Several lines of evidence identify the small Dinocephalosaurus specimen in the abdominal region of LPV 30280 as an embryo of the maternal specimen. First, the embryo is completely enclosed within the body of the maternal specimen, and this excludes the possibility of superposition. The second line of evidence comes from the orientation of the embryo, with the neck pointing forward. This is evidenced by the fact that the cervical rib head of the embryo and the prezygapophyses of the dorsal vertebrae of the adult point in the same direction (Fig. 3). In aquatic amniotes, prey is usually swallowed head-first and this orientation is maintained during digestion and disarticulation13. Indeed, the partially digested perleidid fish (Fig. 3e) preserved in the abdominal region of LPV 30280 is oriented in the head-backward position. Therefore, the neck-forward position of the embryonic skeleton suggests that the included skeleton was not ingested prey, but was an embryo. Finally, we note that the embryo demonstrates the curled posture typical for vertebrate embryos. The neck of the embryo slightly curves towards the forelimbs and ribs (Fig. 3c,d), while in all other known adult specimens of Dinocephalosaurus, the neck curves towards the dorsal side26.

In all archosauromorphs and turtles, the eggshells are well calcified, though many turtle eggs are also pliable27. There is no trace of preserved eggshells near the LPV 30280 embryo, while many delicate calcareous fossils are preserved in the same horizon23. This is consistent with the eggshell morphology of extant viviparous reptiles. Although an eggshell membrane initially forms around the developing embryo in viviparous reptile species, it does not become calcified28. Altogether, these lines of evidence suggest that the embryo was likely contained in soft, uncalcified membranes, as in living viviparous reptiles, although the taphonomic absence of a calcified shell cannot be excluded.

The second line of evidence for viviparity in LPV 30280 is that the bones of the embryo are well ossified, indicating a relatively advanced embryonic stage. Living archosauromorphs, crocodilians and birds, all lay eggs very early in embryonic development, the neurulation and blastulation stages, respectively29. The tuatara and turtles lay eggs even earlier, in the gastrula stage29. Thus, it is unlikely that oviparous protorosaurs, which are archosauromorphs (Fig. 4), laid eggs with embryos in late developmental stages.

Our conclusion that Dinocephalosaurus had live birth is consistent with its functional morphology22. The anatomy of Dinocephalosaurus demonstrates that it was a fully marine reptile, representing the climax of aquatic adaptation of protorosaurs22. This is supported by the presence of hyperphalangy in the LPV 30280 specimen (Fig. 3). Further adaptations to marine living include the large paddle-like limbs and extremely elongated neck, both of which make it unlikely that Dinocephalosaurus could function comfortably on land or easily build terrestrial nests similar to those of sea turtles. Reptilian eggs cannot be incubated underwater; amniote embryos in shelled eggs must exchange respiratory gases with the environment across the eggshell, and this exchange is much slower in water than in air30. Therefore, viviparity would have been highly adaptive for Dinocephalosaurus to reproduce in the sea. We also note that the sacral ribs are separated from the sacrum in Dinocephalosaurus26, indicating a movable pelvis, a character that is interpreted as evidence for viviparity in numerous other Mesozoic marine reptiles6,7.

Although archosauromorphs originated around 260 Ma in the Late Permian18, the earliest evidence of reproductive biology in this group only came from the Early Jurassic dinosaurian embryos associated with calcified eggshells reported from South Africa20 and China21. Thus, there is a gap of roughly 70 million years between the origin of archosauromorphs and the earliest evidence of reproductive biology in this group. Now the discovery of live birth in the Middle Triassic Dinocephalosaurus fills this gap and extends the previous understanding of reproductive biology in archosauromorphs by roughly 50 million years. Our discovery is also the only information available for the reproductive biology of the stem-group archosauromorphs.

Roughly one third of extant amniote species are archosauromorphs (mostly birds). Given the phylogenetic, morphological, and ecological diversity of extant archosauromorphs—birds in particular—the absence of viviparity in this group is striking, especially when compared with lepidosauromorphs (tuatara, lizards and snakes). Multiple hypotheses have been put forward to explain the absence of viviparity in birds, including the biomechanical demands of flight, oviduct physiology and lack of the selection pressure to evolve viviparity31,32. Our discovery of a pregnant Dinocephalosaurus demonstrates that ancestrally there was no genetic or developmental impediment to evolve live birth in this diverse group. The reasons why live birth is absent in extant archosauromorphs may therefore be because of lineage specific constraints and adaptations rather than an attribute of the wider groups’ underlying biology.

Sex determination in Archelosauria (turtles plus crocodilians and birds)33 is diverse, with temperature-dependent sex determination in crocodilians and some turtles, the ZW genetic system in birds and some turtles, and the XY genetic system also in some turtles34. For nearly all amniotes, the evolution of live birth is dependent on the prior evolution of genotypic sex determination (GSD), with some skinks as the notable exception35. Phylogenetic modelling (Fig. 5) using this evolutionary relationship and the presence of live birth in the new specimen shows that Dinocephalosaurus likely had GSD (0.95 posterior probability). This suggests the presence of GSD in marine protorosaurs, consistent with the hypothesis that GSD and live birth were present in early diapsid lineages, and that both features are necessary to facilitate the land-water transition in lineages of obligate marine amniotes35.