1. Fossils in Sedimentary Rock Layers: Clues to Earth’s Past
Prepared by Mrs. Terri Reed
Brookhollow Elementary
January 2005

2. Scientists learn about Earth’s history by studying rocks and fossils.
The word fossil literally means “dug up.”
Scientists who study fossils are called paleontologists.

3. What’s a Fossil?
Fossils are the recognizable remains, such as bones, shells, or leaves, or other evidence, such as tracks, burrows, or impressions, of past life on Earth.

4. How do fossils form?
Usually when an animal or plant dies, it is eaten by another animal or decomposed.

5. Sometimes the remains of organisms are buried by sediment before they are eaten or decomposed.
Over thousands of years, the sediment is compacted into rock.

6. 100 million years ago
Scallops and other invertebrates inhabit the sea in this area.
When they die, they are covered by sand that settles on them.

7. 75 million years ago
Pressure compacts the loose sandy sediments into hard sandstone.
A dinosaur now lives in the area. When it dies, its body is also covered by sand.

8. 50 million years ago
There is a tremendous volcanic eruption in the area. The ash thickly covers the land, and for a while the land cannot support much life.

9. 20 million years ago
A Parahippus, a 3-toed horse common at the time, dies.   Like the other creatures before it, its body is buried in sand and sediments by wave action.

10. 3 million years ago
The massive force of an earthquake causes layers of sediments to tilt and push upward.

11. 2 years ago
Forces of erosion, such as water moving against rock for millions of years, gradually wears away the land surface and a few of the rock layers beneath it.  
The fossilized bones of the horse, hidden for millions of years, are exposed above the ground.

12. Since sedimentary rocks are formed in layers, scientists believe that the oldest rock layers are found at the bottom.

13. Thus by studying sedimentary rock layers, scientists can “see” into the past!

14. What Scientists Learn from Fossils in Sedimentary Rock Layers

15. Different life forms have existed
By studying fossils, scientists learn what kind of plants and animals lived long ago and how they have changed over time.
Extinct relative of starfish

16. This is a time scale collage that shows the different types of life that have existed on Earth through the last 600 million years.
The oldest fossils are at the bottom and youngest at the top. Why?

17. The deeper you dig, the older the fossils
Snail
15 million years old
Triceratops
66 million years old
Dimetrodon
270 million years old
Trilobite
570 million years old

18. Fossils determine age of rocks
A rock sample can be identified and dated (given a time when it was formed) depending on the fossil it contains.
This helps to date rocks above and below the fossil-containing layers.

19. Each rock layer holds a group of fossil that's different from those in every other layer.
No matter where in the world they are found, rocks with matching groups of fossils are the same age.

20. Rocks also help date fossils
Scientists can also do this the other way around – by dating the fossils from the types of rock in which they formed.
Geologists and paleontologists work together to fit rocks and fossils into one time scale.

21. Clues to past environments
If scientists know the kind of environment preferred by the organism, then they can determine what the past environment was like when the plant or animal lived.
If you found this fossil in your backyard, what could you infer about the past environment and climate of your area?

22. Fossil Butte National Monument
Today this area in Wyoming is hot and dry, but scientists have learned that at one time, it was almost tropical!
These rocks contain traces of plants and animals from a lake that covered the area over 50 million years ago.

23. Based on the fossil evidence, scientists believe the area used to look like this.

24. This palm leaf fossil suggests the lake was surrounded by palm trees.

25. Fossils of many species of fish, insects, birds, and even crocodiles have been discovered in the area.

28. Dinosaurs The word fossil makes many people think of dinosaurs.
These were the dominant animals on Earth for well over 100 million years.
By around 65 million years ago, all dinosaurs were extinct, and scientists still aren’t sure why.

29. Oldest animal with feathers
The fossil Archaeopteryx lithographica was a Jurassic animal with the skeleton of a reptile, but was covered with feathers.
Scientists believe this animal is a link between reptiles and birds and that birds are descended from reptiles.
The specimen is about 45 centimeters long.
Photograph courtesy of the National Museum of Natural History, Smithsonian Institution.

30. Although dinosaurs
are very well-known,
they actually only represent a very small portion of all the millions of organisms that have lived on Earth.

31. The majority of fossils are the remains of animals with shells and small plants.
Many of them lived long before the dinosaurs.

32. Each fossil discovery represents a clue – a clue to help solve the mystery of Earth’s past.

33. Sources http://www.flmnh.ufl.edu/natsci/vertpaleo/fhc/layer2.htm
shell fossil from Grand Canyon 12/6
Science Project Book of The Earth; by Steve Parker; © 1993 Marshall Cavendish Limited

34. Sources http://techcenter.davidson.k12.nc.us/spring026/fossil.htm