Phatfossils - What is a fossil?

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What is a Fossil?

What is a Fossil?

What is a fossil? What is the definition of fossil? The simple answer, the preserved remains of a living organism or traces of its activity. Think of it link this, you can find a bunch of T-Rex bones(preserved remains) or you can finds its footprints(Traces of activity), both are fossils. Another example would be that you can either find a piece of petrified wood(Preserved remains) or you can find bark imprints in hardened mud(Traces of activity). To be considered a fossil, a specimen typically must be greater than ten thousand years old.

How Do Fossils Form?

How are fossils made you might ask? Fossils themselves are made in several different ways, whether it be an animal walking a receding lake shore, leaving foot prints in the still soft yet hardening mud or an animal that and ended up at the bottom of the lake it all begins with a plant or animal having interacted with the sediment in some manner. One this interaction occurs, some sort of event then causes the interaction to be preserved. In the event of the foot prints in the mud, perhaps a drought ensued and the mud hardened to rock before years later new rains filled the lake and slowly covered the hardened layer of sediment containing the footprints. For the critter laying on the lake bottom, it could have been either buried by slow sedimentary deposition or perhaps a volcano blew depositing tons of ash in the area. For beter hopes of fully articulated remains, a quick burial to prevent scavenging or elemental decay is typically needed.

Once the specimen is buried it begins to undergo some chemical or geological changes. If a specimen is being preserved through permineralization the specimen itself is being replaced with surrounding minerals. Carbonized specimens are being crushed and compressed so greatly, only a residue is left behind. Cast fossils are decaying and then the space they leave behind is being filled with surrounding sediment.

After tens of thousands to billions of years, do to shifting continents, continental upheaval, water and wind erosion, mining and construction work and several other factors these once buried fossils now become exposed again. Whether it be under a river, on the side of an eroding hill or the bottom of a quarry hole the fossils lay awaiting to be found before the elements destroy it never to be seen again. The video below depicts the process

Despite fossils being so seemingly common, they are in fact, exceedingly rare in the grand scheme of things. It was once explained to me that we get 1% of 1% of 1%. Meaning, less than 1% of all living things get fossilized in some type of sediment. Less than 1% of all fossil bearing sediments will ever be exposed for searching. And then less than 1% of what is coming out of the exposed formations will ever get found. In effect, despite being able to go to the beach and pick up bags of shark teeth and shells, what you are seeing is only the smallest fraction of what really existed. Below are some pics of fossils just starting to expose from the formation they have been entombed in.

Here we have a picture of a fossil tortoise first seeing daylight from its final resting place millions of years ago
Here a fossil megalodon shark tooth is starting to expose, showing where it ended up millions of years after falling out of the sharks mouth

Types of Fossils

When trying to understand what is a fossil, it helps to understand the modes of preservations that can create a fossil. The following are ways a fossil can be formed, each mode of preservation leaves behind a little something differenrt.

Permineralization and Replacement

Accountable for the formation of the most dramatic fossils in this authors opinion, permineralization and replacement are both preservation forms that leave the fossil appearing as 3d life objects, shark teeth and skulls are nice examples of this. Replacement is where a fossil is formed by it being completely replaced by minerals in the surrounding sediments. Permineralization is when minerals seep into pores of items such a bone or wood.

Really awesome fossil pig skull from the Oligocene of Nebraska
large fossil megalodon shark tooth from the eastover formation of Virgina

Molds and Casts

Many times when specimens die, they themselves rot away but the space they took up or their internal cavities are replaced by sediments that are left behind in the exact shape of the item they filled. This is very common in fossil shells where the shell would deteriorate away however the internal sediments would harden inside of the shell and be left behind.

Fossil depicting the internal cavity of Paleocene turitellas. The internal portion of the shell filled in, hardened, and the shell then deteriorated leacing behind the internal cast
Cast fossil depicting a Miocene clam


During the carbonization process the fossil is formed under extreme pressure and heat. The specimen itself is completely crushed and heated to a point where nothing but the carbon remains. This is commonly how some plant fossils are preserved.
fossil leaves from St. clair PA, formed by the carbonization process

Unaltered Preservation

With unaltered preservation it is exactly what it says, the specimen is unaltered. These specimens had to be preserved in a unique manner such as being trapped in tree sap or frozen into a pond. The most common example of this type of fossil would be fossils trapped in amber, mammoths recovered from the frozen tundra or fossils such as those from the tar pits.

Fossil insect trapped in tree sap millions of years ago. Perfectly preserved by the sap turned to amber.

Trace Fossils

Trace fossils are not remains of a living specimen itself, instead its interaction with its surrounding area. The most common type of trace fossil is a footprint

Trace fossil of a footprint left in pliestocene bog iron, perhaps left by a fox