An artist's impression of a living ammonite

An artist's impression of a living ammonite © Esteban De Armas/

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Life in the Jurassic ocean

As ancient ammonites arrive at the Museum from Dorset, we take a look at the 199 million-year-old ecosystem they would have called home.

All we have left of ammonites are their spiral-shaped, grey shells, found scattered across southern England's beaches and cliffs.

These extinct sea creatures are relatives of today's squid and nautilus.

But about 199 million years ago, during the Earth's Jurassic Period, they would have been found in abundance in shallow, tropical ocean waters.

A piece of fossilised seabed featuring ammonites from that period has just been added to the Museum's collections. It was found on the Jurassic coast, a 95-mile stretch of coastline with rocks recording 185 million years of our planet's history.

But what was the water like when these cephalopods were common under the waves?

Tropical predators

The sea around the land mass which is now Britain was the home of marine reptiles, large snails, ancient crustaceans and sea urchins.

An illustration of plesiosaurs swimming

An illustration of how plesiosaurs might have looked. The predators lived in the same oceans as ammonites. © Catmando/


Today the sea is cold and dark, but in the time of ammonites, it would have been significantly warmer.

If you travelled back in time, the coastline itself would be unrecognisable to modern eyes. In the Jurassic Period, most of what later became Britain was under the sea, apart from Scotland, East Anglia and a series of small islands in the southwest.

Ammonites ranged in size from just a few centimetres to as large as a bicycle wheel. They were predators, stalking and feeding on crustaceans and other sea creatures, using long arms that would extend from their hard shells.

Large reptiles would have lived alongside ammonites, including ichthyosaurs and long-necked plesiosaurs. Dinosaurs including the armoured Scelidosaurus would have wandered the land on nearby islands.

A fossil ammonite

Fossilised ammonites can be found all over the Dorset coastline © Vladimir Wrangel/


Museum palaeontologist Prof Richard Twitchett says, 'At this time, the marine ecosystem would have been relatively diverse, with large snails, brachiopods, burrowing creatures, fish and reptiles.

'It would have been a shallow-shelf sea floor. It was probably a calm environment, away from the shore and below any wave activity.'

Protecting future ecosystems

Richard is studying the fossils on the Jurassic coast to understand how future changes to the climate could impact marine ecosystems.

The ammonites were living about two million years after a large mass extinction event called the Late Triassic extinction. It was caused by volcanic activity as the ancient landmass Pangaea started to break up.

The volcanoes released large amounts of carbon dioxide into the air. It is estimated that more than 50% of marine genera were wiped out, including some large marine reptiles and reef-building creatures.

Aerial view of Jurassic-age UK coastline, showing fields on top of cliffs that descend to sandy beaches

Researchers are studying fossils from along the Jurassic coast © Martin Kemp/


These fossils help researchers understand how ecosystems recovered following sudden environmental change.

Richard says, 'This slab is almost a sea floor frozen in time, and it was a really critical time in the Earth's history.

'It preserves evidence of burrowing animals like worms and shrimps alongside the ammonites.

'Burrowers can be found in modern ecosystems too. They are important because they flush water into the sea bed and help recycle nutrients, supporting other creatures.

'Burrowing animals are affected particularly badly by environmental changes like increased carbon dioxide and warmer water.

'As the seas get warmer, the animals get smaller. Modern burrowing animals could also be affected by climate change, which would have an impact on today's oceans.'

Richard can track data from historic extinction events and periods of warming in our planet's history. By looking for patterns and similar responses in each affected ecosystem, he can begin to understand how climate change might impact marine ecosystems in the future.