How come dinosaur bones can survive so long?

When you learn about fossils, you start seeing dates thrown around like “265 million years old” and “going back to the Jurassic” as if it’s no big deal. Fossils can be incredibly, unimaginably old, but how can they survive for so long while animal bones decay in just a few years?

It all comes down to the circumstances in which an animal died, and what happened to their remains after they kicked the bucket. IFLScience got to see first-hand the fossilized bones of a giant sea monster that was crushing things 150 million years ago. As fossil expert Steve Etches MBE told IFLScience, part of the reason the pliosaur – an ancient marine reptile – was so well preserved was because it rolled to death and kept its smile while lying face down in the mud.

“It’s a good thing it was upside down because this is actually the best side,” Etches said. “When you find a fossil in a ledge or whatever, we always turn it upside down because if it dies, the piece that ends up in the mud remains undisturbed. It is always the top end that starts to degrade faster.”

Soft tissues usually decompose quite quickly, although we find rare examples of preserved pieces of skin (even if some of them turn out to be forgeries). As for what keeps these bones seemingly so intact for millions of years, it all comes down to fossils.

What is fossils?

Fossilization can take many forms, but the best fossils are usually formed when the animal becomes quickly encased in sediment – ​​like our upside-down giant sea monster – which can be in the form of mud or volcanic ash. As this sediment builds up, it becomes petrified, meaning it turns to stone and holds the animal in place.

Permineralization is the most common form of fossilization that occurs when water from the ground, lakes, or ocean transports minerals into organic tissues. Eventually, enough deposits build up to create a kind of internal cast, which can be made of calcite, iron or – as in the case of this stunningly beautiful opalescent plesiosaur – silica.

The minerals replace the organic materials in the bone, such as collagen and other proteins, until the fossil contains more mineral crystals than the original bone. The chalky ammonites found along the Jurassic Coast fossil hunter’s haven are filled with calcite, while the astonishing gold specimens of the Astonishing Fool are made of pyrite.

Microraptor gui fossil

A copy of Microraptor gui with bones and feathers, exhibited in the Paleozoological Museum of China.

Image credits: Captmondo, own work, copyright-free use, via Wikimedia

If you want a fun way to demonstrate how minerals can create an internal cast for eager young scientists, the National Park Service has a great exercise where you can try making internal casts of sponges using salt water.

Are all fossils bones?

No. A fossil can be any trace or remnant of a past life, whether it is a footprint, a burrow, a piece of skin or a bone. As the Australian Museum explains, the word fossil is derived from the Latin word fossilswhich simply means ‘excavated’, it is not specific to bone.

Decayed animal bones?

A little thing called the decomposition ecosystem makes quick work of animal remains exposed to the elements, something Dr. Devin Finaughty told IFLScience all about during his talk at CURIOUS Live in 2023. “Decomposition is technically defined as the consumption of organic matter by other organisms, [and is] to be distinguished from the physical breakdown of organic remains by physical, erosive forces, such as water. The decomposition ecosystem revolves around the dead body as a resource and that is mainly for food, but many organisms will also use it as a breeding ground, as a nursery and as a shelter.”

Once the decomposition ecosystem fills up, all that’s left is usually bone, and we can learn a surprising amount from skeletal remains that linger longer than your typical corpse. However, they won’t last forever, as even animal bone eventually disintegrates. It may take several years, but the delicious collagen found in the bones is a food source for bacteria and fungi that will eat away at it until the remains eventually crumble.

So if you want to survive as a fossil for the next hundred million years, it’s time to write down very elaborate funeral wishes.

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