The embryonic individual of Dinocephalosaurus preserved inside LPV 30280 from Luoping can be identified as such for several reasons. First, it is enclosed entirely within the body cavity of the adult. Its cervical ribs - which are long, like that of the adult - face the same direction as the dorsal vertebrae of the adult, which is in contradiction to the typical head-first method of swallowing prey among amniotes. It is also preserved with its neck curling towards its forelimbs, a posture which is seen among vertebrate embryos but not adult Dinocephalosaurus specimens, which generally have the neck bent upwards. The absence of hands associated with the forelimbs in the embryo may be an artifact of preservation, but it may also be due to the sequence in which the limb bones of tetrapods ossify. Although the presence of an embryo per se would be equivocal regarding this issue, the conditions in which the embryo were preserved provide strong evidence that Dinocephalosaurus was viviparous. There is no preserved calcified eggshell surrounding the embryo, despite the presence of delicately-preserved calcified elements from other animals at Luoping. This is consistent with the eggshells surrounding the embryos of viviparous reptiles being non-calcified membranes. Furthermore, the relative proportions of the humerus and the fibula in the embryo, compared with the maternal individual and IVPP V13898 from Panxian, indicate that the embryo is around 12% of the size of its mother. Combined with its ossified bones, this suggests that the embryo was at an advanced developmental stage, whereas crocodilians, birds, turtles, and tuataras lay eggs at very early developmental stages. Overall, the evidence provided by the embryo suggests that Dinocephalosaurus was viviparous, making it the first viviparous archosauromorph asides from possibly metriorhynchids. This is consistent with the separation of its sacral ribs from its sacrum, which indicates a mobile pelvis that could have functioned in giving birth. While the otherwise absence of viviparous archosaurs has been historically attributed to common attributes inherited from the archosaurian stem-lineage, the discovery that Dinocephalosaurus was viviparous suggests that this phenomenon is due to lineage-specific lifestyle restrictions. Although the sex-determination systems among living archosauromorphs are diverse, with crocodilians and turtles using temperature-dependent sex determination, phylogenetic modelling suggests that Dinocephalosaurus retained the basal condition of genotypic sex determination from early diapsids, and that this system facilitated its transition to an obligately marine lifestyle alongside viviparity. Paleoecology Until at least the end of