Burial Process: Covering Dead Organisms with Sediments
The Essential Ingredients for Burial
For a dead organism to have a chance at becoming a fossil, it must be buried quickly. The key ingredients for this process are a source of sediment, a method of transport, and a suitable environment where accumulation can happen. Sediments are small, solid particles like sand, silt, clay, and gravel that are moved by wind, water, or ice.
| Sediment Type | Particle Size | Common Source | Example Burial Environment |
|---|---|---|---|
| Clay & Mud | Very Fine | Weathered rocks in river floodplains | Deep ocean floor, lake bottoms |
| Silt | Fine | River deltas | Lagoons, river floodplains |
| Sand | Medium | Deserts, beaches, riverbeds | Sand dunes, shallow seas |
| Gravel | Coarse | Mountain streams, glaciers | River channels, alluvial fans |
Imagine a fish dying in a lake. Its body sinks to the bottom, where fine mud is constantly settling out of the water. Over days, weeks, and years, more and more mud covers the fish. This mud protects it from being eaten by scavengers and from decaying quickly because it cuts off the oxygen that bacteria need to decompose the soft tissues. This rapid burial is the critical first step that can lead to the fish becoming a fossil.
From Soft Mud to Solid Rock
Burial is just the beginning. The layers of sediment, now containing the buried organism, continue to pile up. The weight of the overlying sediments creates immense pressure, squeezing the lower layers. At the same time, the sediments are often saturated with water containing dissolved minerals like calcite ($CaCO_3$) or silica ($SiO_2$).
This entire process, from burial to lithification, is called diagenesis. The original sediment, like sand, becomes sandstone. The mud and clay become shale. And the remains of the organism trapped inside undergo their own transformation, which we will explore next.
What Happens to the Buried Organism?
Not all buried organisms become fossils. Most simply decay away. Fossilization is a rare event that requires specific conditions. The fate of the buried remains depends on what parts of the organism are present and the chemistry of the surrounding environment.
Hard Parts vs. Soft Parts: Bones, teeth, shells, and wood have a much higher chance of being preserved than skin, muscles, or organs. This is because hard parts are often made of durable minerals like apatite (in bones) or calcite (in shells). Soft tissues usually decompose too quickly unless burial is extremely rapid and in an environment without oxygen, like the sticky tar of the La Brea Tar Pits or the fine mud that preserved the feathered dinosaurs of Liaoning, China.
The main types of preservation after burial are:
- Permineralization: This is the most common process for dinosaur bones and petrified wood. Mineral-laden water (like groundwater) seeps into the tiny pores and spaces within the bone or wood. The minerals then precipitate out of the water, filling the spaces. This hardens the object and makes it heavier, but it preserves the original structure in incredible detail. The chemical formula for the common mineral calcite is $CaCO_3$.
- Replacement: In this process, the original material of the shell or bone is completely dissolved away by groundwater and simultaneously replaced molecule-by-molecule with a new mineral, such as pyrite ($FeS_2$, also known as "fool's gold") or silica ($SiO_2$).
- Carbonization: This often happens with leaves and delicate organisms. After burial, pressure squeezes out the liquids and gases of the organism, leaving behind only a thin film of carbon. This carbon outline is the fossil.
- Molds and Casts: If a shell is buried and then later dissolved away by water, it leaves a hollow imprint in the surrounding rock called a mold. If this mold later gets filled with other minerals, it forms a cast—a replica of the original shell.
Fossil Fuels: The Ultimate Result of Plant Burial
The burial process is not only responsible for the skeletons we see in museums but also for the fuels that power our world. Fossil fuels—coal, oil, and natural gas—are the altered remains of ancient life, transformed by heat and pressure over millions of years.
Coal formed in vast ancient swamps from the burial of enormous quantities of plants. When these plants died, they fell into the oxygen-poor water, which slowed decay. They were buried by sediments, and over geologic time, the increasing heat and pressure drove off gases and moisture, concentrating the carbon. The progression from peat to lignite to bituminous coal, and finally to anthracite, represents increasing carbon concentration and energy content.
Oil and Natural Gas primarily formed from the remains of tiny marine organisms like plankton and algae. When these organisms died, they sank to the seafloor and were buried under layers of mud in an oxygen-poor environment. Over millions of years, the heat from the Earth's interior "cooked" this organic material, converting it into hydrocarbons—the compounds that make up oil and gas.
A Tale of Two Fossils: Dinosaur and Trilobite
Let's follow the journey of two different organisms to see the burial process in action.
The Dinosaur: A large Triceratops dies near a river. Scavengers eat the soft parts and scatter the bones. The next rainy season brings a flood, which washes a layer of silt and sand over the bones, burying them. Over thousands of years, the river changes course, depositing more and more sediment layers. The weight compacts the sediments, and minerals from groundwater cement the sand grains together into sandstone, while also permineralizing the bones. Millions of years later, erosion from wind and water wears away the rock, exposing the fossilized skeleton for a paleontologist to discover.
The Trilobite: This ancient, shrimp-like creature lived on the seafloor. When it died, its body settled into the soft, muddy bottom of the ocean. The calm, deep water allowed fine mud to gently cover it completely, preserving its entire articulated form. The mud eventually turned into a fine-grained rock called shale. Because the burial was so rapid and gentle, the trilobite fossil is exquisitely detailed, showing its legs and antennae.
| Organism | Environment of Death | Burial Sediment | Resulting Fossil/Rock |
|---|---|---|---|
| Dinosaur (e.g., Triceratops) | River Floodplain | Sand & Silt | Permineralized bones in Sandstone |
| Trilobite | Deep Ocean Floor | Fine Mud & Clay | Carbonized impression in Shale |
| Ancient Plant | Swamp | Peat & Silt | Coal |
Common Mistakes and Important Questions
Q: Are fossils the actual bones of the animal?
Often, no. While sometimes the original bone material is preserved, it is much more common for the bone to have been permineralized or replaced by other minerals. The original bone has been turned to stone, which is why we call them "fossils."
Q: Why are fossils so rare?
Fossilization requires a perfect set of conditions. Most dead organisms are eaten, decomposed, or scattered before they can be buried. Even if buried, the rock containing the fossil could be destroyed by Earth's tectonic forces or erosion before it is ever found. For every one fossil we find, millions of organisms left no trace at all.
Q: Can soft tissues like skin ever become fossils?
Yes, but it is extremely rare. This requires very special conditions that almost completely halt decay, such as freezing (like woolly mammoths in ice), desiccation (drying out in caves), or preservation in anoxic (oxygen-free) environments like amber or acidic bogs. These are exceptional cases; the vast majority of fossils are of hard parts.
Footnote
Definitions of key terms used in this article:
1. Diagenesis[1]: All the physical, chemical, and biological changes undergone by a sediment after its initial deposition, and during and after its lithification, excluding weathering and metamorphism.
2. Lithification[2]: The process of converting loose, unconsolidated sediment into solid rock, primarily through compaction and cementation.
3. Permineralization[3]: A fossilization process where mineral deposits from groundwater fill the microscopic pores and cavities within an organism's hard parts (e.g., bone or wood).
4. Fossil Fuels[4]: Combustible fuels such as coal, oil, and natural gas, derived from the buried remains of ancient plants and animals and transformed by geological processes over millions of years.