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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