Warm Up An example of a geologic event that could be recorded in rocks is ____. a. a lunar eclipse c. a civil war b. a volcanic eruption d. the occurrence of sun spots Nicolaus Steno proposed the most basic principle of relative dating, the law of ____. a. gravity c. superposition b. fossil succession d. correlation If a layer of sandstone is in contact with a mass of granite that contains small pieces of the sandstone, which rock is older? a. Both are the same age. b. the granite c. the sandstone d. It is not possible to determine which rock is older from this information. Answers: 1) b. 2) c. 3) c.

Fossils: Evidence of Past Life Chapter 12, Section 2

Fossil Formation Fossil – the remains or traces of an organism preserved from the geologic past The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried Some remains of animals—such as teeth, bones, and shells—may not have been altered hardly at all over time (unaltered remains) The remains of an organism are likely to be changed over time (altered remains) Fossils often become petrified, mineral-rich water soaks into small cavities and pores of the original organism, and replace the original material

Molds and casts are another common type of fossil; it is created when a shell or other structure is buried under sediment and then dissolved by groundwater Carbonization is effective in preserving leaves and delicate mineral forms, occurs when an organism is buried under fine sediment and subjected to pressures which squeeze out its liquid and gases Trace fossils are indirect evidence of prehistoric life (tracks, burrows, dung, etc.) Two conditions are important for preservation: rapid burial and the possession of hard parts

Fossil Formation

Fossil Types

Concept Check What are three types of fossils? Petrified remains, molds, casts, impressions, and trace fossils

Fossils and Correlation In the late 18th Century, William Smith found that fossils were not randomly distributed throughout the rock layers, but each layer contained a distinct assortment of fossils that did not occur in other layers The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order; any time period can be recognized by its fossil content Based on the rock record from around the world, geologists have identified an order of fossils: an Age of Trilobites, an Age of Fishes, an Age of Coal Swamps, an Age of Reptiles, and an Age of Mammals

Index Fossil – a fossil that is associated with a particular span of geologic time Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers Their presence provides an important method of matching rocks of the same age Fossils can also be used to interpret and describe ancient environments

Fossils and Correlation

Index Fossils

Assignment Read Chapter 12, Section 2 (pg. 343-346) Do Chapter 12 Assessment #1-33 (pg. 259-260) For Section 2: Do #’s 3, 6, 16-18, 25, 27, 28, 30, & 31