Live birth has evolved many times independently in vertebrates –– such as mammals and diverse groups of lizards and snakes. However, live birth had not been known before in the major clade Archosauromorpha, a group that first evolved ~260 million years ago and is represented today by birds and crocodilians. Authors [attached] report herein the discovery of a pregnant long-necked marine reptile (Dinocephalosaurus) from the Middle Triassic (245 million years ago) of southwest China showing live birth in archosauromorphs.
This discovery pushes back evidence of reproductive biology in the clade by roughly 50 million years, and shows that there is no fundamental reason that archosauromorphs could not achieve live birth. Phylogenetic models have indicated that Dinocephalosaurus determined the sex of their offspring by sex chromosomes, rather than by environmental temperature –– such as what is seen in crocodilians. These data herein therefore provide crucial evidence for genotypic sex determination facilitating land-water transitions in amniotes. [Amniotes represent the clade of tetrapod vertebrates –– composed of reptiles, birds, and mammals –– that lay their eggs on land, or retain the fertilized egg within the mother. Amniotes are distinguished from the Anamniotes, which typically lay their eggs in water, e.g. turtles, frogs.]
Nature Commun 2o17; 8: 14445