Researchers have found many foetuses inside ichthyosaur bodies. They have now studied them
"This provides new opportunities to understand reproduction in animals that have long been extinct," says a researcher.
A pregnant female of the species Stenopterygius quadriscissus with ten foetuses. Three of them can be seen on the underside of the body.(Photo: Feiko Miedema)
Ichthyosaurs were large animals that lived in the sea at the same time as dinosaurs ruled on land.
Like dolphins and whales, ichthyosaurs gave birth to live offspring. Palaeontologists have found a number of fossils of baby ichthyosaurs and even foetuses.
"Even though we know they produced live young, very few studies have examined how the babies actually developed before birth," says Feiko Miedema, a researcher at the Natural History Museum in Oslo.
He and Erin Maxwell at the Stuttgart State Museum of Natural History in Germany have now cracked the code.
They show that it's possible to determine how close ichthyosaur foetuses were to being born by looking at their vertebrae.
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This knowledge can probably be applied to other animal groups and thus be used to study developmental stages in, for example, pterosaurs and dinosaurs, Miedema explains.
Feiko Miedema is a palaeontologist at the Natural History Museum.(Photo: Private)
What do you do if the head is missing?
It all started when Miedema studied ichthyosaur foetuses of the species Stenopterygius quadriscissus as part of his doctoral work.
From this species, there are an unusually large number of fossils of mothers with foetuses – nearly one hundred.
Miedema and his colleagues identified four developmental stages based on the skulls of the foetuses. They saw that bones in the skull developed at different times, such as bones around the eye forming before those around the brain.
"But what do you do if you only have a few specimens, or the babies don't have heads?" he asks.
The head is often missing. The bones most commonly found are vertebrae, because they are the most numerous bones in the body, Miedema explains.
"We wondered whether vertebrae could be used to determine the stage of development," he says.
Close-ups of ichthyosaur foetuses(Photo: Feiko Miedema)
Close-ups of ichthyosaur foetuses(Photo: Feiko Miedema)
Close-ups of ichthyosaur foetuses(Photo: Feiko Miedema)
The hole in the vertebra becomes smaller
The researchers studied ichthyosaur foetuses that are mainly stored in museums in the UK and Germany.
They observed that the canal for the notochord in the middle of the vertebrae becomes smaller as the foetus develops. This canal appears as a hole.
The vertebral body, the cylindrical main part of the vertebra, first forms as cartilage and is later converted into bone from the outside inwards. The hole in the centre becomes smaller as bone formation continues.
"We did something quite simple. We measured the size of the hole compared with the size of the entire vertebral body," says Miedema.
This approach worked just as well for identifying the stage of development as using skulls.
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However, the canal is not the same size along the entire spine. It is larger in the tail than in the neck and back. Researchers must take this into account when determining the developmental stage.
Here you can see the holes in the vertebrae.(Photo: Feiko Miedema)
Vertebrae.(Photo: Feiko Miedema)
Vertebrae from ichthyosaur embryos with observable holes.(Photo: Feiko Miedema)
Can the same thing be seen in dinosaur eggs?
Miedema says the study will make it easier for ichthyosaur researchers around the world to determine the developmental stage of fossil foetuses.
"This is important because you want to know how close the baby was to being born, for example if you want to say something about whether the mother migrated to a specific birthing area," he says.
The method can also be used to study differences between species.
"From a broader perspective, it's cool that all vertebrates share this type of development, so in principle the method could be applied to other animal groups," he says.
Embryos of dinosaurs and pterosaurs have been found preserved inside eggs.
"I hope colleagues who study dinosaurs or pterosaurs will try it and see whether it works for them," says Miedema.
Can help researchers study growth and behaviour
The findings by Miedema and Maxwell provide a way to determine how far a fossil ichthyosaur foetus had progressed in its development based on vertebrae, Lene Liebe Delsett writes in an email to Science Norway.
Lene Liebe Delsett is a researcher at the Natural History Museum.(Photo: University of Oslo)
She is a vertebrate palaeontologist at the Natural History Museum and studies the evolution of large marine vertebrates. She was not involved in the study.
"This provides new opportunities to understand reproduction in animals that have long been extinct, where we still have major gaps in knowledge despite the fact that pregnancy and birth are so important," she writes.
Ichthyosaurs had land-living ancestors, similar to modern whales.
"There's much we still don't know about how reproduction evolved in these groups, and these findings may help answer some of those questions," she writes.
The method can also help identify foetal fossils among other material and be used to create growth curves for developing foetuses, Delsett explains.
"In a broader sense, this could improve our understanding of reproductive behaviour. Hopefully the method can also be used for other extinct animal groups," she writes.
