Watch. In the snow, a woolly mammoth with a lot of hair kneels to rest. By a river, a dinosaur eats a plant and a leaf falls into the mud. In the fields, a large dinosaur with sharp teeth creeps up on a herd of plant-eating lizards. In the ocean, a very strange-looking crab washes onto the beach.
Now jump forward to our time! In a museum, we find the whole hairy elephant. We find some very old eggshells, a large lizard footprint, and a clay copy of the strange crab. How did they die? Better question, how did these things last all these years without being eaten, rotting, or washing away so we can have the joy of looking at them?
Before we start, let's remember that a fossil is any plant or animal that has stayed around as a print in stone. The living animal or plant is long gone, but we still have an idea of what it looked like or how it lived. Fossils can be anything from bones to feathers to plants. They can be as big as a tree-eating dinosaur or as small as a single cell!
Let's start with the largest find. How could a woolly mammoth stay whole, with fur, eyes, even the food in its mouth? Cold can keep things from breaking down. Do your parents put meat in the freezer to keep it fresh? The same thing happens with animals, even if they are thousands of years old. A body fossil is a fossil with a real piece of the living thing. Things like this do not happen very often. Normally, water, dirt, and bacteria break down the skin and bones until over time, nothing from the living thing is left. We are very lucky to have this woolly mammoth. This one is easy to figure out: it froze to death and stayed that way for thousands of years until someone found its tusk sticking out of the ice.
Let's look at our t-rex footprint. Even though we do not have the animal itself, we can still learn a lot. Its toenail is longer than your foot! Its stride is as long as a car! A trace fossil is a fossil that tells us something about a living thing or how they lived but is not a part of the living thing. If we look near the t-rex's prints, we can see prints from the four-footed animal it was chasing. Footprints can tell us how fast they ran! They can tell us how many feet they walked on. They can even tell us what kinds of shoes they wore (they didn't wear any).
The dinosaurs didn't exactly vanish without a trace.
If you have ever stepped in the mud with your foot, you have made a mold. Now imagine no one touched that for tens of thousands of years. It became hard, and one day someone came across your footprint and said "Wow, this person had beautiful feet!" What happened with the shell we found on the beach? It left a print in the mud. We don't get to see the shell itself, but we can see the space where the shell used to be, just like your footprint in the mud. A mold fossil is made when a living thing makes a mark that is the shape of its body, and the print stays for us to find later on. This can also happen with plants.
You only have one chance to make a first impression.
At last, we come to our strange crab. This crab is so big! It probably died of old age and it laid in the mud. Somehow, no hungry animals saw it and ate it. Like our footprint, this crab made a mold in the soft mud before washing away, and then something else happened. This hole filled with sand. Like the sand inside the eggs, this sand became hard over hundreds of thousands of years. It made a little statue of the crab. A cast fossil is made when the print of a living thing fills in with rock, making the shape of a living thing.
If you wait long enough even a snail can make something of itself.
Fossils tell the end of an animal's story. Whether it's a piece of skin, some eggs or food, a picture in mud, or a statue made of sand, we can see a small piece of how they lived. It's up to scientists to put the story together using these clues.
Fossil Facts and Finds. "How Fossils Are Made" Fossil Facts and Finds, 2005.
Trilobites. "What Are Trilobites?" Trilobites Info, 2003. <http://www.trilobites.info/>
Melbourne Museum. "What Can We Learn From Dinosaur Eggs?" Melbourne, 2010. <http://museumvictoria.com.au/melbournemuseum/discoverycentre/dinosaur-walk/videos/sauropod-eggs/>
eHow. "How Mold and Cast Fossils Are Made" eHow, 2011. <http://www.ehow.com/about_6497177_cast-mold-fossils-made.html>
How Stuff Works. "How Fossils Work" Discovery, 2009. <http://science.howstuffworks.com/environmental/earth/geology/fossil.htm>