How Fossils Form

Becoming a fossil takes a lot of luck. For a creature to survive as a fossil, several specific things usually have to happen in sequence: quick burial, undisturbed time underground, the right chemistry, slow turning to stone, and finally being brought back to the surface where someone can find it. Most creatures that have ever lived left no trace at all: a sad fact for the creatures, but the reason any fossils survive is exactly because the few that do are caught in unusual conditions.

  • Four main types of fossilisationPermineralisation, replacement, mould, preservation
  • Best buried inMud, sand or ashQuickly, before scavengers arrive
  • Time to fossiliseTens of thousands of yearsSometimes much longer
  • % of species fossilisedFar less than 1%99%+ of life leaves no trace
  • Best preservationAmber, ice, tar pits, peatWhere decay is impossible
  • Most common rocks for fossilsSandstone, shale, limestoneSedimentary rocks

The five steps to becoming a fossil

  1. Die: this is the easy part, although where and how you die matters a lot.
  2. Avoid being eaten or rotting: scavengers, bacteria and weather destroy most dead bodies within weeks. The body needs to be quickly buried, ideally under water in soft mud, sand or ash.
  3. Get buried deeper and deeper as more layers of sediment pile up over thousands of years.
  4. Be slowly turned to stone by minerals seeping in. The original tissue is replaced with rock.
  5. Come back to the surface. Hundreds of millions of years later, the rock containing your fossil needs to be pushed up by tectonic forces and eroded away by weather, exposing the fossil at the surface where someone can find it.

The four main types of fossilisation

  • Permineralisation: minerals dissolved in groundwater seep into the pores of bones, wood or shells, filling them in. The original structure stays, but gets harder and heavier. Most dinosaur bones are permineralised.
  • Replacement: the original material slowly dissolves away, and different minerals take its place molecule by molecule. The result is a stone "copy" of the original, sometimes in surprising materials. Petrified wood is often replaced with brightly coloured silica.
  • Casts and moulds: the original material rots away completely, leaving a hollow shape in the rock (the mould). Later, the hollow may fill with minerals, creating a cast.
  • Original preservation: occasionally, soft parts of the body actually survive. This needs very special conditions: ice (woolly mammoths in permafrost), amber (insects trapped in tree resin), tar (mammals in the La Brea Tar Pits), peat bogs (preserved bodies thousands of years old), or extremely dry deserts.

Why most life leaves no fossils

A typical animal dies and is reduced to nothing within weeks. Bacteria, fungi, scavengers, sunlight and weather all attack the body. Bones, teeth and shells can last longer than soft tissue, but even they break down within a few hundred years in normal conditions. The only way to escape this fate is to be removed from oxygen quickly: buried deep underwater, frozen, or sealed in tar or amber.

This is why some types of creature are over-represented in the fossil record (sea creatures with hard shells, swamp-dwellers with bones) while others are under-represented (jellyfish, worms, insects, soft-bodied things). Whole groups of organisms have probably existed for hundreds of millions of years and left almost no fossils.

Fact One of the best preservation environments in the world is amber: hardened tree resin that has trapped insects, spiders, frogs and even feathered dinosaurs in almost perfect condition for tens of millions of years. The oldest known amber is 320 million years old. Inside, the structures of trapped creatures can be preserved in such detail that scientists have studied individual cells, pigment patterns and even DNA fragments.

Special preservation: amber, tar and ice

  • Amber (fossilised tree resin): preserves insects, small animals and plant parts in 3D, sometimes with original colours and pigments intact. The world's richest amber deposits are in the Baltic, the Dominican Republic and Myanmar.
  • Tar pits: like the La Brea Tar Pits in Los Angeles, where sticky natural asphalt has trapped thousands of Ice Age animals over 40,000 years.
  • Permafrost: in Arctic Siberia and Alaska, woolly mammoths have been found frozen with skin, fur and stomach contents intact, despite being 30,000+ years old.
  • Peat bogs: in Denmark, Ireland and England, the chemistry of cool acidic peat has preserved human bodies (the "bog people") with skin, hair and clothes intact for over 2,000 years.
Did you know? Some of the best fossils come from natural disasters. The volcanic ash that buried Pompeii in 79 AD preserved Roman streets, houses and even the shapes of people in stunning detail. Sudden flash floods, ash falls and ice age conditions have all produced exceptional fossil sites. A disaster for the creatures involved, but a goldmine for the palaeontologists.
Deeper dive: lagerstatten, where many fossils come together

Most fossil sites give us scattered hard parts: a bone here, a tooth there. But occasionally, a particular combination of conditions preserves an entire ecosystem in beautiful detail, including soft-bodied creatures that almost never fossilise. Sites like this are called lagerstatten (German for "store") and they are some of the most important fossil sites in the world.

Famous lagerstatten include:

  • Burgess Shale (Canada, 508 million years old): an entire Cambrian ocean ecosystem with soft-bodied creatures preserved in fine detail. Many had no hard parts at all.
  • Solnhofen Limestone (Germany, 150 million years old): fine-grained lagoon mud preserving Archaeopteryx (the first known bird), pterosaurs, fish and even rare jellyfish.
  • Messel Pit (Germany, 47 million years old): a former lake that preserved early mammals, birds, fish and insects in fine detail.
  • Yixian Formation (China, 125 million years old): preserved feathered dinosaurs in colour, showing for the first time what they really looked like.
  • La Brea Tar Pits (USA, 40,000 years old): trapped Ice Age mammals including sabre-toothed cats, dire wolves and giant ground sloths.

Each new lagerstatten typically rewrites part of palaeontology, showing creatures and behaviours we never knew existed. The next major discovery could be just one quarry-cut away.

For more, see what are fossils and famous fossil sites.