician Life
Extinctions Large extinction events
limited diversification Cambrian mass
extinctions End of Ordovician
mass extinction
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
Taconic Orogeny
Fossils of different fauna but same age
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
Reefs
Colonial reef building rugose
corals
Glaciation and Mass Extinction
Ordovician glaciation
Glaciation and Mass Extinction
North African glaciation