How are fossils formed? The short answer
Most fossils form when a plant or animal dies and is buried quickly by mud, sand or volcanic ash, before it can rot away.10 More layers of sediment pile on top and slowly harden into rock.1 Meanwhile, groundwater seeps through the buried bones or shells and leaves minerals behind in their tiny holes, turning them to stone.110 Much later, moving tectonic plates can lift the rock up, and wind, rain, ice and rivers wear it away until the fossil is exposed.1
A fossil is any evidence of prehistoric life preserved in rock: the remains themselves, like bones, teeth and shells, or traces such as footprints.13 The word comes from the Latin for "dug up".10
Why most living things never become fossils
Fossils are rare. Most things that die rot away completely and leave nothing behind.1 Microbes and insects move in almost right away to eat a dead body, and scavengers take what they can.91 Two things tip the odds toward becoming a fossil:312
- Rapid burial. Being covered by sediment soon after death protects remains from scavengers and weather. Underneath the sediment there is very little oxygen, so rotting slows down.13
- Hard parts. Bones, teeth, shells and wood last far longer than skin, muscle or leaves.3
That is why around 99% of the fossils we find come from sea animals such as shellfish and sharks: on the seafloor, sand and mud bury remains quickly.1 Animals from rivers, lakes and floodplains are also common as fossils, while animals from mountains and forests are rare.3
How are dinosaur fossils formed over time?
Dinosaurs lived on land, so how did they get buried fast enough? Scientists at the Natural History Museum in London explain that most dinosaur fossils come from animals that lived near a lake or river. Some died just before a flood covered them in mud and silt; others were washed into rivers by heavy rain.1 From there, the steps are the same for a dinosaur as for a fish:
- The animal dies. Soft parts start to rot, and scavengers may eat some of the body.1
- It gets buried. Mud, sand or silt covers the body. Often only the bones and teeth are left by now.1
- Layers pile up. More sediment builds up on top. The weight squeezes the lower layers until they become sedimentary rock: sand becomes sandstone and mud becomes mudstone or shale.15
- Bone turns to stone. Water seeps into the bones and leaves mineral crystals behind in the spaces.1
- It comes back up. Uplift, weathering and erosion eventually bring the fossil to the surface, where someone can find it.110
Fossilizing a big bone can take thousands of years or more. For small soft-bodied animals, it can happen in months or even days.2
Permineralization: how bone turns to stone
Bone is full of tiny holes, like a sponge. When groundwater carrying dissolved minerals flows through buried bone, the minerals crystallize inside those holes. This is called permineralization, and it is the most common way fossils are preserved. It works for teeth, bones, shells and wood, and fossils made this way are often called "petrified".1012 Because the crystals fill in the spaces, dinosaur fossils often still show the honeycomb texture of the bone inside.1
Sometimes the original bone or shell is replaced completely by other minerals, a process called replacement.12 But a fossil bone usually still holds some of the animal's original bone mineral.9 Petrified wood forms the same way: water rich in silica soaks into the wood, which can keep its growth rings and even the outlines of its cells.91
Types of fossils
Moulds and casts are the most common way shelled sea animals, like the coiled ammonites that died out with the dinosaurs, became fossils.1 Trace fossils record behaviour: a footprint in soft mud acts as a natural mould, and new sediment fills it to make a cast.14 Amber is special because it can keep delicate, soft-bodied creatures like ants and flies whole and in three dimensions,11 and amber fossils up to about 100 million years old are known.10 Very rarely, soft tissue survives in other ways, such as mammoths frozen in ice.12
Where are fossils found?
Fossils form in sediment, so they are found in sedimentary rocks like sandstone, shale and limestone.12 They come to the surface in two stages. First, uplift: where tectonic plates collide they push rock upward, sometimes into mountain ranges. That is why fossils of sea animals are found near the top of Mount Everest, and ammonites turn up in the Himalayas.111 Second, weathering and erosion: wind, rain, ice and rivers break rock apart over millions of years until a fossil is exposed.1 Cliffs, quarries and dry, rocky hillsides are good places to look.
How do scientists know how old a fossil is?
Sedimentary rocks build up one layer at a time, so in an undisturbed stack each layer is older than the layers above it. This is the Law of Superposition, first worked out by Nicolas Steno in the 1600s.5 It gives the order of events, but not a number of years.
For numbers, scientists use radiometric dating. Some minerals contain radioactive atoms that change into other atoms at a steady, known pace. The time it takes for half of them to change is called the half-life. Measuring how many have changed tells you the rock's age.6 Fossil-bearing sedimentary rocks are usually dated by finding dated igneous rocks, such as volcanic ash layers, above and below them.7 The fossil must be younger than the layer beneath it and older than the one above. Some common, short-lived species make useful index fossils for matching rock ages from place to place.212
older ash layer below > fossil age > younger ash layer above
How are fossils formed? (for kids)
Imagine a fish dies in a lake. Mud quickly covers it, like a blanket. Over a very long time, more and more mud piles on top and gets squashed into rock. Water trickles through the bones and leaves tiny crystals inside them, until the bones are made of stone. Millions of years later, the land is pushed up and rain wears the rock away, and one day someone spots a stone fish sticking out of a cliff.
Common misconceptions
"Every dead animal becomes a fossil." Most rot away completely. Fossils need quick burial and usually hard parts.
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"Dinosaur fossils are the original bones." They are mostly minerals that filled or replaced the bone, though some original bone mineral often remains.
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"Fossils are only bones." Shells, moulds, casts, footprints, burrows, fossil poo and insects in amber all count.
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"Most fossils are dinosaurs." Dinosaurs are a tiny part of the fossil record. The most common fossils are shelled animals and microscopic skeletons of sea life.
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"Deeper means we know the exact age." Depth only gives the order. Actual ages come from radiometric dating of layers such as volcanic ash.
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FAQ
How are fossils formed, in short?
A plant or animal dies and is buried quickly by mud, sand or volcanic ash before it rots away. More layers pile on top and slowly harden into rock, while groundwater leaves minerals inside the bones or shells and turns them to stone. Millions of years later, uplift and erosion can bring the fossil back to the surface.
Are dinosaur fossils actual bones?
Yes, but turned to stone. Minerals from groundwater filled the tiny holes in the bone, and sometimes replaced parts of it, so a dinosaur fossil is real bone that has become rock. Some of the original bone mineral is usually still there, and the crystals keep the bone's spongy, honeycomb inside.
How long do fossils take to form?
It varies. Big bones such as dinosaur bones usually take thousands of years or more to fossilize, while small soft-bodied animals can be preserved in months or even days. Scientists generally call remains fossils once they are more than about 10,000 years old.
What are the main ways fossils form?
The most common are permineralization (minerals fill the pores in bones, shells and wood), moulds and casts (a buried shell dissolves, leaving a hole that fills with minerals or mud), trace fossils such as footprints and burrows, and rare whole-body preservation in amber, ice or tar.
Can fossils be created today?
Yes. Fossilization never stopped: animals and shells are being buried in mud on lake beds and seafloors right now. The same slow processes of burial, mineral-rich water and time could turn some of them into fossils in the far future.
Why are most fossils from sea animals?
On the seafloor, sand and mud can bury dead sea creatures quickly, before they rot or get eaten. The Natural History Museum estimates that around 99% of the fossils we find are from marine animals.
Where are fossils found?
Almost always in sedimentary rocks, such as sandstone, mudstone, shale and limestone, which form from layers of sediment. Places where these rocks have been lifted up and worn away, like cliffs, quarries and badlands, are the best spots to look.
Sources
- Natural History Museum (London), How are dinosaur fossils formed?
- Natural History Museum (London), What is a fossil?
- U.S. National Park Service, How Fossils Form
- U.S. National Park Service, Trace Fossils
- USGS, Fossils, Rocks, and Time: Rocks and Layers
- USGS, Fossils, Rocks, and Time: The Numeric Time Scale
- USGS, Geologic Time: Radiometric Time Scale
- USGS, Fossils, Rocks, and Time: Fossils and Rocks
- Smithsonian (Anna K. Behrensmeyer, National Museum of Natural History), How Do Fossils Form?
- Australian Museum, How do fossils form?
- National Geographic Education, Fossil
- Encyclopaedia Britannica, Fossil