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Largest marine reptiles may have been wiped out by hidden Triassic extinction
Over 230 million years ago, long-necked plesiosaurs and dolphin-shaped ichthyosaurs dominated the oceans, from open water to the shallow coasts.
But within 30 million years, many of these marine reptiles went extinct. New research is shedding light on what might have happened.
A series of extinction events over 200 million years ago may have sealed the fate of many marine reptiles.
During the Triassic, between 252-200 million years ago, marine reptiles rapidly diversified into many different forms as they spread around the world. Some of the best known are the plesiosaurs and ichthyosaurs, which quickly became the top predators in the oceans.
Their rapid expansion was brought to a halt at the end of the Triassic 200 million years ago by two extinction events in quick succession. Despite the effects of the second extinction event already being known, new research shows that plesiosaurs and ichthyosaurs were more vulnerable because of a previous event several million years earlier.
While both groups survived as a whole, many species went extinct. Among the casualties were the shastasaurid ichthyosaurs, a group of whale-sized marine reptiles that could reach up to 21 metres long. Ichthyosaurs would never reach such massive sizes again for reasons that, according to PhD student Antoine Laboury, are “still a mystery”.
“These animals were at the top level of their food chains, so we expect that they would be highly susceptible to major changes in ecosystems and to extinctions affecting other species,” says Antoine, who led the new research. “However, we don’t have direct evidence to support this.”
“We’re currently lacking the data to show exactly what was happening to the shastasaurids at the end of the Triassic, and whether their extinction was abrupt or staggered. Finding new fossils would help us to shed light on this fascinating period of the history of ichthyosaurs.”
The findings of the study were published in the journal Evolution.
The marine reptiles gallery was scanned in the morning before it opened to the public. © The Trustees of the Natural History Museum, London
Scanning our specimens
To answer questions about the evolution of marine reptiles, the researchers needed access to a variety of different specimens from the end of the Triassic and start of the Jurassic. Some of the most important are mounted on the wall of our Fossil Marine Reptiles gallery, which includes fossils collected by Mary Anning.
As these specimens are displayed high up on the wall, and passed by thousands of visitors every day, it’s simply not possible to take them down. Instead, the scientists had to scan the specimens where they are.
Dr Marc Jones, the curator who looks after our fossil marine reptiles, says, “One of the biggest challenges was to scan the specimens behind glass, because the barrier can cause the scanner problems.”
“The height is also a problem. But, with the help of a genie scissor lift and a portable scanner, we were able to get Antoine up to the specimens so he could start to digitise them.”
In total, 21 specimens from the Fossil Marine Reptiles gallery have now been scanned, with the data made publicly available online. Marc hopes that this will make them more accessible to researchers and members of the public all around the world.
“There are all kinds of possible uses for these scans, including further research but also science outreach and art,” Marc says. “The Museum of Lyme Regis, for example, is already 3D printing some of the skulls their education programmes, and this is just the start.”
“In the future, we hope to get all of the fossil marine reptiles in the Natural History Museum scanned so they can be used by everyone.”
A portable scanner allowed the specimens to be scanned while still on the wall. © Marc Jones
How did marine reptiles change in the Triassic?
The digitised specimens have been used by Antoine and the rest of the study’s authors in combination with hundreds of scans of plesiosaurs, ichthyosaurs and their relatives from other museums. This allowed the team to track the changes these groups experienced in the Triassic and into the early Jurassic.
After peaking in diversity during the Middle Triassic, and filling a wide variety of environmental niches, the researchers saw rapid changes in the fins of plesiosaur ancestors known as pistosaurs and the parvipelvian ichthyosaurs.
“The fins of these marine reptiles became more compact, which is characteristic of organisms highly adapted to open-ocean life,” Antoine explains. “This would have enhanced the ability of plesiosaurs to ‘fly’ underwater, and cruise over long distances.”
“In fact, this adaptation likely enabled early plesiosaurs and the pelagic parvipelvians to survive sea-level fluctuations in the Triassic, playing a significant role in their future evolutionary success.”
The scans will allow researchers all over the world to use the specimens in their research. © The Trustees of the Natural History Museum, London
There were also changes in the heads of these animals, especially in ichthyosaurs. While by the Middle Triassic plesiosaur skulls had already evolved many different shapes, ichthyosaurs continued to experiment with head shape in a more complex way, often revisiting old forms.
“It’s commonly stated that ichthyosaurs in the Jurassic, and beyond, were very distinct from, and not as diverse as, their Triassic predecessors,” Antoine says. “However, that hypothesis has never been comprehensively tested.”
“Our research showed that post-Triassic ichthyosaurs were still diverse, and that certain head and body plans recurred in distantly related species before and after the end-Triassic extinction. This explains why different ichthyosaurs can be found together at the same time, as their different morphologies allowed them to exploit different resources.”
While the plesiosaurs and ichthyosaurs were able to survive the end-Triassic extinction, they didn’t last forever. The last ichthyosaurs died out around 90 million years ago during the Cretaceous after they were unable to adapt to changes in the ocean and the extinction of some of their food sources.
Plesiosaurs, meanwhile, lasted until the end of the Cretaceous. They were eventually wiped out during the same extinction event that killed the non-bird dinosaurs.
