Why Are Fossils Only Found in Sedimentary Rocks?

The trick to a successful search for fossils is finding the proper place. After all, paleontologists rarely select their sites at random. Instead, they consider an assortment of areas — comparing the age and the accessibility of the rock — prior to picking up their trowels.

Most important in their considerations is the type of stone at a prospective search site, which is almost always sedimentary rather than igneous or metamorphic. The question is, why?

While the formation of sedimentary rock is perfect for the preservation of fossils, the formation of igneous and metamorphic rock isn’t so suited, resulting in the removal of the traces of ancient animals and plants.

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

Specialists say that there are three main types of rock, including igneous, metamorphic and sedimentary rock, which are all created through their own characteristic physical processes.

For instance, while igneous rocks form when the molten material from the planet’s core cools and sets as a single, solid mass, metamorphic rocks form when intense temperatures or pressures inside or outside of the planet’s crust transform the structure of an already formed stone.

Alternatively, sedimentary rocks take shape when the tiny, weathered fragments of stones are compacted and cemented together.

The process begins when rocks, minerals and organic materials — such as shells — are broken down into bits 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’re compressed so tightly that they turn into a solid stone.

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While the molten and partially molten material that make igneous rocks tend to wipe away the traces of ancient organisms, the temperatures and pressures producing metamorphic rocks typically obliterate any organic materials that they touch. As such, fossils usually only occur in sedimentary stones, where calmer conditions allow for the preservation of past life.

Specialists say that these sedimentary fossils form when sediments such as silt, sand and shards of shell settle over the remains or other remnants of ancient animals and plants. That said, they add that it is also possible to find the occasional fossil in igneous and metamorphic stones, though it is a remarkably rare occurrence.

For instance, volcanic blasts produce plumes of ash that occasionally bury organisms. Before their bodies are burned away by the torrents of magma, that are spewed onto the surface of the planet, they form a fossil in igneous rock. Alternatively, fossils sometimes appear in slabs of stone that are in the middle of metamorphosing. (As this transformation progresses, though, these fossils become increasingly distorted until they’re destroyed).

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Overall, out of all of the separate subtypes of sedimentary rock, fossils are most abundant in shale, sandstone and limestone, though they also appear in conglomerates and breccias.

Shale, first and foremost, forms when the super-fine fragments of stones and minerals that are classified as “silt” settle and solidify. With fragments around 0.002 to 0.05 millimeters across, this type of sedimentary stone can preserve the traces of ancient animals and plants in incredible detail.

According to specialists, shale tends to set in areas with shallow, slow-moving water. As a result, the fossils found in shale are typically small, aquatic animals and plants, such as bryozoans, brachiopods and arthropods.

Similarly, sandstone is fashioned when the semi-fine fragments of stones and minerals that are classified as “sand” are collected and compressed. These fragments are typically around 0.05 to 2.0 millimeters in diameter, meaning that the fossils found in sandstone deposits demonstrate far fewer fine details than those found in shale.

Appearing in an assortment of terrains, including seas, streams, beaches, deserts and dunes, sandstone tends to feature a wider array of fossils, including small aquatic animals as well as larger land animals.

Alternatively, limestone forms when the accumulations of calcium carbonate, found inside bodies of water, crystalize or when the fragments of shells and corals (which are mostly made out of calcium carbonate) fuse together.

Frequently featuring the traces of shelled sea creatures, limestone is commonly considered the most fossiliferous material on the planet. In fact, some chunks of limestone are almost completely composed out of fossils.

Finally, conglomerates and breccias are created whenever coarse fragments of stone are consolidated together.

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

But in both cases, the sediments that compose the structure of conglomerates and breccias measure more than 2.0 millimeters across and are not as prone to preserving fossils as the constituent materials of shale, sandstone and limestone.

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While this all provides paleontologists with an initial idea of where to look for fossils, these ancient treasures aren’t always so simple to identify. In fact, there are all sorts of fossils, which differ in their appearance depending on their process of preservation.

For instance, body fossils are the fossils that feature bits of the actual bodies of ancient animals and plants, and they’re what people tend to imagine when they think of fossils. They typically form when an organism’s body is buried by sediment and saturated with water. Once saturated, the organic material in the body is steadily supplemented or swapped with minerals from the water, which slowly solidify, turning the body to rock.

Being best suited for the preservation of bone and shell, these particular processes of fossilization tend to pass over the softer bits of the body, such as skin. That being said, there are some instances of softer tissues being preserved, with different fossils demonstrating different degrees of decomposition.

Other fossils form when organisms are buried before decaying, creating casts of their bodies. Others still form when the traces of an animal’s activity, such as its footprints or its feces, are preserved rather than the remains of the animal itself.

Respectively referred to as impression and trace fossils, these types of fossils represent a few of the most prevalent processes of preservation, alongside body fossilization. And, of course, despite all of the differences between these processes, specialists stress that all three types of specimen are best found by searching through the sediment.

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