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Fossil Friday: There are Many Different Kinds of Fossils! Part 4

Updated: Aug 12, 2022

Quiz (answers to quiz are at the end of this blog).

1) Body Fossils include all of the following except:

a) Shells

b) Indentations

c) Teeth

d) Bones

2) Permineralization refers to:

a) Soft water in lakes

b) Fingernails and tusks

c) Hollow places in organisms

d) Leaf impressions left in clay

"Petrified Fossils and More!"

By Synthia Durrant

In Part 4 of this series on Kinds of Fossils, we are going to talk about "Body Fossils", these are the kinds of fossils that you can hold in your hand and pass around without worrying very much about damaging them. These kinds of fossils are often rock-like in appearance and appear impervious to their environment like rocks or minerals. Usually, this type of fossil consists of the "hard parts" of an organism that is left behind after it dies. This usually means the bones, teeth or shell (the outer casing) of an organism (1).

A close-up image of a man holding a shark's tooth in one hand and a megalodon's tooth in the other. The sharks tooth is much smaller and is a natural white. The megalodon's tooth is much larger and is colored black and gray.
A shark's tooth compared to a megalodon's. Not only are the two hugely different in size, but the megalodon's tooth has become mineralized (as you can see by its dark color) and the shark's tooth is still unaltered making this photograph a particularly good example of unaltered and mineralized fossils (A).

Some of these are the literal, unchanged hard parts that are left behind from a once-living organism. These are called unaltered hard parts, and consist of the actual bones, teeth, or shell of a once living organism. This kind of fossils relies on the strictest definition of a fossil as "any part of a once living organism that is left behind". Many people have never considered that bones, teeth, or shells are "fossils", but they are.

A closeup of a man's hand holding a boars tooth or tusk. It has a hole drilled in the top and a silk string pulled through, so the tooth can be worn as a necklace.
This is an example of tooth jewelry. This is a boar's tooth (or tusk). It is typical of the kind of tooth jewelry that is common even today. This is a kind of unaltered fossil (B).

If you have every seen a skeleton used to teach human or animal anatomy then you have seen this kind of unaltered (unchanged) fossil. Another kind of unaltered fossil you may have already seen is any kind of tooth jewelry such as shark's tooth which is very common. Occasionally jewelry is made of claws from an animal, as well. This is also a kind of unaltered fossil. The main thing to remember with an unaltered fossil is that it's structure is exactly the same as it was when it was part of a living organism.

Other kinds of "body fossil" include permineralized fossils and petrified fossils. These kinds of fossils may look similar to one another, but they are made in slightly different ways. They are both considered replacement fossils. This means that something about their structure was replaced compared to their original, organic structure. This replacement ensures they will not decay, they have become stable.

An example of a permineralized tree fossil. The outside and hollow spaces are filled with mineral, but the tightly packed tree rings still appear to be wood.
An example of a permineralized tree fossil. If you look closely you can see wood tree rings and also mineral/rock edges. It is a good example of how the hollow places get filled with minerals first. Only after they are filled does the tree fully mineralize and become petrified. Perminalization is on the way to petrification, but not all permineralized fossils will become petrified (D).

Perminerlized fossils usually occur with spongey, porous material such as bones, shells, and trees. Permineralization occurs when their spongey interior has been slowly filled and replaced with with calcium carbonate, or other mineral. This usually occurred when living organism died in watery environments that were prone to flooding. Permineralized fossils are usually much heavier than their living counterparts because their empty spaces have been filled with minerals (2 & 4).

An example of a fully petrified tree stump. It is red, gold, and black. The grain of the wood and the tree rings are apparent.
This is an example of a fully petrified tree. Amazingly, the tree rings and the grain of the wood still appears obvious (E).,

A petrified fossil is also a replacement fossil. In the case of a replacement fossil, their cells have been replaced one-by-one with a mineral, this process is also called "recrystallization". The main difference between a recrystallized (or petrified) fossil and permineralized fossil is that a permineralized fossil has had it's pores filled with a mineral. A recrystallized or petrified fossil, on the other hand, has had it's entire cell structure replaced with a mineral (4). A recrystallized fossil is also much heavier than it's living counterpart.

A 3-dimensional remnant of an octopus-like creature partly emerging from stone. It's 5 legs coil gently around itself as it was preserved when it settled on the bottom of the sea-floor. It is delicate and beautiful.
An ancient octopus-like creature has left it's cast at what was once the bottom of a sea. A cast is made when the organic parts disappear leaving a hollow in the ground that gets filled with minerals (F).

There are other kinds of fossils that are considered "body fossils", these are called casts. A cast occurs when the organic material of a once living organism decays and leaves a hollow in the ground in the exact shape of the organism. This hollow can be filled filled with minerals that eventually become a fossil of the organism (2)

3-dimensional fossils that we can pick up and examine closely, feeling their texture and size are some of the most compelling fossils for us as individuals. We can connect with them because we can touch them. These body fossils, whether they are unaltered as they were in life, or replaced with minerals, or simply cast replicas of the original, often make the deepest impression on us. Many of us keep such a fossil as a sentimental attachment to a time that came before us and a piece that will be around long after we are gone.

Quiz Answers

1) Body Fossils include all of the following except:

b) Indentations

2) Permineralization refers to:

c) Hollow places in organisms


cast--an exact replica of a living organism, in a fossil this occurs when an organism gets buried and it's organic body decays and is replaced by minerals

permineralized--hollow places within a once living organism are filled with minerals

petrified--all the cells have been replaced from the once living material to minerals

recrystallized--the replacement, cell-by-cell, from organic material into a mineral; another word for a petrified fossil

shell--the outer casing of an organism

stable--will not decay


unaltered hard parts--bones, teeth, or shell that has not become mineralized













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