Fossils and The Geologic Time Scale

What are Fossils?

• Fossils are remains or traces of ancient plants and animals preserved in the Earth's crust.
• Actual body parts are rarely preserved; often, we have imprints or traces like burrows.
• Fossils indicate the presence and type of life in a past geological period.

Fossilization Process

• The process of turning organic remains into fossils is known as fossilization.
• Requires rapid burial to prevent decay and scavenging.
• Hard parts like bones, shells, and teeth are more likely to be preserved.
• Marine organisms are overrepresented due to the rapid burial in ocean sediments.

Types of Fossilization

1. Actual Preservation: Rarely occurs; involves entire body preservation, e.g., insects in amber or dinosaur bones.
2. Molds: Impressions left after the organism's body parts dissolve, leaving external or internal molds.
3. Casts: Formed when a mold is filled with minerals or sediments, creating a reproduction of the organism.
4. Permineralization: Replacement of organic molecules with minerals, preserving the shape with new mineral material.
5. Carbonization: Common in plants; organic material leaves a carbon silhouette after being compressed.
6. Trace Fossils: Indirect evidence such as footprints, burrows, or coprolites showing an organism's presence.

Index Fossils

• Used for correlating rocks over large geographic areas.
• Must be easily recognizable, widely distributed, and from a limited time period.
• Examples include trilobites, ammonites, and dinosaurs.

Geologic Time Scale

• A calendar of Earth's history, divided based on fossil records rather than equal time intervals.
• Principle of Fossil Succession: Unique fossil assemblages correlate with specific geological times.

Main Divisions

1. Precambrian: 4.56 billion to 542 million years ago; mostly single-celled organisms.
2. Paleozoic: 542 to 251 million years ago; first fish, land plants, amphibians, reptiles, dominance of marine invertebrates.
3. Mesozoic: 251 to 65 million years ago; dinosaurs, mammals, birds evolve.
4. Cenozoic: 65 million years ago to present; mammals become dominant, rise of humans.

Mass Extinctions

• Permian-Triassic Extinction (250 million years ago): Largest, likely due to Siberian volcanic eruptions.
• Triassic-Jurassic Extinction (200 million years ago): Likely due to Atlantic volcanic eruptions.
• Cretaceous-Paleogene Extinction (65 million years ago): Asteroid impact, extinction of dinosaurs.

Correlation in Geology

• Rocks can be correlated by stratigraphy and fossil assemblages across different regions.
• Used to understand and date geological layers and events.

Conclusion

• Geologic time scale originally based on fossil records, now dated using radiometric methods.
• It continues to be refined with ongoing fossil discoveries.