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Fossil of marine reptile giving birth raises new questions

Paleontologists have unearthed plenty of fossilized eggs and prehistoric reptile babies over the years, but one team in China recently discovered a fossil of an actual birth under way. The study, which was published this week in the journal PLOS ONE, describes an unusual fossil of a marine reptile Chaohusaurus giving birth to triplets.

The fossil is an estimated 248 million years old and depicts a baby exiting from the pelvis of the mother. A second baby was lying next to the mother, and a third was still inside her. The mother appears to have died in labor.

The remains themselves were a surprise. Ryosuke Motani, University of California-Davis paleobiologist who undertook the study, said that he and his colleagues were initially at the site to dig up the partially exposed fossil remains of a fish called Saurichthys. Only as the slab came loose did the researchers realize that this bigger Chaohusaurus find lay below it.

The even bigger surprise was the nature of the birth itself, however. The offspring was emerging head-first, a behavioral trait that only appears among animals that give birth on land. When whales, sea cows, and most other marine mammals give birth, their babies emerge tail-first. According to Motani and colleagues, this finding suggests that Chaohusaurus likewise gave birth on land, as well.

Since Chaohusaurus was a reptile, its ancestors would have been land-dwelling creatures. The species followed a reverse evolution of sorts, transitioning from animals that lived on land back to ones that lived in the sea. If this fossil is an accurate indicator, though, the trait of reptilian live birth itself probably evolved on land, also and not in the water.

Chaohusaurus was a dinosaur precursor, having lived in the early Triassic period in the waters off of present-day China. It may have been one of the ichthyosaurs, a large family of marine reptiles. But whereas many ichthyosaurs resembled porpoises and dolphins in their long snouts and stubby heads, the Chaohusaurus had a long neck and a more generally lizard-like in appearance. Like dolphins, though, it had flippers and no dorsal fin. The adults measured about 70 to 80 centimeters in length and weighed around 10 kilograms on average.