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 O 2 ) 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

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

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 Africa tillites

Glaciation and Mass Extinction North African glaciation