The research shows that humans are far from being the first to evolve this form of birth called viviparity.
The study, published in the Historical Biology: An International Journal of Paleobiology, focuses on mesosaurs, which were among the world's first aquatic reptiles.
They lived in what are now South America and South Africa at a time when these two landmasses were united and part of the giant supercontinent Pangaea, the Discovery News reported.
Despite the fact that the embryos were dated to around 280 million years ago, researchers found them in a remarkably well preserved condition.
Mesosaurs, and even their earlier ancestors, possibly "were not able to produce hard shelled eggs, at least for the first several million years of their evolution," said lead author Graciela Pineiro, a paleontologist at Uruguay's Facultad de Ciencias.
"After the recent discovery of mesosaur embryos, we can state with a high degree of confidence that embryo retention developed early in amniote evolution, given that mesosaurs are among the basal-most reptiles and that they date from the Early Permian around 280 million years ago," Pineiro said.
Researchers recently discovered the exceptionally well-preserved mesosaur embryos at sites in Uruguay and Brazil. The environmental conditions at the locations allowed for the preservation of soft tissues, nerves and blood vessels, she said.
"We think that the retention of the eggs may have appeared in amniotes as a useful strategy to avoid predation and increase survivorship chances for the embryos," Pineiro said.
There is also compelling evidence that giant, carnivorous, four-flippered reptiles known as plesiosaurs gave birth to live young as well.
Robin O'Keefe of Marshall University and team discovered a big embryonic marine reptile contained in the fossil of its 15.4-foot-long mother, which lived 78 million years ago.
"The embryo is very large in comparison to the mother much larger than one would expect in comparison with other reptiles. Many of the animals alive today that give birth to large, single young are social and have maternal care," O'Keefe said.
"We speculate that plesiosaurs may have exhibited similar behaviours, making their social lives more similar to those of modern dolphins than other reptiles," O'Keefe added.
