When an animal or plant dies it is usually completely destroyed — either by another animal eating it or by decaying on the ground or in the water. But sometimes the animal is buried before it is destroyed. And when that happens and conditions are just right, the remains of the animal are preserved as fossils.
Here is a simple definition of fossils: Fossils are the naturally preserved remains or traces of ancient life that lived in the geologic past.
Most fossils are invertebrates, that is, animals without backbones. Worms, insects, and clams are all invertebrates. 95% of all living animals are invertebrates. There were even more in the past.
Here are the really important facts about fossils:
Fossils represent the remains or traces of once-living organisms.
Most fossils are the remains of extinct organisms — that is, they belong to plants or animals that are no longer living anywhere on Earth.
The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time.
Body Fossils include the remains of organisms that were once living. Usually only the hard parts of animals like shells and teeth and bones become fossilized. However, feathers, fur, and skin have also been found.
Trace Fossils are the signs that organisms were once present. Trace fossils can be tracks, footprints, trails, burrows, eggs, nests, leaf impressions, and feces. Take a peek at these images of trace fossils.
There are also fossil plants. The oldest fossils of land plants visible with the naked eye — that is, without a microscope — are about 425 million years oldpale. That's OLD!
Sedimentary rocks are rocks that formed when layers of sediment such as clay, mud, silt, or sand hardened over millions of years. These types of sediments settle down in lakes, swamps and oceans. This is one of the reasons why most fossils are the remains of animals who lived in or near the water.
Read about the Burgess Shale, a special location because so many different kinds of soft-bodied, invertebrate animals had parts preserved as fossils.
What Do Fossils Tell Us?
Rock layers can tell us Earth's history because they preserve past events. Fossils help scientists determine the age of the layered rocks.
Fossils also tell us what happened in Earth's history and when it happened. Fossils can be used to recognize rocks of the same or different ages. They are clues to former life.
Probably the most familiar fossil is the ammonite. Ammonite were predators belonging to a group called cephalopods, which include squid, octopus and nautilus. Ammonites lived in the seas between 240–65 million years ago, then became extinct along with the dinosaurs.
Another group of invertebrate fossils are called Trilobites. Trilobites inhabited the Earth during the Paleozoic era. They were arthropods (like insects, spiders, and shellfish) who lived in the ocean.
Earth is about 4.6 billion years old. The oldest known fossils are from rocks from about 3.5 billion years ago. At Shark Bay, Australia, you can see stromatolites — literally layered rocks — which are examples of what the earliest life on earth looked like.