By Sid Perkins
Reptiles that took to the seas during the Age of Dinosaurs succeeded evolutionarily in large part because their ancestors determined the gender of their offspring by genetics rather than by incubation temperature.
![](https://i0.wp.com/www.sciencenews.org/wp-content/uploads/2009/09/11034.jpg?resize=300%2C191&ssl=1)
Today, the 60 or so known species of sea snakes are the only reptiles that spend their entire lives at sea, even giving birth there. Other marine reptiles, such as sea turtles and saltwater crocodiles, must return to land to lay their eggs, says Chris Organ, an evolutionary biologist at Harvard University. But there’s another big difference between the fully aquatic reptiles and their bound-to-shore kin: In sea snakes, the gender of the young is determined genetically, not by incubation temperature.
Now, in the Sept. 17 Nature, Organ and his colleagues propose that this same trait enabled all three major groups of ancient marine reptiles — the porpoise-shaped ichthyosaurs, the long-necked plesiosaurs and the snake-shaped mosasaurs — to successfully break free of the land. The fossil record already indicates that all three groups gave birth at sea.