What are mineralized fossils

How are fossils formed?

The creation of fossils from dead organisms is a complex process that takes an unimaginably long time. Under the right circumstances, however, remains or traces of animals and plants are preserved for millions of years.

Past life testimonies

When an animal or plant dies, the dead organism is usually eaten by other animals and broken down by organisms such as bacteria and fungi. It finally dissolves and disappears without leaving any trace. But if the circumstances are right, it can also become a fossil and thus last for millennia. All remains that are older than 10,000 years are considered fossils. The oldest fossil discovery (a type of bacteria) is estimated to be 3.4 billion years old! Life on earth probably originated at this time. The science of fossils is called paleontology.

Much disappears without a trace ... but not everything!

Fossils are all called testimonies and evidence of prehistoric life. These can be body remains (such as part of a dinosaur skeleton or a tooth), but also imprints (e.g. from leaves) and animal tracks - i.e. everything that tells us something about previous life on earth. But very few living things ever become fossils. To do this, their remains must be located in a place where they will not be completely decomposed by scavengers or by weathering. This can be the case, for example, buried under sand, mud or clay. A good place for this is, for example, a river bed or the ocean floor. Embedding it in sediments prevents air from reaching the body and thus slows down decomposition.

Why are the fossil skeletons of prehistoric creatures so strangely twisted?

Achim Reisdorf gives the answer in this Science Slam video!

The embedded remains are mineralized

Over time, the dead body is buried deeper and deeper as new layers of sediment build up on it. The pressure created in this way slowly compresses the mud and sand into stone. The increasing pressure also affects the buried remains of living beings. This compresses these and squeezes all of the water out of them. Water seeps through the soil with substances that dissolve all remaining organic components of the body and replace them with minerals - this is called recrystallization. The result is a “stone copy” of the remains, a fossilization that may eventually become visible again after many years and can be found by a keen-eyed fossil collector.

Soft parts, hard parts, stone cores and trace fossils

The soft parts of a body - muscles, fat, organs, etc. - only survive the transformation into fossils in extremely rare cases. Most of the time, they are broken down by bacteria despite being embedded in mud or sand. What remains are the hard parts that decompose much more slowly, i.e. bones, teeth and shells.

In shellfish or shellfish such as mussels or snails, the decomposition of the soft tissues sometimes creates a cavity (since the shell is still there), which is then filled with minerals. The shell then dissolves. This creates an internal imprint on the stone that has filled the housing and is called the stone core.

In addition to body remains, there are often traces that can tell a lot about the life and behavior of the animals. Such trace fossils include, for example, imprints of excrement, feathers, creeping, breeding and feeding tracks and, of course, animal tracks.

Plants also leave marks or, under the right circumstances, can even completely fossilize. This is how petrified forests are created, for example.

Not all fossils are created equal

Only when the remains have been completely converted into rock is one speaks of "petrification". There are other types of fossils as well. Instead of being embedded in sand or mud, animals and plants can also be frozen in ice, such as mammoths in the permafrost soil in Siberia. Or they get trapped in tree sap, which turns into amber over time.

So that the body of a living being does not completely decompose after its death, it must be buried under the exclusion of oxygen. Sedimentation, a little chemistry and a lot of time may turn it into a fossil. You can find out what fossils can be read in the article "What do fossils tell us?"

In this short video you can see what a fossil taxidermist does and what it takes to become one yourself.

You can get information on training from the Swiss Association of Natural Science Taxidermists.