A Fossil Is Not a Bone Anymore

Nobody has ever seen a living dinosaur. Every single thing anyone knows about them comes from fossils: bones and teeth and footprints that turned to stone and stayed in the ground.

Four words in that sentence are carrying the whole subject, and they are never explained. Turned to stone. Turned how? Bone does not go hard by sitting still. Leave a bone in a field and in a few years there is no bone.

The answer is stranger than it sounds, and it starts with something about your own skeleton.

A bone is not solid

Pick up a chicken bone and look at the broken end. It is not a solid rod. It is full of holes — a honeycomb of little spaces, with channels running through them, and a hollow down the middle.

Bone is built like that on purpose, because a hollow tube is nearly as strong as a solid one and weighs far less. Every bone you have is mostly empty space.

That empty space is the reason a fossil can happen at all. It gives something somewhere to go.

Water gets in

The bone has to be buried quickly, before anything eats it or it rots. Once it is down there, water starts moving through the ground around it, and that water is never pure. It has been through rock, and it carries dissolved minerals — tiny amounts of stone, small enough to travel invisibly in water.

That water seeps into all those holes.

Then it does what a puddle on a windowsill does when it dries: it leaves its cargo behind. Speck by speck, mineral settles out of the water and stays in the gaps. The holes begin filling with stone.

At this stage you still have a bone. It is heavier and harder than it was, and it now has rock packed into every space, but the bone itself is still bone.

Then the swap

Here is the part that catches everybody.

Bone does not last, even underground. Very slowly, the original bone material dissolves away into the passing water and leaves. And as each speck of bone goes, a speck of mineral settles into the space it left.

Out goes a speck. In goes a speck. Again, and again, for thousands of years at the very least and usually far longer.

At no point is there a bone-shaped hole. The swap happens speck by speck, so the shape is held the whole way through — held by everything that has not been swapped yet.

Most people put the dissolving first, because that is how a mould works — the thing rots away, leaves a hole, and the hole fills up. That is a real process, but it is not this one, and it would lose almost everything. A hole cannot remember the fine detail of what made it.

The filling has to come first. The stone that arrives early is what holds the shape steady while the bone leaves.

What you are looking at in a museum

So when you stand in front of a dinosaur skeleton, you are not looking at old bones. You are looking at rock.

Not rock shaped like a bone by somebody. Rock that arrived speck by speck into the exact positions the bone specks vacated, copying the shape as it went — right down to details finer than you can see. Some fossils preserve the tiny channels that carried blood through the living bone, because a mineral speck went into each of those too.

The animal is completely gone. What is left is an accidental cast of it, made by water, in slow motion, over a length of time nothing alive could sit through.

Footprints work the opposite way round

A footprint is not a copy of a foot. There was never anything of the animal in it to copy.

The dinosaur pressed a hollow into soft mud. The mud dried and hardened. Later, sand or silt washed in and filled the hollow, and that filling turned to rock as well. What gets dug up is a stone shape that matches the foot, made entirely of material the animal never touched.

Which is why a footprint tells you things a bone cannot. Bones tell you how the animal was built. Footprints tell you what it was doing: whether it was walking or running, whether it was alone or in a group, and which way it was going.

That weight is the giveaway, and it is worth remembering the next time somebody says fossils are old bones. Old bones are light and dry and crumble. A fossil is a stone that once had an animal's shape poured into it, molecule by molecule, by nothing but patient water.