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Fossils are the remains of animals or plants that lived a long time ago. When we think of fossils, the first things that come to mind are the bones of dinosaurs; but a fossil can be anything. Footprints, flowers, even droppings; all of these things can be turned into fossils under the right conditions. A fossil only becomes a fossil when it has reached a certain age. Though there is no one age, a good rule is anything over 10,000 years old can be considered a fossil. The oldest fossils ever discovered are more than 3.5 billion years old. Fossils are found all over the world and on every continent. A paleontologist is a scientist that studies fossils.
Types of Fossils
There are two main types of fossils: body fossils and trace fossils. A body fossil is the physical remains of an animal. Body fossils can be bones, teeth, or shells. Some very small organisms, called microbacteria, leave behind microscopic particles called filaments. Trace fossils are not the remains of an animal, but something that shows how it lived or died. Footprints, tools, or plant matter are different types of trace fossils.
The two fossil categories can be broken down further. A cast fossil is a fossil made when an organism that is buried in sediment decays and leave behind a mold, or hollow impression in its shape. A cast fossil is like a mold fossil, but in the case of a cast fossil the mold has been filled with minerals. Imprint fossils are made when organic – or living – material is pressed into sediment, but does not decay like with a cast or mold, and the organic material leaves an impression in the formed rock. Some organic material, like tree trunks, can be petrified. Petrification happens when the organic material is slowly replaced by minerals. Some fossils are the actual remains of a plant or animal that has been frozen in ice or trapped in a substance like sap. This type of fossil is called a whole fossil.
How Are Fossils Made?
Making a fossil is tricky and the conditions have to be just right. Despite this, there are several different ways for a living thing to become a fossil. If the fossil was once an animal, the animal has to die. After that, depending on where the animal dies, the process of fossilization can occur in different ways. For most fossils there are three basic things that are needed: sediment, pressure, and time.
Mold and Cast
To create a mold and cast fossil, the animal has to die in a place where it will be covered with dirt, or sediment, fairly quickly, like an animal that dies in the ocean and sinks to the sea floor. The remains are then covered in sediment, or layers of sand and mud. Over time the soft parts of the animal decay, leaving only the harder parts like teeth and bones. More sediment settles on the remains, and time and pressure turn the sediment surrounding the bones and teeth to stone. Groundwater seeps into the newly made stone and dissolves the bone, leaving behind a mold. The mold is an empty cavity, and can be filled with minerals that become a cast for the long dissolved bone.
There are some rare places where a whole animal can be preserved. An animal that dies in a cold place, like the tundra, can be frozen. So long as the temperature remains below freezing, the animal stays frozen, sort of like a natural freezer. Paleontologists have found woolly mammoths perfectly preserved in permafrost.
Bogs are another place where animal remains can be well preserved. A bog is a type of wetland where a large amount of dead plant matter accumulates. The soil in a bog is very low in nutrients and oxygen. Animals that die in bogs and sink beneath the surface are protected from the elements, other animals, and bacteria that cause decay.
How Fossils are Dated: Superposition, Carbon-14, and Radiometric Dating
Imagine you’ve found a fossil in your backyard and you want to know how old it is. How would you go about figuring it out? A long time ago scientists used the concept of superposition, which states that the oldest layer of sedimentary rock is found at the bottom of the strata, or layers, and the youngest at the top. While they didn’t have exact dates for how old these strata were, they could determine which fossils were older than others based on which layer they were found in.
In the 1940s scientists developed a better tool for determining how old fossils were by using radioactive carbon isotopes. An isotope is an atom that has the same number or protons as a given element, but a different number of neutrons. Unlike a normal atom, an isotope undergoes decay, or breaks down. Isotopes break down at a steady rate, which makes it easy to determine how old an object is. The first element used to determine the age of fossils was carbon-14. Carbon-14 has a half-life of 5,730 years, which means that it takes that long for half the amount of carbon-14 in an object to decay. All living things consume carbon-14 in some form. When they die this consumption stops, but the C-14 continues to break down. By determining just how much carbon-14 is in a sample, scientists can make a fairly accurate estimation on how old it is. Carbon dating is only useful for organic material that is less than 50,000 years old, as the amount of C-14 left in an object is too small to detect after that age.
Carbon dating is useful for something that was once alive, but what about something that was never alive, like rocks or our cast and mold fossils? Scientists still use some of the concepts of superposition to date objects by dating the material around a fossil. For dating older objects scientists can use other isotopes like potassium or argon. These isotopes have half-lives in the millions of years, and have been used to date the oldest rocks found on Earth!
- Carbon Dating: What Is It and How Does It Work?
- Scientific American: Carbon Dating Gets a Reset
- Radioactive Dating Game (interactive)
- What is Carbon Dating?
Fossils give us an amazing glimpse into the history of our world. From the tiniest of organisms to the king of the dinosaurs, fossils show just how varied life can be on our planet. They show how animals have evolved to deal with changes in their environment. Every fossil, no matter how large or small, has a story to tell. Paleontologists are modern-day detectives trying to figure out puzzles millions of years in the making.
- Fossil Fabricator (interactive)
- The Fossil Plant Garden at the University of Florida
- Paleontology Stuff at the American Museum of Natural History
- What is Paleontology?