Scientists in China have discovered a fossil capturing what could be the earliest known live reptilian birth.
The fossilised remains show a giant sea dinosaur known as a Chaohusaurus that died in labour. One of the mother's three babies can be seen exiting her pelvis headfirst.
This Chaohusaurus mother (known as AGM I-1) was one of 80 dinosaur skeletons unearthed from Majiashan quarry in Anhui province, central China, in 2010.
AGM I-1 was hidden in a rock slab that has contained a Saurichthys fish fossil from a different time period, and was only found then when fish fossil was being prepared in the lab.
The Chaohusaurus is the earliest ancestor of the giant prehistoric sea predators called ichthyosaurs, and according to a research article published on the online journal PLOS One, researchers say that this is the first proof that these predators birthed their babies headfirst, and that reptilian live birth only occurred on land.
"The reason for this animal dying is likely difficulty in labour," Ryosuke Motani, lead study author and a paleobiologist at the University of California, told Live Science.
The researchers believe that the first baby was born dead, and the mother may have died of complications in labour with the second, which is stuck half-in, half-out of her body.
Ichthyosaurs were sleek, streamlined swimmers that could grow to the length of a bus. They were air-breathing carnivores that evolved from land reptiles that moved into the water from land during the early Triassic period (251-247 million years ago), and had teeth-filled snouts and enormous eyes.
Fossils of pregnant ichthyosaurs have been found before, showing that the reptiles carried live embryos instead of eggs, but researchers were not sure whether they gave birth headfirst or tail-first.
Most air-breathing marine mammals that bear live young, such as dolphins and whales, give birth to their babies tail-first to ensure that the newborns do not suffocate during labour, but this is in the water.
On land, babies are usually born headfirst. The earliest whales, which evolved from land mammals, also birthed their new-borns headfirst.
The Chaohusaurus is on display at the Anhui Geological Museum in Hefei.