Ancient reptile's birth fossilised

Marine reptile fossilised giving birth Green highlights the mother's skeleton, yellow the unborn embryo inside, purple the animal being born and red the remains of another

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A rare fossil has revealed how marine reptiles evolved to give birth to live young, scientists say.

The fossilised Chaohusaurus was discovered in central China's Anhui Province and includes the remains of three babies.

The animals lived 248 million years ago and are the earliest of the marine reptiles from the Mesozoic Era.

Experts suggest the position of the babies shows that giving birth to live young evolved on land, not in the sea.

The findings were published in the journal PLoS One.

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The preserved partial skeleton shows the mother's tail extending to the right, her ribs on the left and her pelvis and hind flipper in the centre.

The skulls of two embryos are visible, one still inside the mother and the other exiting her pelvis.

A third newborn's bones were also preserved beneath the mother's tail.

The fossil is the earliest of its kind and one of only two rare records of Icthyosaurs giving birth.

Previous studies have revealed that live births were common in marine reptiles that could not lay eggs at sea or walk on land but scientists have debated where this style of reproduction originated.

"Being reptiles, their ancestors lived on land. What happened during the transition from land to the sea is not well understood, and Chaohusaurus holds a key to [unlock] the mystery," said Dr Ryosuke Motani from the University of California, Davis, US who undertook the study.

A team of researchers from the US, Peking University and Anhui Geological Museum, China, analysed the fossil after it was uncovered in the laboratory when they were investigating a different specimen of a predatory fish.

"The slab was collected for a fish called Saurichthys that was partly exposed in the field. We later realized that there was Chaohusaurus with babies on the same slab," explained Dr Motani.

The biologists noted that the preserved reptile's babies were born head first which is unusual given that the majority of marine live births are tail first, possibly to avoid suffocation.

In land-based animals, giving birth head first is the norm.

"We always assumed that live-bearing in marine reptiles evolved after they invaded the sea, partly because of this difference. Now the new fossil shows that the most primitive marine reptile gave birth head first," said Dr Motani.

"This strongly suggests that they inherited live-bearing from their land ancestors."

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