Earth Systems 3209

1. Earth Systems 3209 Reference: Chapters 6, 8; Appendix A & B Unit: 2Historical Geology

2. Unit 2: Topic 3.1 Fossils and Geologic Time Focus on . . . • define fossil and explain how fossils are used to subdivide geologic time.

3. Fossils and Geologic Time • Fossils:- are the remains or traces of organisms found in sedimentary rocks. • When plants and animals die they get buried in sediment and the soft parts usually decay with the hard parts being fossilized when the sediment turns to solid rock. • Fossils provide the basis by which the subdivisions of the Geologic Timescale are divided. Text Reference: Pages 183 - 186

4. Fossils and Geologic Time • The record of life in the geologic past may be biased: • the fossil record shows an abundance of organisms that contained hard parts and lived in environments of high sedimentation. However, only a glimpse of the numerous other life forms exist in the fossil record. • In the early 1800’s an English scientist named William Smith noticed that the same fossils were identified in the same rock types. This evidence was the background work for one of the fundamental principles of historical geology known as the principle of fossil succession. • This principle states, “fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.”

5. Fossils and Their Importance in Geology What is the importance of fossils to geologist? And …. What does a fossil indicate? Fossils indicate the age of sedimentary rocks. • Within each division of time there are many subdivisions based on certain species of fossils. For example, the divisions of the geologic time scale is subdivided according to the presence and absence of fossils. • This same succession of fossils within sedimentary rock layers is seen on every major landmass.

6. Fossils and Their Importance in Geology What is the importance of fossils to geologist? And …. What does a fossil indicate? 2. Fossils indicate the environments in which rocks formed. • Knowing the nature of life-forms that existed at a particular time may indicate the environment in which the sedimentary rock formed. • Past environments can be indicated by studying the nature and characteristics of sedimentary rocks and the fossils they contain. For example, if clam shells are found in limestone, a geologist could assume that the region was covered by a shallow sea, because that is where clams are found today. This assumption coincides with the idea of uniformitarianism. • Fossil characteristics reveal what type of environment the organism lived in the past.

7. Fossils and Their Importance in Geology What is the importance of fossils to geologist? And …. What does a fossil indicate? 3. Fossils are used to correlate (match up) rocks. • Once fossils were recognized as time indicators, they became a useful means of correlating rocks of similar age in different regions. • Scientist use fossils called index fossils which are widespread geographically and limited to a short span of geologic time. The presence of these fossils are important when matching rocks of the same age. • If index fossils are not present, then groups of fossils in the same rocks are used to correlate rocks of the same age.

8. Fossils and Their Importance in Geology What is the importance of fossils to geologist? And …. What does a fossil indicate? 4. Fossils provide the basis to subdivide the Geologic Timescale. • By studying characteristics of certain fossils and the type of fossils present in sedimentary rocks, different aspects of the geologic past can be interpreted by geologist. Things such as, temperatures, climate, type of environment, etc….

9. Fossils and Their Importance in Geology What is the importance of fossils to geologist? And …. What does a fossil indicate? 5. Fossils can also indicate evolutionary pathways. • With an understanding of the principle of fossil succession, when fossils are arranged according to their age by applying the law of superposition, fossils in the rocks show a progressive change demonstrating the evolution of life through time. • For example, an Age of Invertebrates, such as the trilobites, are recognized early in the fossil record. Then, in succession, paleontologist recognize an Age of Fishes, an Age of Amphibians, an Age of Reptiles, and an Age of Mammals. • Thus, it is thought that Invertebrates evolved to form Fish, which evolved into Amphibians, which evolved into Reptiles, and finally Mammals.

10. Example 1: Which is used to conclude that Layer B is the same age as layer D? (A) fossil correlation(B) plate tectonics (C) superposition(D) uniformitarianism

11. Example 2: • Which best describes the progression of life forms from Precambrian to Cenozoic? • bacteria – dinosaurs– trilobites– mammoths • bacteria – trilobites– dinosaurs – mammoths • dinosaurs – bacteria– mammoths– trilobites • dinosaurs – mammoths – bacteria – trilobites

12. Your Turn . . . Take the time and complete the following questions . . .(Solutions to follow) Question: • Which is characteristic of an index fossil? • limited to a short span of geologic time • only found on continental landmasses • represent the only species of its kind • restricted to a small geographic area

13. Solutions . . . Questions: • Which is characteristic of an index fossil? • limited to a short span of geologic time • only found on continental landmasses • represent the only species of its kind • restricted to a small geographic area

14. Summary . . . Overview of Points covered: Fossils - are the remains or traces of organisms found in sedimentary rocks Importance of Fossils in Geology: Fossils indicate the age of sedimentary rocks. Fossils indicate the environments in which rocks formed. Fossils are used to correlate (match up) rocks. Fossils provide the basis to subdivide the Geologic Timescale. Fossils can also indicate evolutionary pathways..