Even though populations of most animals have males and females, the way sex is determined in offspring varies. Some animals rely primarily on sex chromosomes, as in humans where two X chromosomes make a female and an X and a Y chromosome make a male. Among living marine species, whales, porpoises, manatees, and sea snakes have chromosomal sex determination.
In sea turtles and saltwater crocodiles, on the other hand, the sex of offspring is generally determined by the temperature at which eggs incubate. These species are also bound to a semi-terrestrial existence because their gas-exchanging hard-shelled eggs must be deposited on land.
"No one has clearly understood how sex determination has co-evolved with live birth and egg laying," Organ says.
Organ, Janes, and colleagues show that evolution of live birth in a species depends on the prior evolution of genetic sex determination. Since the fossilized remains of pregnant mosasaurs, sauropterygians, and ichthyosaurs show that these species gave birth to live young, they must also have employed genetic sex determination, a point on which the fossil record is silent.
|Contact: Steve Bradt|