1. 18.2 Precambrian Time: Vast and Puzzling
Precambrian History
18.2 Precambrian Time: Vast and Puzzling
 The Precambrian encompasses immense geological time, from Earth’s distant beginnings 4.56 billion years ago until the start of the Cambrian period, over 4 billion years later.
 Precambrian Rocks
• Shields are large, relatively flat expanses of ancient metamorphic rock within the stable continental interior.
• Much of what we know about Precambrian rocks comes from ores mined from shields.

2. Geologic Time Scale
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3. Remnants of Precambrian Rocks
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4. 18.2 Precambrian Time: Vast and Puzzling
Precambrian History
18.2 Precambrian Time: Vast and Puzzling
 Earth’s Atmosphere Evolves
• Earth’s original atmosphere was made up of gases similar to those released in volcanic eruptions today—water vapor, carbon dioxide, nitrogen, and several trace gases, but no oxygen.
• Later, primary plants evolved that used photosynthesis and released oxygen.
• Oxygen began to accumulate in the atmosphere about 2.5 billion years ago.

5. 18.2 Precambrian Time: Vast and Puzzling
Precambrian History
18.2 Precambrian Time: Vast and Puzzling
 Precambrian Fossils
• The most common Precambrian fossils are stromatolites.
• Stromatolites are distinctively layered mounds or columns of calcium carbonate. They are not the remains of actual organisms but are the material deposited by algae.
• Many of these ancient fossils are preserved in chert—a hard dense chemical sedimentary rock.

6. Stromatolites
Stromatolites of Hamelin Pool

7. 18.2 Paleozoic Era: Life Explodes
Early Paleozoic
18.2 Paleozoic Era: Life Explodes
 Following the long Precambrian, the most recent 540 million years of Earth’s history are divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

8. 18.2 Paleozoic Era: Life Explodes
Early Paleozoic
18.2 Paleozoic Era: Life Explodes
 Early Paleozoic History
• During the Cambrian, Ordovician, and Silurian periods, the vast southern continent of Gondwana encompassed five continents (South America, Africa, Australia, Antarctica, and part of Asia).

9. Gondwana and the Continental Landmasses
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10. 18.2 Paleozoic Era: Life Explodes
Early Paleozoic
18.2 Paleozoic Era: Life Explodes
 Early Paleozoic Life
• Life in early Paleozoic time was restricted to the seas.

11. Life in the Ordovician Period
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12. 18.2 Paleozoic Era: Life Explodes
Late Paleozoic
18.2 Paleozoic Era: Life Explodes
 Late Paleozoic History
• Laurasia is the continental mass that formed the northern portion of Pangaea, consisting of present-day North America and Eurasia.
• By the end of the Paleozoic, all the continents had fused into the supercontinent of Pangaea.

13. Late Paleozoic Plate Movements
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14. 18.2 Paleozoic Era: Life Explodes
Late Paleozoic
18.2 Paleozoic Era: Life Explodes
 Late Paleozoic Life
• Some 400 million years ago, plants that had adapted to survive at the water’s edge began to move inland, becoming land plants.
• The amphibians rapidly diversified because they had minimal competition from other land dwellers.

15. Armor-Plated Fish
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16. Model of a Pennsylvanian Coal Swamp
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17. 18.2 Paleozoic Era: Life Explodes
The Great Paleozoic Extinction
18.2 Paleozoic Era: Life Explodes
 The world’s climate became very seasonal, probably causing the dramatic extinction of many species.
 The late Paleozoic extinction was the greatest of at least five mass extinctions to occur over the past 500 million years.

18. 18.2 Mesozoic Era: Age of Reptiles
 Dinosaurs were land-dwelling reptiles that thrived during the Mesozoic era. (Jurassic Period)
 Mesozoic History
• A major event of the Mesozoic era was the breakup of Pangaea.

19. 18.2 Mesozoic Era: Age of Reptiles
 Mesozoic Life
• Gymnosperms are seed-bearing plants that do not depend on free-standing water for fertilization.
• The gymnosperms quickly became the dominant plants of the Mesozoic era.

20. Canadian Rockies Were Formed Throughout the Cretaceous Period
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21. 18.2 Mesozoic Era: Age of Reptiles
 The Shelled Egg
• Unlike amphibians, reptiles have shell-covered eggs that can be laid on the land.
• The elimination of a water-dwelling stage (like the tadpole stage in frogs) was an important evolutionary step.

22. 18.2 Mesozoic Era: Age of Reptiles
 Reptiles Dominate
• With the perfection of the shelled egg, reptiles quickly became the dominant land animals.
• At the end of the Mesozoic era, many reptile groups became extinct. (end of the Cretaceous)

23. The Flying Reptile Pteranodon
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24. Fossil Skull of an Extinct Crocodile
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25. 18.2 Cenozoic Era: Age of Mammals
Cenozoic North America
18.2 Cenozoic Era: Age of Mammals
 The Cenozoic era is divided into two periods of very unequal duration, the Tertiary period and the Quaternary period.
 Plate interactions during the Cenozoic era caused many events of mountain building, volcanism, and earthquakes in the West.

26. 18.2 Cenozoic Era: Age of Mammals
Cenozoic Life
18.2 Cenozoic Era: Age of Mammals
 Mammals—animals that bear live young and maintain a steady body temperature— replaced reptiles as the dominant land animals in the Cenozoic era.
 Angiosperms—flowering plants with covered seeds—replaced gymnosperms as the dominant land plants.

27. 18.2 Cenozoic Era: Age of Mammals
Cenozoic Life
18.2 Cenozoic Era: Age of Mammals
 Mammals Replace Reptiles
• Adaptations like being warm blooded, developing insulating body hair, and having more efficient heart and lungs allow mammals to lead more active lives than reptiles.

28. Fossils from La Brea Tar Pits
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29. 18.2 Cenozoic Era: Age of Mammals
Cenozoic Life
18.2 Cenozoic Era: Age of Mammals
 Large Mammals and Extinction
• In North America, the mastodon and mammoth, both huge relatives of the elephant, became extinct. In addition, saber-toothed cats, giant beavers, large ground sloths, horses, camels, giant bison, and others died out on the North American continent.
• The reason for this recent wave of extinctions puzzles scientists.

30. The Geologic Time Scale
Paleozoic Era (six periods) Means “Ancient Life”
Cambrian Period (marine invertebrates) (540 million years ago)
Trilobites
Brachiopods
Ordovician Period (primitive fish) vertebrates (490 million yrs. ago)
Ostracoderms (bony plated fish)
No plant life on land
Silurian Period (marine vertebrates and invertebrates) (443 mya)
Eurypterids
Earliest land plants and animals such as spiders and
Millipedes

31. The Geologic Time Scale
Devonian Period (Age of Fishes)(417 mya)
Lung fish
Ichthyostega (first true amphibian)
Land plants such as giant horsetails, ferns and cone bearing plants
Carboniferous Period(Mississippian 354 mya,Pennsylvanian 323 mya)
Forests and swamps cover much of the land
Coal formation begins
Amphibians and fish thrive
Crinoids (relative of modern sea star) thrive
Reptiles appear at the end of this period
Permian Period (290 mya)
Mass extinction of numerous life forms
Appalachian Mountains form
Trilobites become extinct
Pangaea comes together

32. The Geologic Time Scale
The Mesozoic Era (three periods) Triassic Period (248 mya) Dinosaurs (Terrible Lizards) Ranged in size from small squirrel to 30 meters long Ichthyosaurs Ammonite (shellfish similar to the nautilus) 1st mammals appear Jurassic Period (206 mya) Apatosaurus, Stegosaurus, pterosaurs(flying reptiles) First birds appear Cretaceous Period (144 mya) Tyrannasaurus Rex Angiosperms (flowering plants) Mass extinction of the dinosaurs

33. The Geologic Time Scale
The Cenozoic Era (The Age of Mammals)
Teritiary Period
Paleocene and Eocene Epochs (65 mya – 54.8 mya)
Lemuroids
Hyracotherium (ancestor of the horse)
Flying squirrels, bats, whales
Oligocene and Miocene Epochs (golden age of Mammals)
Largest known land animals existed at this time
Raccoons, wolves, foxes, saber toothed cat
Modern polar ice caps began to form
Pliocene Epoch
Bear, dog, cat became fully evolved
Continental ice sheets began to spread
Bering Land bridge appeared as sea levels fell

34. The Geologic Time Scale
Quaternary Period Pleistocene Epoch Glaciation over Eurasia and North America Large mammal extinctions as humans entered the Holocene Epoch Ice age ends, sea levels rise 140 meters Great Lakes form Humans developed agriculture

35. Questions Why are fossils rare in Precambrian rocks?
Because possible fossils have been destroyed by weathering, erosion, volcanic activity and metamorphism and because early life forms lacked the hard parts that normally fossilize well
How did the formation of Pangaea affect Paleozoic life-forms?
The shallow inland seas disappeared, causing many species of marine invertebrates to die out
How did the ice ages affect animal life during the Cenozoic Era?
Warm-blooded, fur-covered mammals survived and became the predominant life-form
Compare the Permian extinction with the Cretaceous extinction.
Both extinctions allowed a new group of animals to become the dominant life-form in the following era Permian extinction=reptiles became dominant
Cretaceous extinction=mammals became dominant