1. Fossils & the Rock Record
Chapter 21
Fossils & the Rock Record

2. Major milestones in life
Crawling, walking, potty training (still in progress?), first day of school, first kiss, first gf/bf, first love/break up, starting high school, driving a car, graduating, full-time job, parenting, retiring
Are there any categories into which these events can be placed?
Could include infancy, toddlerhood, childhood, adolescence, adulthood
How do these categories help people communicate and analyze their life histories?
- Not everyone reaches the same milestone at the same time. It is using to have references for communicating events

3. 21.1 The Rock Record Objectives
Explain why scientists need a geologic time scale
Distinguish among eons, eras, periods, and epochs
Characterize the groups of plants and animals that dominated eras in Earth’s history
Main Idea: Scientists organize geologic time to help them communicate about Earth’s history
Unknown vocab: fossil—the remains, trace, or imprint of a once-living plant or animal

4. Organizing Time Some of these layers contain fossils
These multicolored layers of rock are called strata
Some of these layers contain fossils
By studying such rock layers and the fossils within them, geologists can reconstruct aspects of Earth’s history and interpret ancient environments
To help in the analysis of Earth’s rocks, geologists have divided history of Earth into time units
Based largely on the fossils contained within the rocks
Time units are part of the geologic time scale—a record of Earth’s history from its origin (4.6 bya) to the present)
Some units have remain unchanged for centuries, while others have been reorganized based on new evidence

5. Geologists organize Earth’s history according to grouping called eons
Each eon contains era, which contain periods, which contain periods, which contain epochs E + P = EP

6. Geologic Time Scale
Enables scientists to find relationships among geological events, env. conditions, and fossilized life-forms preserved in rock record
Eons:
Largest of time units and contains all other units of time
Consist of Hadean, Archean, Proterozoic, and Phanerozoic Eons
3 earliest eons make up 90% of geologic time, known as Precambrian
During Precambrian, Earth was formed and became hospitable to modern life
Simple life-forms began in Archean Eon and that by end of Proterozoic Eon, life had evolved to point that some organisms might have been able to move in complex ways

7. Precambrian Most fossils from this eon were soft-bodied organisms
Others had bodies with rigid parts
All life-forms had soft bodies without shells or skeletons
Era time units are defined by the different life forms found in the rocks
Some periods are named for the geographic region in which the rocks or fossils characterizing the age were first observed and described

8. Eras, Periods, & Epochs Eras Periods Epochs
All eons made up of eras—next largest unit of time
Usually tens to hundreds of millions of years
Periods
All eras divided into periods
Generally tens of millions of years in duration
Epochs
All periods are divided into epochs
Generally hundreds of thousands to millions of years in duration
Rocks & sediments from epochs of Cenozoic Era are most complete due to less time for weathering & erosion to remove evidence

9. Succession of life-forms
During Phanerozoic Eon, multicellular life began to diversify
First era of this eon—Paleozoic—oceans became full of many different kinds of organisms
Small, segmented animals called trilobites were among first hard-shelled life-forms
Dominated oceans in early part of Paleozoic Era
Land plants appeared later, followed by land animals
End of Paleozoic is marked by largest mass extinction event in Earth’s history
During a mass extinction, many groups of organism disappear from rock record at about same time
At the end of the Paleozoic, 90% of all marine organisms became extinct

10. Conditions toward end of Paleozoic era
Pangea had been in its final stages of formation
Conditions became dry on Pangea
Even though ocean covered most of Earth, Pangea was so big that the interior did not benefit from the ocean waters
Swamp lands dried up—so many plants died out as well as amphibians that depended on the swamps
other theories include
Increased volcanic activity
Comets/meteors
Whatever the cause, life on Earth would never again look as it had during the Paleozoic Era

11. Age of dinosaurs Followed the Permian extinction
Era known as the Mesozoic
Large predatory reptiles and corals in the oceans also appeared during this era
Mammals continued to evolve and diversify
Flowering plants & trees emerged as well
End of the Mesozoic is marked by a large extinction that eliminated many groups of organisms—including the non-avian dinosaurs and large marine reptiles

12. Rise of mammals Era the followed the Mesozoic—known as the Cenozoic
Mammals increased in both number & diversity
1st primates, emerged in epoch called the Paleocene
Modern humans appeared in the Pleistocene Epoch