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Life in the Jurassic ocean
What was the ocean like when dinosaurs roamed the Earth?
Following the rescue of a section of ammonite pavement in Lyme Regis, we take a look at the 199 million-year-old ecosystem these animals would have called home.
All we have left of ammonites are their spiral-shaped shells, found scattered across the UK’s beaches and cliffs.
These extinct sea creatures were cephalopods, making them relatives of today’s octopuses, squids and nautiluses.
About 199 million years ago, during the Earth’s Jurassic Period, ammonites would have been found in abundance in shallow, tropical ocean waters.
In 2017, a piece of fossilised seabed featuring ammonites from that period was added to our collections. It was found on the Jurassic coast, a 95-mile stretch along Britain’s southern coastline. The rocks of this World Heritage Site record 185 million years of our planet’s history.
But what was the water like when these shelled cephalopods were common under the waves?
What was the ocean like in the Jurassic Period?
In the Jurassic Period, 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 how plesiosaurs might have looked. The predators lived in the same oceans as ammonites. © Catmando/Shutterstock
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 marine reptiles would have lived in the ocean alongside ammonites, including ichthyosaurs and long-necked plesiosaurs. Dinosaurs including the armoured Scelidosaurus would have wandered the land on nearby islands.
True ammonites lived during the Jurassic and Cretaceous periods, from about 200 million years ago. Today fossilised ammonites, such as this early Jurassic Asteroceras, can be found along the Dorset coastline.
Our palaeontologist Professor 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.’
How can ammonites help us protect 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.
Researchers are studying fossils from along the Jurassic coast © Martin Kemp/Shutterstock
Fossils, such as those in the slab rescued from the Jurassic Coast after it was damaged by storms, 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 seabed 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.
