In this topic you will learn about fossils and what they tell about the past.

Scientists use fossils to learn about the past. By studying fossils they can learn about past events, past environments, and past organisms. Hard parts of living things, such as bones and shells, are most likely to form fossils. Footprints can also form fossils.

Most fossils are found in sedimentary rocks. If the remains of a living thing are buried by sediments, they may become fossils if they are not disturbed as the rocks form.

Other fossils are known as molds. A mold is a hollow form with a particular shape. Molds form when water seeps into the rocks where a shell is buried. The water dissolves the shell and leaves a hollow space. Another type of fossil is known as a cast. A cast is a fossil that forms in a mold. Minerals fill the mold and the fossil takes the shape of the original shell. Sometimes organisms leave tracks, prints, or body outlines called imprints.

Not all fossils are found in rocks. Sometimes insects get caught in the sap of trees. The sap hardens over time, forming amber. Mammoths, which are relatives of modern elephants, have been preserved in ice. Many fossils are discovered in tar pits. Some saber-tooth tigers and mammoths became stuck in tar pits and died. Their remains were found later. Wood and bones may be preserved by being petrified. Petrified means turned to stone. Minerals slowly seep into the pores of bones or wood and fill in these spaces.

Once the fossil is found, it is collected, carefully packaged, and sent to a museum or university. In the museum, rocks in the fossil are removed and missing parts replaced, and it is then finally showcased in an exhibit.

Fossils supply many clues. Growth rings of petrified trees tell the age of the tree when it died. Footprint size is a clue to the animal's size. Distance between footprints may tell if an animal was walking or running. Footprints may also tell if an animal walked on four legs or two legs. Teeth tell what an animal ate. Meat eaters had strong jaws and pointed teeth. Plant eaters had weaker jaws and flat teeth. Fossilized stomach contents tell what an animal ate.

Fossils can also tell about past environments. Fossilized organisms tell where lakes, rivers, and oceans were. Fossils also tell that parts of the world were hotter or colder than they are today. Fossil ferns mean that an area had a warm, moist climate. Fossilized evergreens show that an area was cool.