The fossil record Chapter 8 section 3

Fossils The remains or traces of plants or animals that lived in a previous geologic time Important source of information for finding relative and absolute ages of rock. What normally happens to dead organisms? Almost all are covered in sedimentary rock- Slow or stop the process of decay Protect it from damage

PALEONTOLOGY The study of fossils WHY? reveal how organisms have changed throughout the geologic past. Provide clues for environmental changes EXAMPLE: Marine fossil found in the desert Use to learn about how environmental changes affected living organisms.

FOSSILIZATION Mummification- found in dry places where bacteria cannot survive Amber – hardened tree sap Tar Seeps- commonly covered in water…animal goes to drink..gets trapped in the sticky tar. Remains covered by tar and preserved Freezing- organisms buried in frozen soil or ice don’t decay because bacteria cannot survive the temperatures

Petrification- Minerals from groundwater replace organic material buried under sediment. Results in almost perfect replica of the original organism Carbon Films- Carbonized residue of leaves, stems, flowers and fish found in sedimentary rock made of mud or clay. After organic material partially decays-leaves behind a carbon film the shows surface features Molds and casts- shells can leave empty spacs in sediments (molds) within hardened sediment. When sand or mud fills the mold and turns to rock a cast is formed.

Coprolite- fossilized waste ;can reveal feeding habits of organisms Gastroliths- stones found in digestive systems of dinosaurs that help grind food. Found near dinosaur remains.

TRACE FOSSILS A fossilized mark that formed in sedimentary rock by movement of an animal on soft sediment Examples- Footprints of animals left in soft mud/clay. Gets gently covered by lighter sediment to protect it and then covered by more sediment; turning into sedimentary rock and preserving the footprint

INDEX FOSSILS Important in estimating the absolute age of specific rock layers. Are fossils that occur ONLY in rock layers of a particular geologic age Must: 1) be present in rocks scattered over a large region 2) must have distinguishable features 3) The organism must have lived during a short span of geologic time (20 million years) 4) occur in fairly large numbers within rock layers.

Rock layers in different areas of the world that contain the same index fossil were formed during the same period of time Used to help locate rock layers that contain oil and natural gas.