What is a Fossil? Facts about Fossils, Fossil Formation, Types of Fossils, and Where to Find Fossils

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Facts about Fossils
The Fossil Gallery
What is a Trilobite?
All about Megalodon Sharks
Prehistoric Shark Gallery
Eurypterid Facts and Information

Where to Find Fossils
Fossil Sites Page
Cavlert Cliffs, MD
Big Brook, NJ
Potomac River, MD
Sylvania, OH
Near Buffalo, NY


Printable Identification Fossil Sheets

Why are Fossils Different Colors?

Shark Evolution

Fossil Great White Shark Gallery

Megalodon Shark Gallery

Fossil Fish Fossil Shark Teeth Fossil Trilobite
What are Fossils? Types of Fossils Education: Recommended Books and Kits Where to Find Fossils Examples of Fossils in Different Rock Types

What is a Fossil? Fossil Facts and Information

A fossil is the preserved remains or trace of a plant or animal from the past. That's the simple answer to "what is a fossil?"

Plants can be preserved with a carbon film on a piece of shale, an animal bone can be naturally replaced by minerals and preserved, or a footprint in the sand can harden into a fossil.

How do Fossils Form:

Fossils usually only form in sedimentary rock. Sediments have to accumulate over the organism in order to preserve it. This could be a rapid burial, such as a volcanic eruption or mudslide, it could be sap from a tree oozing over an insect, or simply a slow sediment accumulation on the bottom of a lake.

Most fossils are marine in origin, or are land animals that fell into a water environment. This is because sediments easily accumulate in water environments (lakes, streams, oceans). Land environments are usually the sites for erosion, not sediment deposition.

Ammointe Fossils
Image of a few Ammonite Fossils. These are the mineralized shells of marine animals, similar to todays Nautiloids.

Let's answer the question "what is a fossil" in a little more detail by going into a specific example about fossils

Think about someone finding a Native American arrowhead in a recently ploughed farm field. That plough disturbed and dug up a few inches of soil. The arrowhead, which was previously buried in the soil popped to the surface. How did that arrowhead end up underground in the first place? A Native American left it on the surface hundreds of years ago. Over time, vegetation grew and died, creating soil. The soil slowly accumulated and buried the arrowhead.

This is similar to how a fossil forms. Let's take that lost arrowhead above and replace it by a leaf. Lets say the leaf falls into a lake with a low oxygen content, so it does not rot away. Now, let it sit there for a year. It will be covered by a small amount of sediment. Another year goes by and more sediment covers it. Now, multiply the time frame by an unimaginable number of years.. millions of years. Instead of the leaf being buried by a few inches of sediment, it is buried by miles and miles of sediment! Something now begins to happen to that sediment. Under miles and miles of pressure it heats up, the leaf literally cooks. Only the carbon ash remains. Chemical processes start to occur under the tremendous pressure, and the sediments compact into a type of rock... sedimentary rock.

Now, add millions of more years, and change the geologic context. Geologic forces thrust that sedimentary rock onto the surface. Wind and weather take its toll. The sedimentary rock outcropping begins to erode away. In one of the pieces of the eroded rock outcropping falls a fossil leaf impression, a preserved carbon footprint of that leaf that fell in the lake millions upon millions of years ago. This is how fossils form.

Preservation methods for fossils vary. Plants are fragile and often cook so that the carbon only remains. Animal bones and hard shelled animals are harder, and often have minerals leaching into them replacing the original bone. Sometimes volcanic ash layers cover the plants or animals and preserve them.

Fossil Fern
A fosssil fern preserved as a carbon film

Types of Fossils and Modes of Fossil Preservation:

There are many ways an organism can become preserved as a fossil. Some of the ways include Casts and Molds, Permineralization, Replacement, and Carbonization. Sometimes the fossil is unaltered, meaning it is the actual organism.

Fossil Casts and Molds:

A cast and mold is created when an organism is buried and rots away. The empty hole where the organism was is filled in with sediments or minerals, that become a cast. The mold is the impression in the rock it left. This is like pouring chocolate into a mold. When the chocolate hardens, it pops out as a chocolate cast, ready to eat.

Fossil shells are often casts, and shell impressions on rock are often molds.
A fossilized footprint is a mold.
Plant fossils and trilobites are often found as casts and molds.

Permineralization and Replacement:

Permineralization is when the organism is buried in the ground, minerals from ground water seep into the organism and slowly fill in the pores in the animal, adding rock forming minerals to the hard parts of the animal.

Replacement is similar, in that minerals seep into the organism. However, the minerals replace the original organic material, as the organic material rots away. In the end, the organism is replaced by minerals.

Many fossils are preserved these ways, from Fossil Sharks Teeth, to Trilobites, to Bone.


Carbonization is the process where only the residual carbon of the organism remains. In nature this usually happens over time when the organism is subject to heat and pressure.
A very common example of carbonization are fossil plants, where only a thin carbon layer is left on a piece of shale. In the Carboniferous time period, fast fern forests created miles of carbon, which we mine today as coal.

Another, more recent example is the fossilized feathers found on dinosaurs in China. These are left as carbon imprints in the shale around the mineralized dinosaur bones.

It's important to note that most fossils are preserved by more than one mode. For example, the fossil leaves are carbonized, but also leave a cast and mold. Fossil ammonites are casts, however, they also are mineralized. Fossil trilobites are often found as casts and molds, but their exoskeletons are mineralized (usually replaced by calcite).

Unaltered remains:

An organism is considered unaltered if there is no change in the original composition of the organism.
A famous example of this is found at the La Brae Tar Pits in Los Angeles. Here, Ice Age animals became trapped and sunk into the tar pits. The soft tissues rotted away, but the original bones still remain. Original bones from Saber Toothed Cats, Giant Ground Sloths, Short Faced Cave Bears, Dire Wolves, and many more strange creatures are all unaltered.

Another example includes insects and small animals trapped in Amber. The sap from a once living tree entombed the animals. This sap eventually hardened, and the original animals are preseved inside the amber.

Trace Fossils:

A trace fossil is a fossil not of an organism, but instead a fossil of an organisms activity. For example, a trace fossil can be a mold or cast of a footprint, or a cast of a fossil burrow. Animal borrows that have been filled in by sediment are very common in many sedimentary rock outcroppings.

The bottom of this page shows various fossils in different types of matrix and preservation modes.

Fossil Turitella shells from the Potomac River. These are perserved as Casts.
These are fossil Turitella shells from the Potomac River. They have been preserved as Casts when sediment filled in the shells. When they pop out of the sedimentary rock, a mold is left.

permineralized fosil porpoise vertebra from Calvert Cliffs, Maryland
This fossil dolphin vertebra from the Calvert Cliffs of MD has been preserved through permineralization. The minerals that replaced it gives it a beautiful rusty-brown color.

cast of a huntonia trilobite - it is also permineralized with calcite.
This is a fossil Huntonia Trilobite. The exoskeleton has been replaced by calcite (permineralization), it was also filled in with sediment (cast).

fossil saber tooth cat from the la brea tar pits in Los Angeles. - It is preserved via original remains.
This is a fossil Saber Toothed Cat from the La Brea Tar Pits in Los Angeles. The skeleton is the original Unaltered Remains. The animal fell in the tar, and the bones are original, just stained a brown color from the tar.

Recommended Books for learning about Fossils:
The book: DK Eyewitness Books: Fossil is a great introductory book about fossils. It explains what fossils are, how fossils form, and how they lived. It is chalk full of spectacualr images of all kinds of fossils, and give the history of fossil discoveries. It's a must have book for any fossil enthusiast.
This next book: Fossil Hunting: An Expert Guide to Finding, and Identifying Fossils and Creating a Collection is another great introductory book about fossils. This visual book concentrates on fossil hunting. It gives a historical perspective and goes over some world famous fossil localities.

Where to Find Fossils

It takes a unique set of circumstances to preserve a fossil, and then to re-expose it for us to find. Each fossil is a miracle. Fortunately, since there have been so many countless life forms over countless years, fossils are more common than you think. Most sedimentary rock outcroppings contain fossils.

Fossils can be found in places opposite as how they form. You want to look for fossils in places of erosion, where sediments do not accumulate, but instead erode away.

Also, to find fossils, you need to look in sedimentary rock outcroppings. Other types of rock, such as granite are not formed by sediments accumulating, so animals will not be preserved in them. Sedimentary rock includes sandstone, shale, mudstone, limestone, and clay.

Included here are some well known places to find fossils. For a complete list of fossil collecting sites from this webpage, go to the FOSSIL SITES SECTION

Calvert Cliffs of Maryland This fossil collecting location contains Miocene marine fauna, including fossil shark teeth.
Calvert Cliffs Fossil Site - Maryland
Calvert Cliffs of Maryland

Big Brook Area, New Jersey This fossil collecting location contains Cretaceous marine fauna, including fossil shark teeth, and the occasional Mosasaur.
Big Brook Area Fossil Site - New Jersey
Big Brook Area Fossil Site - New Jersey

Potomac River, Maryland This fossil collecting location contains Paleocene marine fauna, including fossil shark teeth, and crocodile material.
Potomac River Fossil Site - Maryland
Potomac River Fossil Site - Maryland

Sylvania, Ohio This fossil collecting location contains Devonian marine fauna, including fossil trilobites and brachiopods.
Sylvania, Ohio Fossil Site
Sylvania, Ohio Fossil Site

Western New York: Near Buffalo This fossil collecting location contains Devonian marine fauna, including fossil trilobites and brachiopods.
Fossil Site near buffalo, New York
Penn Dixie Fossil Site Near buffalo, New York

Pittsburgh, Pennsylvania This fossil collecting location contains Carboniferous fern fossils.
Ambridge, Pennsylvania Fossil Site
Ambridge, Pennsylvania Fossil Site

Examples of Fossils in Different Types of Sedimentary Rock
fossil porpoise skull as found in the PCS phosphate mine, Aurora, NC - fossilguy.com fossil porpoise skull after being prepped - fossilguy.com
This is a fossil porpoise skull found at Aurora, NC. It is in coquina matrix. Coquina is a type of limestone composed entirely of cemented shell fragments
The porpoise fossil is preserved by permineralization. The original bone has probably been replaced by phosphate.
This fossil appears to have partially decomposed before the fossilization process was complete, as the remaining skull is compressed vertically and severely fragmented.

fossil shark tooth as found in coquina matrix from aurora, NC - fossilguy.com fossil shark tooth in coquina matrix from aurora, NC - fossilguy.com
This is a fossil shark tooth (Carcharocles subauricualtus - predecessor of the Megalodon) found at Aurora, NC. It is also in a coquina matrix. This was found at the contact layer between the Pungo river and the Yorktown formations. It is miocene in age.
Here, the fossil root is permineralized, probably by phosphate. The enamel however is original material. Enamel is made of a very dense substance called dentine, and readily survives the fossilization process. The color of the enamel is from the leeching of ground minerals.

fossil porpoise skull being excavated by the Calvert Marine Museum - fossilguy.com I am excavating a fossil porpoise skull for the Calvert Marine Museum - fossilguy.com fossil porpoise skull in a plaster jacket after being excavated - fossilguy.com
This is a fossil porpoise skull I am helping excavate for the Calvert Marine Museum from the Calvert Cliffs. The fossil is permineralized. The calvert cliffs are composed of layers of many very soft sedimentary rock, including, but not limited to: diatomaceous clay, sandy clay, siltstone, diatomite, marl, silty clay, sandstone, and greensand.
Please DO NOT DIG IN THE CLIFFS if you go to this area. It is illegal and downright dangerous. If you see a fossil of scientific value embedded in the cliffs, contact the Calvert Marine Museum.

fossil trilobite in matrix from New York - Phacops rana - fossilguy.com
This is a Phacops trilobite from Western New York. It is in a Mudstone Matrix. Mudstone is composed of a fine grained, hardened silt. It is a very soft type of rock.
Most fossils are fount like this, still almost entirely in the matrix. Care is needed to extract them from the rock matrix.
This trilobite is preserved as a Cast. The exoskeleton is also Permineralized by calcite.

fossil Lepidodendron, scale tree, root - stigmaria from the carboniferous - fossilguy.com
This is part of a fossilized tree called a Lepidodendron. This fossil is about 7 inches long. It is from a weird "Scale Tree" which does not exist today. In the Carboniferous, these trees created vast "Coal swamps" which have created much of coal we mine today. The actual fossil is from the tree root, called the stigmaria. It is a cast. Mud filled in the hollow roots, harden and fossilize.

fossil fern plate from St. Clair area. It's Carboniferous in age - fossilguy.com
This is a fossil fern plate, about 2 feet across, from a famous locality called St. Clair. These fossils are in a shale created by a fine sediment. The fossil fern leaves have been buried, carbonized by heat and pressure to leave a black carbon imprint on the black shale. However, this is when the cool fossilization part begins! At some later time, through oxidation and replacement reactions, the Pyrite was replaced by a white substance called Pyrophyllite. This caused many of the orange fern fossils to turn white. The result is white fern imprints on black shale.

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