Fossils are the remains or traces of living things from long ago, such as bones, shells, leaves and footprints, kept in rock, amber, tar or ice. Most form when a plant or animal is buried quickly in mud or sand and minerals slowly turn it to stone over millions of years, and you can find them yourself on beaches like Charmouth in England or Calvert Cliffs in Maryland.
A Body Sinks Into the Mud
Picture an ichthyosaur, a sea reptile shaped a bit like a dolphin, swimming in a warm shallow sea about 200 million years ago where the south coast of England is today. It dies, and its body sinks to the seabed.
Most dead animals never become fossils. Scavengers tear them apart, bacteria rot the soft parts, and waves scatter the bones. Our ichthyosaur lands on soft mud with very little oxygen, where few scavengers live, and fine mud settles over it layer after layer.
That is the first rule of fossils: quick burial. Animals that died in rivers, lakes and shallow seas had the best chance, because water carries mud and sand that cover a body fast. This is why so many fossils are shells, sea lilies and fish, and why land animals are rarer. A dinosaur that died on a dry hilltop was usually eaten long before anything could bury it. Hard parts such as bones, teeth, shells and wood also survive far better than skin or feathers, which rot within days.
Stone Takes the Bones' Place
As more mud piles on top, the weight squeezes the lower layers. Water is pressed out, grains stick together, and over thousands to millions of years mud hardens into shale, sand into sandstone and lime-rich mud into limestone. These sedimentary rocks hold almost every fossil ever found. Rocks made from lava, or squashed and heated deep underground, usually destroy anything inside them; the rock cycle explains how each kind is made.
Meanwhile, groundwater trickles through the buried bones. A bone is full of tiny holes, like a sponge. The water carries dissolved minerals, mainly silica (the stuff of quartz), calcite and sometimes pyrite, nicknamed fool's gold. Grain by grain, these fill the holes and can replace the bone itself. Scientists call this permineralization and replacement.
The result has the exact shape of the bone but is now mostly stone, which is why fossil bones are heavy and often dark brown or black. Petrified wood in Arizona's Petrified Forest National Park formed the same way: its trees fell more than 200 million years ago, were buried by river mud and volcanic ash, and turned to silica in shades of red, yellow and purple.
Wind and Waves Bring It Back
A buried fossil has to come back up before anyone can find it. Two forces do this.
The first is the movement of Earth's crust. Over millions of years, the plates that make up the surface push together and lift old seabeds into mountains. The Burgess Shale fossils in Canada formed on a seafloor about 508 million years ago and now sit on a ridge in Yoho National Park, high in the Rocky Mountains.
The second is erosion: rain, rivers, frost and waves wearing rock away. On a crumbling sea cliff, each winter storm pulls down more rock and washes out whatever was inside it. If nobody picks up a fossil soon after it lands on the beach, the sea grinds it to pieces too. That is why the best fossil beaches have the fastest-falling cliffs.
A Girl With a Hammer at Lyme Regis
The last step in a fossil's life is being found, and some of the greatest finders were not trained scientists.
Mary Anning was born in 1799 in Lyme Regis, on the Dorset coast of England. Her family was poor and sold "curiosities" from the cliffs to tourists. In 1811 her brother Joseph found the skull of a large creature, and over the next year Mary, aged about 12, dug out much of the rest. It was an ichthyosaur, one of the first studied by scientists. In 1823 she found the first complete plesiosaur, a long-necked sea reptile, and in 1828 Britain's first pterosaur, a flying reptile now called Dimorphodon. As a woman she could not join the Geological Society of London, and others often took the credit.
A more recent find came on 12 August 1990 near Faith, South Dakota. While her team's truck was being fixed, Sue Hendrickson walked to a cliff and spotted bones sticking out of the rock. They belonged to the largest and most complete Tyrannosaurus rex found up to that time, about 67 million years old, named Sue after her. In 1997 the Field Museum in Chicago bought it at auction for about $8.4 million.
Bones, Footprints and Bugs in Amber
Not every fossil is a stone bone. Scientists sort them into a few main kinds.
- Body fossils are parts of the living thing: bones, teeth, shells and wood. Shark teeth are among the most common, because sharks shed thousands of teeth in a lifetime.
- Molds and casts form when a shell dissolves and leaves a hollow in the rock (the mold), which later fills with minerals to make a natural copy (the cast). Ammonites, coiled shellfish related to squid, are often found this way.
- Trace fossils show what an animal did: dinosaur footprints at Dinosaur Valley State Park in Texas, burrows, bite marks and fossil poo, called coprolites.
- Amber fossils are creatures trapped in sticky tree resin that hardened. Baltic amber, roughly 40 to 50 million years old, holds ants, spiders and flies. In 2016 scientists described a feathered dinosaur tail in amber from Myanmar about 99 million years old.
- Tar fossils come from the La Brea Tar Pits in Los Angeles, where sticky asphalt seeps from the ground. Over roughly the last 50,000 years, animals got stuck and sank, and scientists have recovered millions of fossils, including thousands of dire wolves and sabre-toothed cats.
- Frozen fossils are animals kept whole in ice. In 2007 a reindeer herder in Siberia found Lyuba, a baby woolly mammoth about 42,000 years old, with her skin and trunk intact.
The Burgess Shale is famous because it saved soft bodies too, such as Opabinia, which had five eyes and a long nozzle with a claw on the end. The oldest fossils of all are stromatolites, mounds built by microbes; some in Western Australia are about 3.5 billion years old.
Where Kids Can Go Fossil Hunting
Charmouth and Lyme Regis, England. These beaches sit on the Jurassic Coast, a 95-mile (about 153 km) stretch of Devon and Dorset that became a UNESCO World Heritage Site in 2001. Ammonites, belemnites (bullet-shaped fossils of squid-like animals) and bits of ichthyosaur turn up among the pebbles, especially after winter storms, and the Charmouth Heritage Coast Centre runs guided walks.
Calvert Cliffs State Park, Maryland, USA. Cliffs along Chesapeake Bay hold fossils from a sea that covered the area roughly 10 to 20 million years ago. Visitors sift sand at the water's edge for shark teeth, occasionally including one from megalodon.
Venice, Florida, USA. The self-styled "Shark Tooth Capital of the World", where small black fossil teeth wash up on Caspersen Beach and people sieve for them with mesh scoops.
Penn Dixie Fossil Park, New York, USA. Near Buffalo, families split shale and keep trilobites and brachiopods from a sea about 380 million years old.
Joggins Fossil Cliffs, Nova Scotia, Canada. The Bay of Fundy's huge tides scour cliffs holding upright fossil tree trunks from swampy forests around 310 million years old.
Go with an adult and check tide times, because the sea can cut off a beach. Stay away from the foot of cliffs, which can fall without warning, and collect only loose fossils. In many protected places, including US national parks and the Burgess Shale, collecting is not allowed at all; the Burgess Shale can be visited only on guided hikes.
Real Fossil vs Museum Cast
When you stand under a dinosaur skeleton in a museum, you may not be looking at real bones. Many displays are casts: copies made by pressing rubber or silicone over the real fossil to make a mold, then filling it with plaster, resin or fibreglass.
Museums have good reasons. Real fossil bones are heavy and fragile. Sue's real skull weighs around 600 pounds (about 270 kg), too heavy for the mounted skeleton, so the Field Museum fits a lighter cast skull and shows the real one in a glass case nearby. Casts can also be shared: in the early 1900s Andrew Carnegie paid for copies of the dinosaur Diplodocus carnegii to be sent to museums around the world, and one, nicknamed Dippy, greeted visitors to London's Natural History Museum for over a century.
Clues for telling them apart:
- Weight and feel. A real fossil is stone, cold and heavy. A cast is lighter and can feel plastic-like.
- Colour. Real fossils take their colour from minerals, so bones may be patchy brown, grey or black. Casts are often painted one even colour.
- The label. Museums usually say whether a specimen is original, a cast or a mix; many skeletons fill missing bones with copies.
- Repeats. If the same skeleton stands in museums in different countries, at least some of them are casts.
Casts are honest copies that let millions of people see rare fossils while the originals stay safe for study. A real fossil, though, is the actual remains of something that once lived, turned to stone grain by grain. To see where the world's richest fossil beds lie, read our guide to fossil geography around the world.