1. Chapter 21 – Earth’s History
21.1 – The Rock Record

2. Why study Earth’s history?
Scientists organize geologic time to help them communicate about Earth’s history.

3. Organizing Time
Fossils = the remains, traces, or imprints of ancient organisms.
By studying rock layers and the fossils within them, geologists can reconstruct aspects of Earth’s history and interpret ancient environments

4. Geologic Time Scale
A record of Earth’s history from its origin 4.6 billion years ago (bya) to the present.
The geologic time scale enables scientists to find relationships among the geological events, environmental conditions, and fossilized life-forms that are preserved in the rock record

6. Visualizing Geologic Time
Eons = is the largest of these time units and encompasses the others
Divided into 4 different eons = Hadean, Archean, Proterozoic, and Phanerozoic
The three earliest eons make up 90 percent of geologic time, known together as the Precambrian.
During the Precambrian, Earth was formed and became hospitable to modern life.
Eons are further divided into eras

7. Eras Usually tens to hundreds of millions of years in duration.
Divided into 4 different eras = Precambrian, Paleozoic, Mesozoic, Cenozoic
Eras are divided into periods

8. Periods Tens of millions of years in duration
Each period in the geologic time scale contains epochs.
Epochs = hundreds of thousands to millions of years in duration

9. http://www. habitat. adfg. state. ak

10. In-Class Assignment/Homework
Your Geologic Time Scale

11. 21.1 – Notes Continued

12. Succession of Life-Forms
Phanerozoic Eon - multicellular life began to diversify
Paleozoic Era - the oceans became full of many different kinds of organisms

13. Succession of Life-Forms
Trilobites = Small, segmented animals among the first hard-shelled life-forms.
They dominated the oceans in the early part of the Paleozoic Era.
Land plants appeared later, followed by land animals.

14. Succession of Life-Forms
End of the Paleozoic is marked by the largest mass extinction event in Earth’s history
In a mass extinction, many groups of organisms disappear from the rock record at about the same time
At the end of the Paleozoic, 90 percent of all marine organisms became extinct

15. Succession of Life-Forms
Mesozoic Era is known for the emergence of dinosaurs
Many other organisms appeared, including large predatory reptiles and corals in the oceans

16. Succession of Life-Forms
During the Mesozoic, water-dwelling amphibians began adapting to terrestrial environments
Insects, some as large as birds, lived
Mammals evolved and began to diversify
Flowering plants and trees emerged

17. Succession of Life-Forms
End of the Mesozoic is marked by a large extinction event
Many groups of organisms became extinct, including the non-avian dinosaurs and large marine reptiles

18. Succession of Life-Forms
Cenozoic Era - mammals increased both in number and diversity
Human ancestors, the first primates, emerged in the epoch called the Paleocene, and modern humans appeared in the Pleistocene Epoch

20. In-Class Assignment/Homework
21.1 WKT (Transparency & Study Guide)