What Are Fossils?
Taught in US schools
Key Takeaways
- A fossil is the preserved remains, imprint, or trace of an organism that lived long ago, typically found embedded in rock.
- Types of fossils include body fossils (actual bones or shells), trace fossils (footprints, burrows), mold and cast fossils, and amber fossils.
- Fossils tell scientists what ancient organisms looked like, how they moved, what they ate, and what the environment was like when they were alive.
What Are Fossils?
A fossil is the preserved remains, imprint, or trace of an organism (plant, animal, or other living thing) that lived a long time ago - typically thousands to millions of years in the past. Fossils are found embedded in sedimentary rock and provide the primary evidence that scientists use to understand ancient life on Earth.
The study of fossils is called paleontology, and scientists who study fossils are called paleontologists.
Types of Fossils
There are several different ways an organism can be preserved as a fossil:
Body Fossils
The preserved remains of the organism itself - bones, teeth, shells, or other hard parts.
Examples: Dinosaur bones, trilobites, ammonite shells, shark teeth, tree trunks
Body fossils are the most familiar type. Soft body parts (skin, organs, muscles) rarely fossilize because they decay too quickly.
Trace Fossils
The preserved evidence of an organism's activity - not the organism itself.
Examples: Footprints, burrows, nests, bite marks, dung (coprolites)
Trace fossils tell us about behavior: how an animal walked, what it ate, where it lived.
Mold and Cast Fossils
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Mold fossil = a hollow impression left in rock after the organism decays
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Cast fossil = a solid replica formed when minerals fill the mold
Together, molds and casts reveal the external shape of ancient organisms.
Amber Fossils
Organisms trapped in tree resin (sap) that hardened over millions of years into amber. Amber fossils can preserve soft tissues, colors, and even DNA in extraordinary detail.
Famous example: Insects, spiders, and plant material preserved in amber - sometimes millions of years old.
Permafrost and Ice Fossils
In cold regions, organisms can be frozen in ice or permafrost, preserving soft tissues. Woolly mammoths have been found nearly intact this way in Siberia.
How Fossils Form (Fossilization)
Most fossils form through a slow process:
- Organism dies and is quickly buried by sediment (sand, mud, silt)
- Hard parts remain (bones, shells, teeth); soft parts decay
- Over millions of years, sediment hardens into rock
- Minerals replace the original material through a process called permineralization
- Erosion eventually exposes the fossil at Earth's surface
- A paleontologist discovers and excavates the fossil
What Fossils Tell Us
Fossils are a window into Earth's ancient past. They tell scientists:
- What organisms looked like (body shape, size, structure)
- How they moved (footprint patterns, body proportions)
- What they ate (teeth shape, gut contents in rare cases)
- What the environment was like (ocean fossils found on mountains = that area was once underwater)
- When an organism lived (determined by the age of the rock layer)
Famous Fossil Examples
Trilobite: Ancient marine arthropod - One of the most common fossils; 250+ million years old
Ammonite: Extinct shelled cephalopod - Beautiful spiral shape; widely found
T. rex: Large carnivorous dinosaur - Most recognizable dinosaur; found in North America
Archaeopteryx: Early bird-like dinosaur - Shows the evolutionary link between dinosaurs and birds
Woolly mammoth: Ice Age elephant relative - Sometimes preserved in ice with soft tissue intact
Practice Activities
- Make "fossil cookies" or plaster casts of shells/leaves to model how cast fossils form.
- Sort picture cards of fossil types into categories: body fossil, trace fossil, mold/cast, amber.
- Research Mary Anning - the self-taught fossil hunter who made key dinosaur discoveries in 19th-century England.
- Use a geological time scale to place key fossils in order - when did trilobites appear? When did dinosaurs go extinct?
- Design challenge: how would YOU preserve an organism for future scientists? Students brainstorm materials and methods.
Frequently Asked Questions
How do fossils form?
Most fossils form when an organism dies and is quickly buried by sediment (sand, mud, or silt). Over millions of years, the sediment hardens into rock and the organism's remains are slowly replaced by minerals - a process called mineralization or permineralization. The result is a stone replica of the original organism. Soft body parts rarely fossilize; hard parts like bones, teeth, and shells are most commonly preserved.
What is the difference between a mold fossil and a cast fossil?
A mold fossil forms when an organism is buried in sediment and the sediment hardens around it - when the organism decays, it leaves a hollow impression (the mold) in the shape of the organism. A cast fossil forms when minerals fill in that hollow mold, creating a solid replica of the original shape. Together, mold and cast fossils show us the external shape of ancient organisms.
What is paleontology?
Paleontology is the scientific study of ancient life through fossils. Scientists who study fossils are called paleontologists. They use fossils to reconstruct what ancient organisms looked like, understand how they lived and evolved, and learn about Earth's history. Famous paleontologists include Mary Anning (19th century, England) who discovered key dinosaur and ichthyosaur fossils.
Free Fossils Worksheets
Curriculum-aligned printable worksheets for 3rd – 4th Grade. Download free.