The oldest embryos of a Mesozoic marine reptile have been unearthed in China, pre-dating the previous record by ten million years, a new study says.

The 248-million-year-old fossil from the Mesozoic era (252 to 66 million years ago) reveals an ichthyosaur baby inside its mother (orange) and another stuck in her pelvis (yellow). A third embryo discovered nearby suggests it was stillborn; scientists believe the mother died during a difficult labor.

The narrow, eel-like ichthyosaur belongs to the genus Chaohusaurus and is the oldest known species of the group. (Also see "Pictures: Oldest Dinosaur Embryos Show 'Big Surprises.'")

The findings were published February 12 in the journal PLOS ONE.

It's not just the age of the Mesozoic-era discovery that is surprising; it's the shattering of the belief that ichthyosaurs—also dubbed sea monsters—gave birth in water, not on land.

The scientists reached their conclusion because the fossil showed the offspring emerging head-first—a behavior found only in animals that give birth on land. The babies of most marine animals, such as whales and sea cows, are born tail-first. (Explore an interactive sea-monster time line.)

"That was a big surprise. Initially I couldn't believe my eyes when I saw that," said study leader Ryosuke Motani, a prehistoric marine reptile expert at the University of California, Davis.

Land Birthing

As a rule, reptiles lay eggs and mammals give birth to live young, a strategy known as viviparity. But new research reveals that over the evolutionary trajectory of reproduction, reptiles veered back and forth between the two strategies before settling on egg-laying.

Live birth in reptiles seems to have evolved more than a hundred times in history. But there are many gaps in scientific knowledge surrounding the phenomenon in ancient sea reptiles.

Since ichthyosaurs evolved first on land before becoming aquatic, some of the first species to live in water, such as Chaohusaurus, were still giving birth as their ancestors had—on land. (Read "When Monsters Ruled the Deep" in National Geographic magazine.)

So "live bearing did not evolve in water as scientists thought," said Motani, who worked with scientists at Peking University and Anhui Geological Museum. "Our assumption was wrong."