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.
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.
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Whether a reptile gives birth or lays eggs is fairly obvious, but scientists have information about how the gender of offspring is determined for only a few dozen species, Organ says. For the new study, he and his colleagues pruned the existing reptile family tree to include only those species. Then, they added some species of birds, which lay eggs and long ago evolved from reptiles, and some mammals, which typically bear live young and share a more distant common ancestor with reptiles.
The researchers found that, with rare exceptions, reptile species that give birth to live young also have sex chromosomes, which typically determine the gender of offspring. When the family tree was expanded to include ancient marine reptiles, all three groups ended up on branches near groups of species with genetically determined gender. The team’s analyses suggest that the species in all three groups of ancient, fully marine reptiles had a better than 90 percent probability of having genetically determined gender, a trait the new research shows is strongly linked to live birth.
“The ability to give live birth was a necessary change for these creatures to become fully marine,” says Mike Everhart, a paleontologist at the Sternberg Museum of Natural History in Hays, Kan. “If they had been unable to give birth in water and were therefore tied to land, they couldn’t have developed the extreme adaptations that they did.”
Rick Shine, an evolutionary biologist at the University of Sydney, says the study is interesting and exciting. For creatures that spend their entire lives immersed in waters where temperature doesn’t vary much, having temperature-determined gender doesn’t make much evolutionary sense, he notes. In such scenarios, sex ratios among offspring would likely be consistently skewed in one direction.
However, he notes, sex chromosomes aren’t infallible. In some reptile species, especially in situations where incubation temperatures are unusually high or low, an individual can end up as one gender when its chromosomes suggest it should be the other.