When Does Something Become a Fossil?

Belemnite fossil

By Adam Kelliher

The short answer: there's no single official cut-off, but many museums use about 10,000 years as a working rule. Remains or traces of life older than that are generally called fossils.[1][2][3] Some use 12,000 years, tied to the start of the Holocene, and the US National Park Service doesn't use an age at all.[4][5]

It's a fair question, and the honest answer is "it depends who you ask". Here's how the main definitions line up.

The 10,000-year rule of thumb

The British Geological Survey describes fossils as preserved remains or traces of life that are typically more than 10,000 years old.[2] The American Museum of Natural History puts it the same way: any evidence of prehistoric life at least 10,000 years old.[3] That number roughly matches the end of the last ice age.

Giant oyster shell fossil from France
A giant fossil oyster shell from France. (photo: Rubble Rock & Gem)

Where it gets grey

The Field Museum gives a good example. Mammoth bones about 4,000 years old from Wrangel Island in Russia are treated as fossils by many palaeontologists, while 4,000-year-old mummified cats from Egypt are not.[1] Museums Victoria uses about 12,000 years instead, marking the start of the Holocene, the epoch we live in now.[5]

The US National Park Service skips age altogether. Its definition is "evidence of life preserved in a geologic context". It even counts a lava tree mold in Hawaii that formed about 1,000 years ago, when lava flowed around a tree.[4]

Turning to stone isn't required

A lot of people think a fossil has to be petrified. It doesn't. The Field Museum notes a fossil can be original shell or bone, mineral that has replaced it, or just an impression.[1] Footprints, burrows and dung count too. Those are trace fossils.[1][3] We cover the types in body fossils, trace fossils and casts, and the process in the journey from life to fossil.

Turritella fossil from Agadir, Morocco
Turritella, a fossil sea snail from Morocco. (photo: Rubble Rock & Gem)

Why it matters to a collector

Most fossils you'll see for sale are millions of years old, so the cut-off rarely comes up. For how ages are actually worked out, read how fossils are dated, and browse all our fossils.

See what's new: browse our newest arrivals for the latest pieces added to the shop.

Sources

  1. The Field Museum: What Is a Fossil? (Silurian Reef).
  2. British Geological Survey: Fossils.
  3. American Museum of Natural History: Dinosaur Bones: Learning from Fossils.
  4. US National Park Service: What Is a Fossil?
  5. Museums Victoria: Know Your Bones: What Is a "Real" Fossil?

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