The mother sea reptile was found with three embryos, one that had already been born, one that was emerging from the birth canal, and one that was still inside her body. (Ryosuke Motani) A fossilized baby reptile emerging from its mother's body during a live birth has been discovered in China.

The fossils, belonging to dolphin-like marine reptiles with huge eyes called Chaohusaurus, date back to 248 million years ago, during the Mesozoic era, before the rise of the dinosaurs.

That makes this the oldest live birth of a vertebrate — an animal with a backbone — known in the fossil record.

The mother appears to have been in the process of giving birth to triplets. The one that died while being born had two siblings nearby — one still inside the mother, and one that had already been born, lying near the mother.

The fossils were described by University of California Davis paleontologist Ryosuke Motani, and American, Italian and Chinese colleagues in a paper published in the journal PLOS ONE this week.

Chaohusaurus is the oldest of a group of marine reptiles called Ichthyosaurs. Ichthyosaurs were already known to give birth to live young, unlike many present day reptiles that lay eggs. Later ichthyosaurs have been found being born tail first, like whales, thought to be an adaptation to prevent suffocation during underwater births.

The fact that the Chaohusaurus baby was being born head first suggests that its ancestors were live-bearing land animals, the researchers said. That implies that reptiles on land evolved the ability to give birth to live young much earlier than thought.