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Why we say "dinosaur fossils", not "dinosaur bones"

Most of what you see in a museum is no longer bone at all, but rock in the shape of bone. Here is how bone turns to stone.

What is a fossil?

The word fossil comes from the Latin fossilis, meaning "dug up". It refers to the remains or traces of ancient life preserved in rock.[3]

Fossils are not only bones. They include teeth, eggs, skin impressions, feather impressions, footprints and even fossilized dung (coprolites).[3]

How bone turns to stone

Living bone is full of tiny pores. When a dinosaur carcass was quickly buried in sediment, groundwater carrying dissolved minerals such as silica, calcite or iron minerals seeped into those pores and crystallized. This process is called permineralization.[1][2]

Over millions of years, minerals may also gradually replace the original bone material (replacement), until what remains has the same shape, size and sometimes even the microscopic structure of the bone, but is made of rock. That is why fossils are much heavier than real bone.[2][3]

Magnified bone (simplified): blue is mineral crystallizing in the pores, grey is mineral that has replaced the original bone.

Why fossils aren't bone-colored

A fossil's color comes from the minerals that got into the bone as it turned to stone, not from the bone itself. So most vertebrate fossils are earth tones such as brown, black or white: brown from silica, for example, or black from iron minerals. Unusual colors like green are very rare.[5]

Many skeletons on display in museums are casts made from real fossils, because the real fossils are very heavy, fragile and irreplaceable if damaged, so museums keep the originals safe for scientists to study.[6]

Is any original bone left?

In a few cases, scientists have reported traces of proteins or soft-tissue-like structures in dinosaur fossils, but these finds are still debated and very rare. In general, what is dug up is rock that records the shape of a bone, not fresh bone, so we call it a "fossil" rather than a "bone".[4]

Sources and further reading

  1. [1]Natural History Museum (UK): How are fossils formed?
  2. [2]Wikipedia: Permineralization
  3. [3]Wikipedia: Fossil
  4. [4]Wikipedia: Dinosaur soft tissue
  5. [5]NBC News (2024): Gnatalie is the only green-boned dinosaur found on the planet (Natural History Museum of Los Angeles)
  6. [6]Museums Victoria: Know your bones – what is a 'real' fossil?

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