What Are Fossils? Lesson 3 Big Shells: Ammonites were animals similar to squids with shells. They lived millions of years ago. The largest ammonite fossil found so far has a shell almost 6.5 feet across. Page 248

Fossils People today know about many plants and animals of the past because of fossils. A fossil is the remains or traces of an organism that lived long ago. Most fossils form in sedimentary rock. First, sediment covers an organism. Then, the sediment hardens into rock, preserving the fossil shape. The soft parts of organisms break down quickly and decay. Because of this, most fossils are formed from only the hard parts of living things, such as shells, bones, and teeth. Page 250

Fossils There are other kinds of fossils. When minerals fill the cells of once-living things, a different kind of fossil forms. Petrified wood is an example. It is the wood of a tree that has been replaced by rock. Another type of fossil is a trace fossil. It doesn’t show how a whole plant or animal looked, but it tells something about it. It could tell us the size of the animal or how it moved. Some fossils are the remains of whole animals. They were trapped in ice or tree sap that hardened. Some are known as an Amber Fossil. Page 251