Fossils in Sedimentary RockSedimentary rocks with fossils

The key to a successful fossil hunt is finding the right place. After all, paleontologists rarely choose their sites at random. Instead, they consider a range of areas – comparing the age and accessibility of the rock – before picking up their trowels.

Foremost in their considerations is the type of rock at the proposed search site, which is almost always sedimentary rather than igneous or metamorphic. The question is why?

While the sedimentary rock formation is ideal for preserving the fossilsthe formation of igneous and metamorphic rocks is not so suitable, resulting in the removal of traces of ancient animals and plants.

Here’s what you’ll want to know about the processes of rock formation and fossilization.

Types of rock

Experts say there are three main types of rocks, including igneous, metamorphic and sedimentary rocks, which are all created by their own characteristic physical processes.

Igneous and metamorphic rocks

For example, while igneous rocks formed when molten material from the planet’s core cooled and solidified as a single solid mass, metamorphic rocks are formed when high temperatures or pressures inside or outside the planet’s crust transform the structure of already formed rock.

Sedimentary rocks

Alternatively, sedimentary rocks they take shape when small, weathered stone fragments are compacted and cemented together.

The process begins when rocks, minerals and organic materials – such as shells – are broken down into pieces and transported to a new area by the forces of wind or water. Over time, as more of these sediments accumulate in the area, they are compressed so tightly that they become solid rock.

Read more: What are the oldest fossils in the world?

Rocks with fossils

While the molten and partially molten material that makes igneous rocks tends to erase traces of ancient organisms, the temperatures and pressures that produce metamorphic rocks usually obliterate any organic material they touch. As such, fossils are usually only found in sedimentary rocks where calmer conditions allow past life to be preserved.

Experts say these sedimentary fossils form when sediments such as silt, sand and pieces of shell settle over the remains or other remains of ancient animals and plants. However, they add that it is also possible to find occasional fossils in igneous and metamorphic rocks, although this is extremely rare.

For example, volcanic eruptions produce ash clouds that sometimes bury organisms. Before their bodies are incinerated by the torrents of magma that are ejected onto the planet’s surface, they fossilize in igneous rock. Alternatively, fossils sometimes occur in rock slabs that are undergoing metamorphosis. (As this transformation progresses, however, these fossils become increasingly distorted until they are destroyed).

Read more: Are the oldest fossils real – or just rocks?

Sedimentary rocks with fossils

In general, from everyone separate subtypes of sedimentary rocksfossils are most abundant in shale, sandstone, and limestone, although they also occur in conglomerates and breccias.


Shale, in the first place, is formed when the super-fine fragments of rocks and minerals that are classified as “silt” settle out and solidify. With fragments about 0.002 to 0.05 millimeters in diameter, this type of sedimentary rock can preserve traces of ancient animals and plants in incredible detail.

According to experts, shales tend to settle in areas with shallow, slow-moving water. As a result, fossils found in shale are usually small aquatic animals and plants, such as bryozoans, brachiopods, and arthropods.


Similarly, sandstone is formed when the semi-fine fragments of rocks and minerals that are classified as “sand” are collected and compressed. These fragments are usually about 0.05 to 2.0 millimeters in diameter, meaning that fossils found in sandstone deposits show much less fine detail than those found in shale.

Occurring in a range of terrains, including seas, streams, beaches, deserts and dunes, sandstone tends to include a wider range of fossils, including small aquatic animals as well as larger land animals.


Alternatively, limestone is formed when deposits of calcium carbonate found in bodies of water crystallize or when fragments of shells and corals (which are mostly calcium carbonate) fuse together.

Often containing traces of shelly marine life, limestone is generally considered to be the most fossiliferous material on the planet. In fact, some pieces of limestone are almost entirely composed of fossils.

Conglomerates and breccias

Finally, conglomerates and breccias are created when coarse rock fragments consolidate together.

While conglomerates tend to contain smooth sediments, breccias tend to contain sharp sediments. This means that the fragments that consist of these two types of rock travel different distances before being deposited and condensed.

But in both cases, the sediments that make up the structure of conglomerates and breccias are larger than 2.0 millimeters in size and are not as prone to fossil preservation as the constituent materials of shale, sandstone, and limestone.

Read more: Take a tour of these incredible living fossils

What are fossils?

While all of this gives paleontologists an initial idea of ​​where to look for fossils, these ancient treasures aren’t always so easy to identify. In fact, there are all kinds of fossils that differ in appearance depending on them preservation process.

Body fossils

For example, body fossils are the fossils that contain fragments of the actual bodies of ancient animals and plants and are what people tend to picture when they think of fossils. They usually form when the organism’s body is buried by sediment and saturated with water. Once saturated, the organic material in the body is constantly replenished or replaced by minerals from the water, which slowly solidify, turning the body into rock.

Because they are best suited to preserve bones and shells, these specific petrification processes tend to go over the softer parts of the body, such as the skin. That being said, there are some cases of preservation of softer tissues, with different fossils demonstrating varying degrees of decomposition.

Footprints and trace fossils

Other fossils form when organisms are buried before they decay, creating casts of their bodies. Still others are formed when traces of animal activity are preserved, such as footprints or droppings, rather than the remains of the animal itself.

Appropriately called imprint and trace fossils, these types of fossils represent several of the most common preservation processes, along with body fossilization. And, of course, despite all the differences between these processes, experts emphasize that all three types of specimens are best found by searching in the sediment.

Read more: We owe our life to 3 billion year old stromatolite fossils

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