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The Cambrian and Ordovician Life: Fossils, Reefs, and Mass Extinctions

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  1. Chapter 13 The Early Paleozoic World

  2. 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?

  3. 444 Million years 488 Million years 542 Million years

  4. Cambrian Explosion • Lowermost Cambrian • Simple skeletal fossils • Teeth

  5. Cambrian Explosion • Large animals with skeletons • Trilobites • Arthropods with calcified segmented skeletons

  6. Cambrian Explosion • Bottom-dwelling forms create scratch marks • Similar to some Neoproterozoic tracks

  7. Cambrian Explosion • Other abundant Early Cambrian animal groups • Monoplacophoran mollusks • Inarticulate brachiopods • Echinoderms

  8. Cambrian Explosion • Chengjiang fauna • Soft- bodied creatures including: • Cnidarians • Predatory worms • Anomalocarids • Huge carnivores (2 m) • Swimmers • Impaled prey

  9. 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

  10. Cambrian Explosion • Stromatolites • Less abundant; more restricted • Weak grazing pressure in inter-tidal zone

  11. Cambrian Explosion • Reefs • Archeocyathids • Suspension feeders • Probably sponges

  12. Cambrian Explosion • Evolutionary experimentation • Bizarre echinoderm classes • Few species and genera • Tried out many body plans

  13. Cambrian Explosion • Middle and Late Cambrian • 15 Million year duration • Expansion of many groups • Trilobites • Echinoderns • Conodonts • Early fish • Isolated bony external plates

  14. Cambrian Explosion • Burgess Shale Fauna • Western No. America • Deep-water setting (low O2) • Chordata • Pikaia: Notochord • Arthropods • Onychophorans • Intermediate between segmented worms and arthropods

  15. Ordovician Life • Great radiation • Graptolites • Nautiloids • Life in sediment • Burrowers expanded • Pump oxygen-bearing water into sediment • Diversification of worms and other soft-burrowers

  16. 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

  17. Ordovician Life • Sediments indicate burrowers flourished

  18. Ordovician Life • Extinctions • Large extinction events limited diversification • Cambrian mass extinctions • End of Ordovician mass extinction

  19. Ordovician Life • Plants may have invaded land • Inconclusive evidence • Probably restricted to moist habitats

  20. Paleogeography • Cambrian • Cratons formed supercontinent early in Cambrian • Progressive flooding of continents • Regression in Middle Cambrian and again in Late Cambrian

  21. Ordovician Life • Transgression • Yields characteristic sedimentary pattern • Siliciclastic sediments • Innermost belt • Carbonate platform • Seaward of siliciclastics

  22. Cambrian Events • Episodic mass extinctions • Shallow- water trilobites

  23. Cambrian Events • Took a few thousand years each • Temporary cooling of the seas

  24. Paleogeography • Gondwanaland nearing south pole • Glacier expanded • Sea-level fell • Mass extinction (2 pulses) • Early Ordovician • Baltica began move from South Pole • End of Ordovician • Baltica moved to tropics

  25. Taconic Orogeny • Ordovician mountain building • Early Ordovician carbonate platform east coast of Laurentia • Mid-Ordovician carbonate deposition stopped; flysch sedimentation dominated

  26. Taconic Orogeny • Flysch overlain by molasse • Clastic wedge tapering towards northwest

  27. Taconic Orogeny • Carbonate platform wedged into subduction zone • Exotic terrane

  28. Taconic Orogeny • Fossils of different fauna but same age

  29. Taconic Orogeny • With continued collision, foreland basin migrated westward

  30. Western Laurentian Margin • Stable continental shelf • Steep carbonate platform edge • Accumulated thick limestone sequences

  31. Western Laurentian Margin • Burgess Shale • Unusual fauna • Collected by Walcott

  32. Western Laurentian Margin • Buried by turbidites • Accumulated in oxygen-poor environment

  33. Tommotian Fauna

  34. Ordovician Oolites

  35. Reefs Colonial reef building rugose corals

  36. Glaciation and Mass Extinction Ordovician glaciation

  37. Glaciation and Mass Extinction North Africa tillites

  38. Glaciation and Mass Extinction North African glaciation

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