74 Fossils-Mid

Fossils FOCUS Question What can fossils tell us about animals and plants that lived long ago?

What Is a Fossil? How do we know dinosaurs existed? In fact, how do we know anything about ancient plants and animals? We can look at rocks! Not just any rocks, though. We look at fossils. A fossil is anything left from an ancient living thing. It might be an animal's whole body, or just one part. It might be a nest full of eggs. It might be the shape of a leaf or animal footprints. It might even be really old poop! Fossils can teach us about plants and animals that lived a long time ago.

Kinds of Fossils Most living things never become fossils. When they die, they get eaten or rot away. Once in a while, dead things are buried in the ground and slowly turn into rock. Over many, many years, they become fossils. There are two main types of fossils: body fossils and trace fossils. A body fossil is a living thing, or part of a living thing, that has turned into rock. Body fossils show the size and shape of living things. A trace fossil shows evidence of the activity of a living thing. Trace fossils can be nests, footprints, or poop that a living thing left behind.

Bones, Teeth, and Shells A body fossil forms when a dead thing gets buried under layers of sediment. Sediment is made of tiny bits of loose material. It can be moved by wind or water. The hard parts of a dead thing are most likely to last and become fossils. That is why the most common body fossils are bones, teeth, shells, and the woody parts of plants. The weight of the layers above slowly presses down on the sediment. The loose sediment becomes solid rock. After thousands of years, all that is left of the dead thing is a fossil in rock.

Fossil Myths Myth: All fossils are from dinosaurs. Science: Countless plants and animals. including insects, have become fossils. Some of the most common fossils are from snails, clams, and other sea creatures with shells. Myth: Fossils are from animals or plants that do not exist anymore. Science: We find fossils of lots of animals and plant species that are still alive today! This fish has lived on Earth for 400 million years. You can see a live fish or a fossil of one from long ago. It has not changed much in all that time!

Feathers, Fur, and Skin How do we know what ancient animals looked like? Changing to rock usually destroys the soft parts of dead things. Sometimes the outside of an animal turns into a fossil. Paleontologists—scientists who study fossils—have found ancient feathers, fur, and even skin. These fossils are very special. They can tell us what an ancient animal looked like. This is a fossil of one of the first birds. Scientists have found fossils of these birds' feathers. Do You Know? Not all fossils are preserved in rock. Sometimes a dead plant or animal gets preserved in tree sap or in ice.