An artist's impression of the Tethys Sea with ichthyosaurs, pterosaurs and turtles

The fossils were found in rocks once at the bottom of a shallow lagoon, but now high in the Alps. Image © Jeannette Rüegg/Heinz Furrer, University of Zurich

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Giant ichthyosaur fossils reveal the last days of Triassic marine reptiles

Fossils discovered high in the Swiss Alps are revealing clues about the last days of some of Earth's largest ever animals.

The remains of giant ichthyosaurs, which measured longer than a bowling lane, show that the animals remained large until they were suddenly wiped out over 200 million years ago. 

A tooth from a giant ichthyosaur the size of a whale has deepened our understanding of these Mesozoic marine monsters.

Weighing as much as 80 tonnes and measuring more than 20 metres long, the marine reptiles were among some of the largest animals to have ever lived. They would have been the top predators swimming in the ocean 205 million years ago.

Despite their size, these animals are often absent from the fossil record, with most remains often being fragments of much larger bones. Teeth are even more uncommon, with this new discovery only the second ever found from a giant ichthyosaur.  

Prof Martin Sander is the lead author of a new paper describing this previously undocumented tooth plus a selection of other ichthyosaur remains. He says, 'From our point of view, the tooth is particularly exciting because this is huge by ichthyosaur standards.

'Its root was 60 millimetres in diameter, significantly more than the largest specimen found to date which was 20 millimetes. That tooth came from an ichthyosaur that was nearly 18 meters long, which may help infer the size of our specimen.

'However, it is hard to say if the tooth is from a large ichthyosaur with giant teeth or from a giant ichthyosaur with average-sized teeth.'

The study was published in the Journal of Vertebrate Palaeontology

Prof Martin Sander (left) holds one of the rib fossils, while co-author Dr Heinz Furrer (right) hold a vertebrae

Late Triassic giant ichthyosaur bones are rare in the fossil record. Image adapted from © Laurent Garbay/University of Bonn and Rosi Roth/University of Zurich

What are giant ichthyosaurs, and how did they live?

The origins of the ichthyosaurs start in the aftermath of the worst ever mass extinction at the end of the Permian Period over 250 million years ago.

The marine reptiles first begin to appear in the fossil record around 240 million years ago, and likely descended from land dwelling ancestors which returned to the ocean. They grew large relatively rapidly, with some reaching the size of whales within a few million years.

One group of these ichthyosaurs are known as the Shastasauridae, which contain the largest ichthyosaurs which ever lived. These include Shonisaurus sikanniensis, which is estimated to have been around 21 metres long, as well as unidentified ichthyosaurs found in Somerset and South Gloucestershire, UK, which may have reached over 25 metres.

The difficulty identifying these specimens reflects the scarcity of intact fossils for these giant ichthyosaurs. Any specimens which are found are generally fragments of jaw bones or ribs, leading to a great deal of uncertainty over the exact size and lifestyle of these ocean titans.

Some hypotheses suggest that because most giant ichthyosaur fossils are found without teeth, the animals may have had sucked in prey such as cephalopods rather than grasping them. Those with teeth may have behaved more like modern sperm whales, being able to eat squid but also vertebrates such fish and other ichthyosaurs.

But while ichthyosaurs in general managed to survive until 95 million years ago, the giant ichthyosaurs mysteriously vanished as the Triassic came to an end around 200 million years ago. The fossils in Switzerland therefore represent some of the last giant ichthyosaurs, and may help to reveal more about the final days of these monsters of the deep. 

Martin Sander and Michael Hautmann look over over the Swiss Alps

The fossils were discovered 2,800 metres up in the Alps. Image © Jelle Heijne/University of Bonn

How did ichthyosaur fossils end up in the Alps?

When the ichthyosaurs were alive, the rocks which now form the Swiss Alps were at the bottom of the ocean known as the Tethys Sea, which lay between the two supercontinents of Gondwana and Laurasia. 

The fossils were found in what is thought to have been a large lagoon or shallow bay at the sea's northwestern edge. As this shallow lagoon wouldn't have been able to support these big predators, the scientists believe that the animals may have either beached there accidentally while pursuing prey or been washed in after dying elsewhere.

After their remains were fossilised, the African and European tectonic plates began pushing into one another. Over tens of millions of years, this collision forced the rocks containing the ichthyosaurs upwards to form the Alps.

The first remains of the marine reptiles were uncovered in 1976, with more discovered over the next 14 years. In total, the fossils consist of multiple spine and rib bones from two individuals. One is estimated to have been around 20 metres in length, while the other was around 15 metres. 

The tooth is thought to be from a separate individual but is missing most of the upper section used for biting, known as the crown, which would help identify it.

While there are some features that set the fossils apart from other known ichthyosaurs, the scientists are not confident enough to either assign them to any existing species or name a new one. The processes that formed the Alps have deformed the fossils, which makes it difficult to link them to the remains of relatives found elsewhere.

The size of the specimens, however, suggests that giant ichthyosaurs survived right up until the end of the Triassic, when an estimated 23% of all marine animals went extinct.

While the causes of this extinction are still debated, it could have resulted from climate change, volcanic activity or a meteorite impact. 

But despite the giant ichthyosaurs fizzling out, their smaller relatives lived on until around 90 million years ago when other marine reptiles such as plesiosaurs are thought to have replaced them as the largest ocean predators for the remainder of the Mesozoic Era.