The Fossil Record

1. Fossils are the remains of organisms that lived in a previous geologic time. 2. The study of these fossils is called paleontology. 3. Most fossils are discovered in sedimentary rock because the sediments cover and protect the body from damage.

4. They are rarely found in igneous or metamorphic rock because the heat, pressure, and chemical reactions involved in the formation of these rocks destroys the body. 5. The fossil record gives us information about the geologic history of the Earth by revealing the way that organisms have changed or evolved throughout the past.

6. Marine animal and plant fossils found far from an ocean tells us that the region was once covered by an ocean in the past. 7. Most dead plants and animals are eaten or decomposed by bacteria. However if the dead organism is buried quickly, it can become a fossil. 8. Normally only the hard parts become a fossil.

9. There are 9 ways fossils can form. In mummification, the remains are formed in very dry locations. 10. Insects can be caught in tree sap and preserved when it hardens. This hardened tree sap is called amber. 11. When petroleum oozes to the surface, it forms tar seeps. If an animal gets trapped in the tar, it can become covered and preserved.

12. Organisms buried in ice or frozen earth become fossils because bacteria can not survive freezing temperatures. 13. Petrification is when minerals replace the original organic material in an organism, resulting in a mineral replica of the original organism.

14. Imprints are made when an organism dies in soft mud or clay and leaves behind a carbon rich film. Shells often leave empty cavities called molds within hardened sediments. 15. When the mold fills with sand or mud, a cast forms. 16. Fossilized dung is called a coprolite.

17. Some dinosaurs had stones in their stomach to help grind the food. These stones called gastroliths often survive as fossils. 18. Trace fossils are evidence of past animal movement and include footprints, burrows, and tracks. 19. Scientists have discovered the ancint footprints of reptiles, amphibians, birds, and mammals.

20. Index fossils only occur in rocks of a particular geologic era. 21. To be an index fossil, a fossil must be scattered over a large region, must be easy to distinguish from other fossils, the organisms from which it formed must have lived during a short geologic span, and 22. it must occur in large numbers in the rock layer.

23. These index fossils are used to estimate absolute age of rock layers. 24. Ammonite fossils show that the rock in which the fossils are found, formed between 180 and 206 million years ago. 25. Index fossils can also be used by geologists to find rock layers that may contain oil and natural gas.