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