The largest and most diverse group of Triassic aquatic reptiles gave birth to live young, researchers report in this week's Nature. The finding in sauropterygians is the first evidence of viviparity in this group of animals, which lived throughout the Mesozoic era, from 250 to 65 million years ago.

Paleontologists have been divided on the issue of whether sauropterygians were oviparous or viviparous. Many contended that the reptiles laid their eggs on the shores—as modern marine turtles do—while others argued that they gave birth to live young in the water, study coauthor Xiao-chun Wu, of the Canadian Museum of Nature, told The Scientist. "Until now, nobody had any evidence [to support either position]," Wu said.

The evidence came in the form of two small, nearly complete, gravid specimens of the sauropterygians Keichousaurus hui from the province of Guizhou in southwestern China. Most of the embryos found within...