A fossil of a pregnant reptile from approximately 245 million years ago is described in Nature Communications this week. This discovery indicates that this archosauromorph reptile, which belongs to the same group as dinosaurs, birds and crocodilians, gave birth to live young rather than laying eggs. Live birth was previously not known in any archosauromorph.
Live birth has evolved independently in many types of animals. However, modern birds and crocodilians all reproduce by laying eggs, which has suggested a biological constraint preventing live birth in this lineage.
Jun Liu and colleagues describe a new fossil specimen of Dinocephalosaurus, a long-necked, aquatic reptile found in China and dating from the Middle Triassic. A developmentally advanced embryo is preserved within the abdomen of the adult Dinocephalosaurus, providing evidence of live birth. This finding suggests that there is no general impediment against live birth in the archosauramorphs.
Analysis of the evolutionary history of Dinocephalosaurus indicates that the sex of offspring was determined genetically. This differs from related species, such as crocodilians and some turtles, in which the sex is determined by temperature. The authors conclude that genetic sex determination may have been a precondition for live birth in Dinocephalosaurus, since internal body temperature regulation is not compatible with temperature-dependent sex determination.