Ichthyosaur foetuses from Svalbard shed light on how these animals evolved
Not long after the greatest mass extinction in history, ichthyosaurs were ready for life in the open sea.
At the Grippia bone bed in Svalbard, researchers have unearthed ichthyosaur remains, uncovering hundreds of vertebrae belonging to the Grippia genus.(Photo: Lene Liebe Delsett)
The smallest vertebra is only four millimetres in height.
At the Natural History Museum in Oslo, there are plenty of vertebrae from early ichthyosaurs.
These bones come in all sizes, from adult and juvenile specimens – even foetuses.
"Having the entire developmental sequence is truly unique. We've been able to see how the skeleton changes from embryo to adult," says palaeontologist Lene Liebe Delsett.
The bones were excavated in Svalbard and include some of the oldest known fossils of ichthyosaur foetuses.
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"We're fortunate to have this extraordinary material from Svalbard, which is both rich and ancient," says Delsett, who is a researcher at the Natural History Museum.
Delsett and colleagues have now studied the spines from two types of early ichthyosaurs. They date back to an exciting chapter in Earth's history – three to five million years after the largest mass extinction of all time.
Lene Liebe Delsett is a palaeontologist at the Natural History Museum.(Photo: University of Oslo)
"This is very early in ichthyosaur evolution, and we see that they already have all the adaptations needed to be fast-swimming predators in the open sea," she says.
Their findings are presented in a new study published in Scientific Reports.
Close to the extinction event
Ichthyosaurs were reptiles that evolved from land-dwelling animals, much like whales. Little is known about their transition from land to water.
They first appeared after the Permian-Triassic extinction 252 million years ago.
Over time, ichthyosaurs developed dolphin-like bodies and spread to seas all over the world. They were successful predators for millions of years, with the largest species exceeding 20 metres in length.
Hans Arne Nakrem, a professor at the Natural History Museum who was not involved in the new study, has also worked on excavations on Svalbard, but focuses on other fossil groups.
“These fossils reveal a biological story that we can now date with much greater accuracy than before," says Nakrem.
He explains that establishing a precise age is crucial, and researchers have addressed past critiques, steadily improving their ability to date these fossils.
"This means that the bones are around 248 million years old. For a long time, it was thought that ecosystems took five to ten million years to recover after the massive extinction," he says.
Now, the discoveries place ichthyosaurs much closer in time to the Permian-Triassic extinction itself. That's an important point, according to Nakrem.
Many fossilised vertebrae unearthed in Svalbard.(Photo: Lene Liebe Delsett)
Focus on swimming
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In the new study, Delsett and colleagues examined a series of vertebrae from two types of ichthyosaurs: Grippia and Cymbospondylus. Both lived around the same time, though their bodies were somewhat different.
Grippia ichthyosaurs were quite small, about 1.5 metres long, while Cymbospondylus could reach several metres in length.
The researchers analysed 10 to 15 vertebrae from each species, from foetuses to adults. They also cut thin slices from the vertebrae to examine them under a microscope.
"That's not something you do lightly, but since we have such a large collection, we can afford to," says Lene Liebe Delsett.
The aim was to understand how these ichthyosaurs swam, since swimming style reveals a lot about how they used their habitats, Delsett explains.
The inside of the vertebrae offered a new perspective.
"It was striking how big the difference was," says Delsett.
Illustration of a group of Cymbospondylus.(Photo: Shutterstock / NTB)
Just like a whale
Cymbospondylus had vertebrae that, on the inside, looked exactly like those of whales, says Delsett. They contained a spongy structure throughout.
"This is the textbook anatomy of a large, fast-swimming predator in the open sea," she says.
By contrast, the slightly older Grippia had vertebrae that looked distinctly different: a dense, compact layer surrounding the whole structure.
These were harder to interpret, says Delsett. The vertebrae did not resemble those of whales, but neither did they look like those of land animals.
"Our interpretation is that they swam differently – perhaps staying closer to the coast and not diving as deep," she says.
These ichthyosaurs swam by moving a larger portion of the spine, more like a lizard.
Cymbospondylus, however, had a stiffer spine and propelled itself mainly with its tail, much like whales do today.
One of the fossilised vertebrae analysed in the study.(Photo: Lene Liebe Delsett)
Grew quickly
The progression from foetus to adult animals also reveals growth patterns and how their skeletons developed.
"By zooming in with the microscope, we can see cell structures, different types of fibres in the bone, and growth marks," says Delsett.
The researchers saw structures that indicate rapid growth.
"This tissue type was abundant in the largest specimens, from foetuses to adults. We also found it in Grippia, though to a lesser extent. This is very interesting and could be an adaptation that allowed Cymbospondylus to grow really large. To grow that big, rapid growth is an advantage," she says.
For the first time, researchers also identified a marker in the bone tissue linked to birth in ichthyosaurs – from when the animals were born.
"We think this could be used in the future to reveal even more about growth," she says.
On rare occasions, palaeontologists find ichthyosaur foetus fossils. This fossil of a mother with foetuses is now displayed in the Natural History Museum's geological exhibition. This specimen comes from the Jurassic period, from Holzmaden, Germany.(Photo: Hans Arne Nakrem)
Life after extinction
The new study shows that ecosystems bounced back surprisingly quickly after the massive extinction 252 million years ago, says Delsett.
Researchers believe the ancestors of ichthyosaurs transitioned from land to sea after that event.
"In that light, our finding is incredibly exciting, because it suggests they took only a few million years to go from land-dwelling animals to complete dominance of the open sea, with all the adaptations required," she says.
Alternatively, the transition to water may have begun before the extinction.
"But that would be a rather radical idea," she says.
Hans Arne Nakrem is a professor and research group leader at the Natural History Museum's Norwegian Centre for Palaeontology.(Photo: University of Oslo)
Body temperature and growth
"We don't yet know which species ichthyosaurs evolved from," says Hans Arne Nakrem.
He adds that researchers will keep searching, perhaps for a form that existed before the mass extinction.
"Ichthyosaurs were already so well-developed at such an early stage that it doesn't quite fit with evolutionary expectations. Something must have happened a couple of million years before the mass extinction too," he says.
Nekrem says that another interesting finding is that the study suggests ichthyosaurs were likely warm-blooded. That means they maintained a stable body temperature, like humans do. Fish and crocodiles, on the other hand, are cold-blooded.
"This is tied to metabolism and activity. Researchers believe that since ichthyosaurs were warm-blooded, they had a more active lifestyle than the reptiles we know today. The evidence comes from microstructures in their bones," he says.
The study also revealed that their growth was episodic – occuring in bursts rather than continuously, Nakrem adds.
"Paints a remarkable picture"
Nakrem explains that the depositional environment where the ichthyosaurs were found – the sediments and other organisms surrounding them – shows that the waters became deeper over time.
"The sediments in the lowest and oldest layers suggest that they were deposited in relatively shallow water. As time went on, further into the Triassic, sea levels rose and the waters around Svalbard became deeper," says Nakrem.
"In that sense, Grippia was more of a shallow-water reptile, while Cymbospondylus was adapted to the open ocean. From the vertebrae, we can see that Grippia swam more like a lizard, wriggling forward, while Cymbospondylus already resembled the ichthyosaurs that appeared toward the end of the Triassic," he says.
The ichthyosaurs of the late Triassic, and especially those of the Jurassic and Cretaceous, had dolphin-like bodies that made them even more efficient swimmers, the professor explains.
"The study paints a remarkable picture of early ichthyosaur evolution. It shows how they lived in a changing environment with deepening seas, while also highlighting internal biological structures tied to warm-bloodedness and growth," says Nakrem.
The study also demonstrates ontogeny – the development of individuals from foetus to old age.
"All these facets make this groundbreaking," he says.
