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 the nearly 12-inch long females were "head backwards," a position thought to be abnormal, which the authors suspect could have caused the death of the two mothers and their young.

"These two specimens speak very nicely, very neatly, and very cleanly [of the fact] that the ability to give birth to live young arose very early on in the evolution of these groups of reptiles," said Michael Caldwell of the University of Alberta. "These results indicate that viviparity is a primitive feature for this group."

To Caldwell, who was not involved in the study, the findings also answer questions about more evolved groups of sauropterygians, such as the giant plesiosaurs. "How did these enormous, 10-ton animals, hold themselves while going to the beach to lay eggs? Well, clearly, they didn't have to." If the capacity for viviparity existed in their tiny little ancestors, he reasoned, it might have also been preserved in their descendants. Giving birth in the water would have been advantageous, since animals thus avoid the risks associated with going on land to reproduce.

One interesting anatomical feature of the Keichousaurus specimens is the presence of a very loose attachment between the pelvic girdle and the sacrum, in contrast with the more solid connection found in land animals. That loose type of joint has been interpreted as an adaptation to aquatic environments, allowing movement and thus reducing physical stress during sudden stops or turns in the water. According to Wu, the chain-like connection present between the pelvis and the sacrum would also have allowed the pelvic girdle to change its shape, maximizing the space of the birth canal.

Robert Carroll, of the Redpath Museum at McGill University, said that the presence of the embryos within the mothers' bodies provides a "solid reason" for that particular type of morphological connection. "Now, suddenly, this makes sense," said Carroll, who was not involved in the study.

The discovery of viviparity in the Keichousaurus hui fossils also enabled Wu and his colleagues to determine the gender of the existing morphotypes—particular specimens that define the characteristics of the group—of this dimorphic species, known until now as "sex X" and "sex Y." The specimens, unequivocally female, led the authors to the identification of sex X as the female and sex Y as the male, based on the structural complexity of the humerus and on the length ratio between the humerus and the femur.

Wu thinks that these findings will help determine the sex of other morphotypes, which will greatly facilitate the study of the thousands of specimens of sauropterygians in collections around the world. "We have so many specimens, and we often don't know which is the female or which is the male," said Wu. "Now we will be able to sex the specimens first, before going any further."