Here’s Stan, one of the largest and most complete Tyrannosaurus rex fossil specimens.

Do dinosaur bones turn to stone?

What is a fossilised bone actually made of? Palaeontologist Lene Liebe Delsett explains.

Researchers have unearthed loads of bones originating from dinosaurs, fish lizards, mammoths, and other extinct animals.

Dinosaur skeletons have been lying in the ground for many millions of years.

What happens to their bones during this time? Is there still anything left of the original bone? Or has the skeleton turned into stone? Or has it been replaced by stone?

Lene Liebe Delsett is a palaeontologist. She studies animals that have lived on Earth a very long time ago.

Delsett has helped excavate 150-million-year-old skeletons of ichthyosaurs and plesiosaurs on Svalbard.

Lene Liebe Delsett is a palaeontologist and researcher at the University of Oslo.

Open spaces are filled

She says that the amount of original bone left in fossil bones varies.

“We palaeontologists aren’t particularly fond of the word petrification. Partly because stone is an imprecise word and because sometimes the original material is preserved,” says Delsett.

You can find old bones that consist of the same material as when the animal was alive.

But often, cavities inside the bone are filled with other substances.

“Skeletons have different cavities inside them that were filled with blood vessels, fat, nerves, or other things found in a living animal,” she says.

Blood vessels and nerves decay when the bones lie in the ground.

“When a skeleton lies in the ground, water may flow through and deposit minerals or other substances in the original cavities,” she says.

It is quite common for fossil bones to consist of ordinary skeletal material, and for other materials to fill the cavities, Delsett explains. The bone then becomes denser and heavier than it originally was.

It is not true that all dinosaur bones are made of stone. Most of the original bone mineral, calcium phosphate, is preserved, according to an article in The Conversation.

Cavities inside fossil bones are often filled with minerals.

Bone replacement or moulds

Another possibility is that the original bone tissue has been completely replaced by other substances over time.

“The bone might have retained its shape, but is now composed of other substances,” says Delsett.

If you perform chemical tests on such a bone, you will see that the bone is not made up of substances found in skeletal remains. It's something else.

This is only possible over an extremely long time, says Delsett. It can happen with dinosaur bones that have been in the ground for some 100 million years.

“That’s enough time for such slow processes where substances are very gradually replaced," she says.

Sometimes bones leave an impression in the ground, which can function somewhat like a mould. The bones rot away, but the imprint is filled in. 

In these instances, nothing remains of the original skeleton. But the imprint can tell us what the animal looked like.

What a fossil bone consists of depends on how old it is, where the fossil was located, and what has happened there.

Steady progress

The younger the bone is, the greater the chance of finding more of the original bone substances – even the remnants of cells and DNA.

Cells and DNA often degrade quickly in the soil.

The record for the oldest DNA ever found is two million years. Norwegian researchers took part in a study about this find in 2022.

Researchers are constantly pushing the limits of what can be found in old bones, notes Delsett. In recent years, there have been significant advancements with both X-ray and microscope technology.

“Better imaging technology and other methods have made it possible for people to find structures and organic tissue in ancient fossils that we didn't think were there,” she says. 

Organic tissue is what living things are made of.

Details in skin and bones

Dinosaur colouration is quite a new area of research in the field.

“In the last 10-15 years, we’ve started to achieve imaging technology that enables us to see certain cells related to pigment,” says Delsett.

Pigments are substances that give animals different colours. They are also what gives humans different skin colours. 

Researchers have looked at microscopic shapes in dinosaur feathers that can indicate their colour.

“The same thing applies to skin. I work with ichthyosaurs, and we have several specimens with skin. With the new microscopes, you can see details in the skin,” Delsett says.

Sometimes researchers claim to have found remnants of cells in dinosaur fossils.  Is there anything left of the original cells, or is the shape preserved and replaced with other substances?

It is not always easy to know, and it’s a topic that generates discussion, according to Delsett.

Open spaces after cells die

Delsett has studied the smallest details in ancient bones. She uses a microscope to study thin slices of bone.

“Inside the bone tissue, we can see some small structures that are black. It looks like a little black thing with arms out to all sides. A bone cell used to be there,” she says.

“The arms on all sides were how the cells communicated with each other. We can see exactly what the bone cell looked like, and we can see the shape. But the cell itself is gone in this case and was replaced with some kind of mineral.”

Lots of fossil types

Most animals that die do not become fossils. They get eaten or decompose, and the substances they were made of return to the cycle of nature.

Occasionally a fossil can form.

Two conditions in particular give the animal a better chance of being preserved as a fossil: the body ends up in a place where oxygen is limited, and it gets covered up quickly, says Delsett.

The animal may, for example, have sunk into clay after it died, or the carcass was quickly covered by sand during a sandstorm.

It is usually the hard parts of the animal that are preserved as fossils, such as bones and teeth, or the shells and exoskeletons of marine animals. Sometimes researchers also find soft tissue, like fur and skin.

Fossils come in many forms and types, not only large animal skeletons. Plants, insects, shells, sea creatures, bacteria, and sometimes fungi can be preserved as fossils.

Fossils are usually divided into two groups: body fossils and trace fossils.

Body fossils are remnants of the actual animal or plant itself. Trace fossils are what the animal has left behind, such as footprints, nests, faeces, or vomit.


Translated by Ingrid P. Nuse

Read the Norwegian version of this article on

Powered by Labrador CMS