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Fossils are handy tools to help determine the age of layers of earth, as, within the same layer of earth, they always appear in a fixed, particular order. Layers containing mammoth bones, for instance, are younger than layers containing remnants of ammonites, which lived hundreds of millions of years ago. Oftentimes, we only dispose of fossils of the traces even older species of animals and plants left behind, e.g. burrow fossils. As these traces tend to be difficult to date, researchers will also investigate the rocks in which they were found.
  • Scientific name: Perisphinctes sp.
  • Found in: formation of Croï, Pointe de la Rochette, Wimereux, France
  • Collection: Department of Geology
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Ammonites are extinct, squid-like molluscs that lived in a spiral shell.

What is deep time?

Deep time means the geological period between the formation of the Earth and present day. To make this long period easier to comprehend, scientists sometimes compare it to a 24-hour day. If the Earth was formed just after midnight, at 00:00:01, the first life emerged at 3 a.m. The dinosaurs did not evolve until quarter to eleven at night, and the first apes appeared at quarter to midnight.

How do scientists measure deep time?

Geologists measure deep time using things that have formed very slowly, such as sediment layers on the ocean floor or minerals that bear traces of slow physical processes, such as the radioactive decay of unstable atoms. They also use fossils, each of which are typical of a certain epoch in geological time.

What is an ammonite?

Ammonites are extinct, squid-like molluscs that lived in a spiral shell. They were highly evolved, good swimmers and efficient predators.

When did the ammonite live?

The true ammonites only lived in the Mesozoic Era, more specifically in the Jurassic and Cretaceous periods, 66 to 201 million years ago.

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