In this artist's rendition, a female mosasaur gives birth. (Dan Varner)

Ancient sea reptiles gave birth to live offspring, and their genes, not their environment, determined the offspring's sex, researchers in the U.S. and U.K. say.

The research means that the gender of these extinct reptiles — mosasaurs, sauropterygians and ichthyosaurs — was not determined by temperature and other environmental factors, as is the case in many modern reptiles, such as alligators and some turtles.

"Determining sex with genetic mechanisms allowed marine reptiles to give live birth, in the water, as opposed to laying eggs on a nesting beach," said Chris Organ, of Harvard's Department of Organismic and Evolutionary Biology, in a news release.

By moving away from land completely, these marine reptiles were able to adapt to life in the sea, evolving fin-like limbs, streamlined bodies and fluked tails. Like marine mammals such as whales and dolphins, these reptiles breathed air but otherwise spent their entire lives underwater.

Mosasaurs, sauropterygians and ichthyosaurs lived in the oceans in the Mesozoic era, between 251 million and 100 million years ago, but aren't considered dinosaurs. The dolphin-like ichthyosaur could grow to more than 20 metres in length and was an important predator.

Fossilized remains of pregnant marine reptiles show that they gave birth to live young.

Most reptiles lay eggs, but the eggs have leathery shells that exchange oxygen and carbon dioxide with the outside environment, so they can't survive underwater. That is why modern marine reptiles that lay eggs, such as sea turtles and salt-water crocodiles, lay them on land. The gender of these reptiles is determined generally by the temperature of the nest in which their eggs are laid.

By analyzing the DNA of living descendants of the extinct marine reptiles, the researchers deduced that the sex of these ancient reptiles was determined by their genes, in much the same way as X and Y chromosomes determine sex in people and other mammals.

In other animals, such as birds and some reptiles, including the modern-day sea snake, sex is determined by Z and W chromosomes. The researchers said it's not known what genetic mechanism the ancient sea reptiles used.

"Modern reptiles exhibit tremendous variability in the organization of their sex chromosomes. Some possess Zs and Ws. Others possess Xs and Ys. At this time, we cannot speak to the condition of sex chromosomes in extinct marine reptiles," wrote Dan Janes of Harvard University in an email.

The researchers said that genetic sex determination has certain advantages over sex determined by temperature and other environmental factors.

"Genetic sex determination is advantageous because it balances the male-to-female ratio in the species and buffers its survival from environmental fluctuations," said Mark Pagel of the University of Reading. It was also required to adapt completely to ocean life.

"The oceans have a relatively stable temperature, so a genetic determination for sex was necessary to enable colonization of this environment," said Pagel.

The research appears in this week's issue of Nature.