Fossils are the preserved remains of plants or animals. For such remains to be considered fossils, scientists have decided they have to be over 10,000 years old.
There are two main types of fossils, body fossils and trace fossils. Body fossils are the preserved remains of a plant or animal’s body. Trace fossils are the remains of the activity of an animal, such as preserved trackways, footprints, fossilized egg shells, and nests.
When asked what a fossil is, most people think of petrified bones or petrified wood. Permineralization is a process. For bone to be permineralized, the body must first be quickly buried. Second, ground water fills up all the empty spaces in body, even the cells get filled with water. Third, the water slowly dissolves the organic material and leaves minerals behind. By the time permineralization is done, what was once bone is now a rock in the shape of a bone. Unlike what you see in cartoons, dogs wouldn’t be interested in these bones.
When an animal or plant dies, it may fall into mud or soft sand and make an impression or mark in the dirt. The body is then covered by another layer of mud or sand. Over time, the body falls apart and is dissolved. The mud or sand can harden into rock preserving the impression of the body, leaving an animal or plant shaped hole in the rock. This hole is called a mold fossil. If the mold becomes filled over time with other minerals the rock is called a cast fossil.
A simple experiment can show you how this works. Take some clay and press a seashell or some other object into the clay. Pull the sea shell out of the clay any you will see a detailed impression of your seashell in the clay. If, over time, the clay hardens into rock the result would be a fossil mold. But really, who has millions of years to wait to make their own fossil?
Here’s the quick way. Pour plaster of Paris, dental stone, or other plaster into the mold. Wait for it to harden and you have just made your own cast fossil.
Another type of fossil is a resin fossil. Resin is sometimes called amber. Plants, mostly trees, secrete sticky stuff called resin. Sometimes insects, other small animals, or bits of plants get stuck in the sticky resin. The resin hardens overtime and is preserved in rock making a fossil.
There are many other types of fossils. Compression fossils form when leaves become trapped between layers of sediment (mud, fine sand, or volcanic ash) and get smashed flat. Over time the sediment hardens into rock and the leaves become fossils. This usually happens with leaves because they are already flat.
Trace fossils are the remains of trackways, burrows, eggs, nests, and fossil coprolites (poop). Trace fossils can tell us a lot about the animals that left them. For example, from trackways we can tell about how fast an animal was moving when they made the tracks. From coprolites we can tell what kind of things an animal ate. Trace fossils are important because they tell us how an animal lived, and what it was like in their environment.
Anyone can find fossils. I have been finding fossils for nearly as long as I can remember, and finding fossils is still one of my favorite things to do. I found many fossils as a small child near my grandmother’s house in Huntington, Utah. Huntington, Utah, is near the famous Cleveland Lloyd Dinosaur Quarry and is surrounded by the Morrison Rock Formation. The Morrison Rock Formation is famous worldwide for having dinosaur fossils in it. Near the home where I grew up I could find Trilobites and the trace fossils made by ancient worms as they burrowed through soft mud at the bottom of an ancient sea.
Fossils are found in sedimentary rocks. Sedimentary rocks are one of the three main types of rock. The other two are igneous and metamorphic rocks.
Sedimentary rocks are formed when other types of rocks are eroded away. Broken down bits of rock, called sediments, pile up in what geologists call beds or layers. Over time sediments get buried by new layers of sediments. Sometimes these sediments harden into rock. These rocks are now sedimentary rocks. Of the three different rock types only sedimentary rocks can hold fossils. Igneous rocks are formed when liquid melted (magma or lava) rock cools and hardens. Metamorphic rocks are formed under intense heat and pressure. Metamorphic rocks can be formed out of either sedimentary or igneous rock; however, the process of forming metamorphic rock would mess up or destroy any fossils that happened to be in the rock.
If you want to find fossils you will need to find the right kind of sedimentary rocks. While digging on the beach in England in 2010, 5-year-old Emily Baldry discovered the fossil of a 160 million year old sea creature. Sedimentary rocks typically do not live in your backyard, so please don’t go digging, unless you have your parents’ permission. When you are outside where the rocks “live,” keep your eyes open, you just might find a fossil.
Even if you don’t find a fossil, rocks are just plain cool.
How Old Are Fossils?
It can be difficult to determine the age of fossils (date the fossils). Because fossil bones aren’t made from the right kind of rocks, you can’t really run scientific tests on fossils to determine their age. Scientists can use scientific tests to determine the age of rocks near the fossils. The types of tests are called radiometric dating. There are certain types of elements that are radioactive. Radioactive elements fall apart overtime and slowly change into another element that is not radioactive. We know how fast radioactive elements fall apart. We know what radioactive elements turn into after they have fallen apart. 
We can compare the amount of radioactive elements in a rock to the amount of specific non-radioactive elements in a rock, do some math and determine how old the rock is. For example, uranium falls apart into lead. So, if we find a rock that has uranium in it, we can compare it to the amount of lead in the rock to find out how old the rock is. If you have the right kind of rocks, this method is very accurate.
The trouble with using this method to date fossils is: Radioactive elements are only found in igneous rocks, and you can’t find fossils in igneous rocks.
So, if you can’t date the fossil directly with scientific tests, how do you date the fossil? You have to use what scientists call relative dating. The relative dating method most commonly used by paleontologists and geologists is called stratigraphy. Stratigraphic dating works like this. Rocks are formed in horizontal (flat, not up and down) beds or layers. These layers are called strata. The oldest layers are on the bottom and the youngest layers are on the top. So, the oldest fossils are on the bottom layer and the newest fossils are on the top layers. If you find a layer of the right kind of igneous rocks you can use the exact dating method to determine and exact date of that layer.
Paleontologists have found certain fossils that are different from all other fossils. These organisms lived for a relatively short amount of time, and they know when these animals lived. Paleontologists call these fossils index fossils. Index fossils can be used to determine approximately how old an unknown fossil is. For example, if you find an unknown fossil in the same layer of rock as one of the index fossils, you know your fossil is the same age as the index fossil.
Geologists and paleontologists have used a combination of dating techniques, which are radiometric dating, stratigraphic dating, and index fossils, to determine the approximate age of rocks all over the world.
Once you know the approximate age of rocks, you can determine the approximate age of the fossil.