Not merely an old rock. Not an attractive geological paperweight. Not something the Earth made solely to tempt us into climbing unstable slopes in inappropriate footwear.
A fossil is a message from the past.
It may be a bone, tooth, shell or leaf. It may be the delicate impression of a feather, the tunnel left by a burrowing animal or the footprints of a creature that crossed a muddy shoreline hundreds of millions of years ago.
It may even be fossilized dung.
The fossil record has never been overly concerned with dignity.
Together, fossils preserve the history of life on Earth: its astonishing experiments, spectacular successes, evolutionary detours and occasional catastrophes. They introduce us to organisms no human ever saw alive and to ecosystems that vanished long before our own species appeared.
But what exactly qualifies as a fossil—and how can something as fleeting as a footprint survive for millions of years?
Let us begin at the beginning.
SO, WHAT IS A FOSSIL?
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| Dinosaur Footprints near Tumbler Ridge, BC |
A fossil can be part of an organism’s body, such as a bone, tooth, shell, leaf or piece of wood. It can also be a mark made by an organism while it was alive: a footprint, burrow, feeding trail, nest or bite mark.
The organism itself does not necessarily need to be present.
This is one of the most important things to understand about fossils. We often picture a magnificent dinosaur skeleton standing in a museum, every rib neatly arranged and every tooth poised for drama. Yet complete skeletons are extraordinarily rare. Most fossils are fragments: a single tooth, part of a shell, a scrap of leaf, one vertebra or a dark smudge of carbon in the rock.
Some fossils are so small that thousands could sit comfortably on the head of a pin. Others belong to animals that stretched longer than a city bus.
All of them are evidence.
We divide fossils into two major categories: body fossils and trace fossils. The distinction is simple, but it opens the door to an astonishing variety of preservation.
BODY FOSSILS: WHEN PART OF THE ORGANISM REMAINS
A body fossil preserves part of the organism itself—or a direct impression, mould or cast of its remains.
The most familiar body fossils include:
- Bones
- Teeth
- Shells
- Exoskeletons
- Scales
- Wood
- Leaves
- Seeds
- Pollen
- Spores
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| Cenoceras Fossilized Shell Material |
This is one reason isolated fossil teeth are much more common than complete fossil animals. Nature produces teeth generously and skeletons only once per customer.
Shells also have an excellent chance of preservation. Ancient seafloors could accumulate the remains of countless brachiopods, molluscs, corals and other marine organisms. Over time, those shells became part of the sediment and, eventually, part of the rock itself.
The most durable portions of an organism dominate the fossil record. According to the National Park Service overview of body fossils, bones, teeth, shells, mineralized plates and wood are preserved far more frequently than skin, organs, flowers or fruit.
This creates an immediate problem.
The fossil record is not a complete census of everything that ever lived. It has a built-in preference for organisms with hard parts, those living in places where burial could occur and those fortunate enough to die under favourable conditions.
A clam living on a muddy seafloor has a better chance of becoming a fossil than a soft-bodied creature living on a windswept mountain. This does not mean the mountain creature was less important. It was simply less likely to leave us its forwarding address. We find many more of the durable creatures than we do the squishy.
TRACE FOSSILS: WHEN BEHAVIOUR BECOMES STONE
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| Theropod Track, Thailand |
They are also called ichnofossils, from the Greek word ichnos, meaning track or trace. Scientists who specialize in studying them are called ichnologists.
Trace fossils include:
- Footprints and trackways
- Burrows and tunnels
- Trails
- Resting impressions
- Nests
- Root traces
- Feeding marks
- Bite marks
- Borings in shells or wood
- Coprolites, or fossilized dung
- Regurgitated material
- Gastroliths associated with an animal
A body fossil can tell us that an organism existed. A trace fossil may reveal what it was doing.
A line of footprints can show whether an animal walked on two legs or four. The distance between prints can help researchers estimate speed. Several parallel trackways may suggest that animals moved together, although such interpretations must be made carefully.
A burrow can reveal how an animal moved through sediment, searched for food or protected itself. Bite marks can document interactions between predator and prey. A resting impression can preserve the outline of an animal settling momentarily onto a soft surface.
Trace fossils often remain in the substrate where the behaviour originally occurred. Unlike bones, which may be carried away by water, scavengers or gravity, a burrow generally records activity at that exact location.
This gives trace fossils an extraordinary quality. They preserve moments. An animal stepped here. A worm burrowed here. Something bit this shell.
A dinosaur crossed this stretch of wet sediment, placing one foot after another beneath a sky no human would ever see. The animal continued on. The moment passed. But the ground remembered.
YES, FOSSILIZED POO IS A REAL FOSSIL
Coprolites are fossilized faeces, and they are among some of our most personal treasures.
They vary enormously in size and appearance. Some resemble pellets, spirals or lumpy stones. Others are difficult to distinguish from ordinary rock without examining their shape, contents or chemical composition.
Coprolites can preserve:
- Bone fragments
- Fish scales
- Plant fibres
- Seeds
- Pollen
- Insect remains
- Parasite eggs
- Chemical evidence of diet
They can help researchers determine whether an animal ate meat, plants, insects, fish—or almost anything unfortunate enough to fit inside it.
The difficulty lies in identifying the producer. Unless a coprolite is found directly associated with a particular animal, assigning it to a species can be challenging. Size, shape, contents and geological context may narrow the possibilities, but we must resist the temptation to point dramatically at every ancient dropping and declare, “Tyrannosaurus!”
Not every large coprolite came from the most famous carnivore available.
Still, fossilized dung offers something bones alone cannot: evidence of a meal passing through an animal’s digestive system. Coprolites are trace fossils that can provide direct clues about ancient diets.
One creature’s private moment has become everyone’s scientific business. Deep time can be terribly indiscreet.
DOES A FOSSIL HAVE TO BE TURNED TO STONE?
No. “Fossil” describes evidence of past life, not one particular style of preservation.
Some fossils are petrified or mineralized. Others preserve original material. Fossils can occur as impressions, carbon films, moulds, casts or remains trapped in substances such as amber, tar or ice.
Let us look at a few possibilities.
Permineralized Fossils
Bones and wood contain microscopic spaces. After burial, mineral-rich water may move through the sediment and deposit minerals inside those spaces.
This process is called permineralization.
The original structure may remain while its pores become filled with minerals such as silica, calcite or iron compounds. The result is denser and heavier than the original material but can preserve remarkable internal detail.
Petrified wood is a beautiful example. Its tissues may become mineralized so precisely that growth rings and cellular structures remain visible.
Replacement Fossils
In some fossils, the original biological material dissolves and is gradually replaced by minerals. The new mineral preserves the form—and sometimes even the microscopic structure—of the original organism.
It is rather like replacing a building one brick at a time while somehow retaining its shape.
Moulds and Casts
Imagine a shell buried in sediment.
The sediment hardens around it, but the shell later dissolves, leaving an empty impression. This cavity is a mould. If minerals or sediment later fill that mould and harden, they create a cast: a three-dimensional replica of the original shell.
An internal mould can form when sediment fills the inside of a shell and hardens before the shell disappears. Such fossils are sometimes called steinkerns, a German term meaning “stone kernel.”
Nothing of the original shell may remain, yet its shape survives. The body is gone. The absence has become the fossil.
Compression and Carbon Films
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| McAbee Fossil Plants |
These fossils can appear as dark, delicate outlines on the rock. Leaves may preserve their veins.
Fish may retain the shapes of scales and fins. Soft-bodied animals can sometimes be recorded in astonishing detail.
They are the geological equivalent of silhouettes—life pressed into stone. Some of the lovely specimens we find at the Eocene McAbee Fossil Beds near Cache Creek, British Columbia, are a great example.
Amber Preservation
Amber begins as sticky resin produced by ancient trees. Small organisms such as insects, spiders, mites and fragments of plants can become trapped in the resin. As the resin is buried and chemically altered over time, it becomes amber.
Amber can preserve extraordinary detail: hairs, wings, compound eyes and delicate structures that would normally decay rapidly.
It can even preserve tiny moments of behaviour. Organisms have been found trapped while feeding, mating, carrying pollen or interacting with parasites.
Amber is not a perfect time capsule, however, and it has not provided usable dinosaur DNA. The mosquitoes of Jurassic Park remain unemployed. If you are looking for some superb examples of amber preservation, check out the collection of Harry Tabiner in the UK. He has a remarkable eye and enviable collection, which he shares online.
Ice, Dryness and Natural Tar
Cold, arid or oxygen-poor conditions can dramatically slow decay. Mammoths preserved in frozen ground may retain hair, skin, muscle and stomach contents. Natural tar deposits can preserve vast collections of bones, although the remains themselves may be stained, altered or compressed.
These forms of preservation remind us that a fossil does not need to resemble a traditional stone skeleton. Fossilization is not a single recipe. It is a vast collection of unlikely outcomes.
WHAT ABOUT FOOTPRINTS? HOW CAN A HOLE BECOME A FOSSIL?
A footprint fossil begins with a perfectly ordinary step.
An animal walks across a surface soft enough to receive an impression but firm enough to retain its shape. Mud and damp sand are excellent candidates.
The track must then be buried before wind, waves, rain, other animals or enthusiastic members of the herd destroy it. A new layer of sediment settles over the surface, filling and protecting the impression.
With continued burial, both layers may harden into rock. Millions of years later, erosion separates them again. One surface may preserve the original footprint as a depression. The layer that filled the print may preserve a raised natural cast.
Tracks are not simply stamps of the foot. The animal’s weight can deform layers beneath the surface, producing structures known as undertracks. These may be less anatomically precise than the original print but can still record the passage of the animal.
Interpreting tracks requires caution. Wetness, sediment texture, erosion and the animal’s movement can all alter a footprint’s appearance. The same animal could leave surprisingly different tracks after walking only a short distance.
We must study the track, the sediment and the entire trackway—not merely find three vaguely pointed marks and immediately summon the media.
HOW OLD DOES SOMETHING HAVE TO BE BEFORE IT BECOMES A FOSSIL?
You may have heard that a fossil must be at least 10,000 years old.
That figure is commonly used as a practical dividing line in educational materials and some regulatory contexts, but nature has not installed an official fossil clock that rings at midnight on an organism’s ten-thousandth birthday.
We may use terms such as subfossil for relatively recent remains that are not fully fossilized or that retain considerable original material. The terminology can vary depending on the field, material and legal jurisdiction.
Age alone is not the only consideration. Geological context, preservation and scientific usage all matter.
An ammonite shell 100 million years old is unquestionably a fossil. A mammoth tusk 30,000 years old is also a fossil, even if some of its original material remains. Very recent bones from an archaeological site may be described as remains rather than fossils.
The boundary is useful, but it is not a magical transformation.
ARE ALL FOSSILS VISIBLE?
Not remotely. Some of the most important fossils on Earth are microscopic.
Microfossils include fossilized pollen, spores, algae, bacteria, protists and the tiny shells or skeletal elements of marine organisms.
They may be smaller than a grain of sand, yet they occur in enormous numbers and can be tremendously useful. Because some microscopic organisms evolved rapidly and lived across wide geographic areas, their fossils can help scientists date and correlate rock layers.
Microfossils also reveal ancient environments. Different species lived under different conditions, so their presence can provide clues about water depth, temperature, salinity, oxygen levels and climate.
Oil and gas exploration has long relied on micropalaeontology to interpret underground rock layers. A microscopic fossil no larger than a speck of dust may help identify the age and environment of rocks buried kilometres beneath the surface.
Dinosaurs receive the posters. Microfossils quietly organize geological time. Alas, always the bridesmaid, never the bride... Dinos get all the love.
CAN MOLECULES BE FOSSILS?
Ancient life can sometimes leave chemical evidence even when no recognizable body or trace remains.
These signals are often called chemical fossils or biomarkers.
Certain molecules are associated with particular groups of organisms or biological processes. Modified versions of those compounds may survive in ancient rocks, providing evidence that particular kinds of life were present.
Stable isotope patterns can also carry biological information. Living organisms use elements in ways that can leave characteristic chemical signatures in their tissues or surrounding sediments.
Chemical fossils require careful interpretation because geological heat, pressure and contamination can alter molecular evidence. But when combined with other data, they can help scientists investigate some of the earliest chapters in the history of life.
Sometimes life leaves a bone. Sometimes it leaves a footprint. Sometimes it leaves little more than chemistry.
IS A DINOSAUR SKELETON IN A MUSEUM MADE OF REAL FOSSILS?
Sometimes entirely. Sometimes partly. Sometimes hardly at all.
Complete dinosaur skeletons are rare. Museum mounts are often composites, combining fossil bones from more than one individual of the same species. Missing elements may be reconstructed as casts.
In some displays, the original fossils are too heavy, fragile or scientifically valuable to suspend in a dramatic pose. Museums may mount lightweight replicas while keeping the genuine fossils safely stored for research.
This does not make the display fake. A high-quality cast reproduces the shape and surface detail of the original fossil while protecting irreplaceable material. Casts can also be shared with museums, schools and researchers around the world.
Labels generally explain which elements are original fossils and which are reconstructed. It is always worth looking.
The glamorous skeleton towering over the gallery may be a carefully assembled ambassador for a far more fragmentary animal.
IS EVERY OLD BONE A FOSSIL?
No. Sometimes an old bone is just that, an old bone. Not a fossil. Age and appearance alone do not prove that something is fossilized. Modern bones can become stained by soil and minerals, making them look ancient. Conversely, genuine fossils do not always appear dramatically different from ordinary bone.
Identification may involve examining:
- Geological context
- Colour and density
- Surface texture
- Internal structure
- Mineral composition
- Associated fossils
- Degree of mineralization
- Evidence of recent organic material
And no, licking it is not the preferred scientific method but, I confess, it is helpful in the field. Not fully diagnostic, but helpful. I have a good friend Skippy who swears by it.
There is an old field trick based on the idea that porous fossil bone may stick slightly to the tongue, but modern bone can do the same, as can many ordinary rocks. The technique is unhygienic, unreliable and particularly regrettable when the mystery specimen is not bone at all.
Use a hand lens. Use comparative anatomy. Use laboratory analysis. Leave your tongue out of the peer-reviewed methodology.
WHY ARE FOSSILS SO RARE?
To become a fossil, an organism must pass through a series of improbable filters. Fossils are not rare per se, as I can think of localities with hundreds spread across the surface and more beneath, but they are rare in the sense that only a small percentage of the organisms that have lived have been preserved this way. To become a fossil is a bit of a process, as you might expect.
First, the organism, dino, plant, or what have you, must die in a place where preservation is possible.
Then its remains must escape scavengers, decay, trampling, weather and destruction. They must be buried. The sediment must remain intact as it becomes rock. The fossil must survive heat, pressure, chemical alteration, faulting, erosion and the slow recycling of Earth’s crust.
Finally, it must become exposed—or be reached by excavation—at precisely the right time for someone to recognize it. Too early, and it remains buried. Too late, and erosion destroys it.
The fossil record is therefore both magnificent and profoundly incomplete.
Most organisms that ever lived left no known fossil. Entire species may have appeared, flourished and vanished without preserving a trace that we have yet discovered.
Soft-bodied organisms are especially underrepresented. Environments such as mountains, forests and tropical soils generally offer fewer opportunities for long-term preservation than lakes, river floodplains and marine sediments.
When we reconstruct ancient ecosystems, we are working with survivors of a second kind—not organisms that survived extinction, but evidence that survived geology.
IS A FOSSIL THE SAME THING AS A ROCK?
A fossil is preserved within a geological material, and many fossils have been heavily mineralized, but “fossil” and “rock” are not interchangeable terms.
A rock forms through geological processes and may contain fossils. A fossil is evidence of past life preserved within rock, sediment, amber, ice or another natural context.
The distinction can become wonderfully complicated.
A fossil shell may retain original shell material. A petrified log may be almost entirely mineral. A mould contains none of the organism but preserves its shape. A trace fossil may consist entirely of disturbed sediment.
So, is a fossil a rock? Sometimes it has effectively become one. But what makes it a fossil is not simply its material. It is the biological information it carries.
WHAT CAN ONE FOSSIL TELL US?
Far more than its name. A fossil’s anatomy may reveal how an organism moved, fed or defended itself. Growth rings can record seasons or age. Microscopic wear on teeth may reveal diet. Bone injuries can preserve evidence of accidents, disease, combat and healing.
The surrounding sediment can tell us whether the organism lived in a river, desert, forest, lake or sea. Associated fossils help reconstruct the plants, animals and microorganisms sharing that environment.
Chemical analysis may reveal temperature, water conditions, migration or food-web relationships. Footprints may preserve speed and direction. Burrows can reveal oxygen levels in ancient sediments. Coprolites can contain the remains of a final meal.
Even the way a body was buried matters. Were the bones scattered by a river? Gnawed by scavengers? Transported by waves? Buried in a sudden flood?
The study of what happens to an organism between death, burial and discovery is called taphonomy. It helps palaeontologists separate evidence of how an animal lived from evidence of what happened after it died. A fossil is not simply an object. It is a scene containing clues.
WHAT IS NOT A FOSSIL?
Not every interesting pattern in rock was created by life.
Mineral crystals, concretions, sedimentary structures and weathered rocks can resemble eggs, shells, bones, skin, feathers or entire mysterious creatures. These geological look-alikes are sometimes called pseudofossils. Or, leaverits, as in leave it right there.
Manganese minerals can grow in branching patterns resembling ferns. Concretions can look remarkably like eggs. Layered rocks may split into shapes suggesting shells or leaves. Some rocks possess suspiciously skull-like features and an unreasonable amount of charisma.
The human brain excels at recognizing familiar forms. This tendency, called pareidolia, helps us see faces in clouds, animals in tree bark and dinosaurs in the landscaping gravel.
None of this makes an unusual rock uninteresting. A pseudofossil can still be geologically beautiful.
It simply tells a different story.
THE FOSSIL RECORD IS BIASED
The fossil record is sometimes imagined as a great stone library containing the complete history of life.
It is more like a library after fire, flood, renovation, several lost catalogues and four billion years of questionable management.
Some chapters are richly detailed. Others contain a few scattered sentences. Entire volumes are missing.
Organisms with shells, bones or teeth are more likely to be preserved than soft-bodied organisms. Aquatic environments are generally better represented than upland habitats. Abundant and widespread species have a greater chance of discovery than rare, geographically restricted ones.
Even human activity introduces bias. Fossils are more likely to be discovered where rocks are exposed, where researchers can travel and where scientific work has been funded.
Palaeontologists must account for these biases when interpreting patterns of diversity, evolution and extinction. An absence of fossils does not necessarily prove that an organism was absent. It may mean that the right rocks were not deposited, did not survive or have not yet been explored.
The fossil record is incomplete. But incomplete does not mean useless. It means we must read it carefully.
A FOSSIL IS A RELATIONSHIP
There is another way to think about fossils. A fossil is not merely a thing that was once alive. It is the relationship between life and the Earth that preserved it. The organism contributed its body or behaviour. The environment supplied mud, sand, ash, resin, ice or minerals. Time transformed the evidence. Erosion returned it to the light. Then someone noticed.
That final part matters. A fossil can lie exposed among a thousand ordinary stones and remain silent until a curious person stops, looks closely and asks a question.
What made this shape? Why is this shell inside a mountain? Who walked here? What kind of world once existed beneath my feet? Every fossil discovery begins with attention. And many great specimens have been found by folk of all ages (many of them too young to vote) who take notice of the world around them.
A MESSAGE THAT SURVIVED
To hold a fossil is to hold a contradiction. It is evidence of death that teaches us about life. It is a motionless object that may record movement. It is a fragment capable of revealing an ecosystem.
It is ancient, yet newly discovered.
The animal, plant or microorganism did not set out to leave us a message. The footprint was not made for our benefit. The shell did not settle onto the seafloor hoping to appear one day in a museum drawer.
And yet the evidence survived.
Across burial, mineralization, shifting continents, rising mountains and relentless erosion, something remained.
A tooth.
A leaf.
A burrow.
A trail across the mud.
A tiny, stubborn piece of deep time waiting for someone to recognize it.
That is a fossil.
