Live Birth in Archosauromorphs 245 Million Years Ago

Modern birds and crocodilians are all oviparous, and the fossil record likewise contains abundant eggs from non-avian dinosaurs and pterosaurs. All of these animals belong to Archosauromorpha, yet viviparity is exceptionally rare within this enormous evolutionary lineage. Live birth has, however, evolved repeatedly in other reptiles. Viviparous species occur among living lizards and snakes, while several groups of Mesozoic aquatic reptiles—including ichthyosaurs, sauropterygians, and mosasauroids—have also yielded fossils preserving embryos within the body.

Time-calibrated phylogenetic tree of amniotes. O, oviparous; V, viviparous（Image source：Liu J et al. (2017)， CC BY 4.0 ）

A fossil of an indeterminate species of Dinocephalosaurus, catalogued as LPV 30280, provides evidence that viviparity also occurred among archosauromorphs. The specimen was discovered in the Luoping biota of Yunnan, China, and dates to the Anisian stage of the Middle Triassic, approximately 244–245 million years ago. Dinocephalosaurus was highly adapted to aquatic life and was characterized by an extraordinarily elongated neck composed of 32 cervical vertebrae, together with paddle-like limbs. Phylogenetic analyses place it among relatively basal archosauromorphs, closely related to protorosaurs such as Tanystropheus.

(a) Geographic location of the Luoping biota in Yunnan, China; (b) stratigraphic column showing the horizon of the indeterminate Dinocephalosaurus specimen（Image source：Liu J et al. (2017)， CC BY 4.0 ）

Specimen LPV 30280 preserves an adult Dinocephalosaurus, but inside its abdominal cavity lies the skeleton of a much smaller Dinocephalosaurus. The presence of a small individual inside the body cannot by itself demonstrate pregnancy. The bones could conceivably have belonged to another animal that became superimposed on the adult after death, or they might represent prey swallowed shortly before the adult died.

(a) Fossil specimen LPV 30280; (b) interpretive drawing of the specimen; (c) the small individual interpreted as an embryo; (d) interpretive drawing of the embryo; (e) partially digested fish fossil（Image source：Liu J et al. (2017)， CC BY 4.0 ）

The smaller skeleton, however, is completely enclosed within the body outline of the adult, making accidental superposition unlikely. Its cervical vertebrae also possess the distinctive morphology of Dinocephalosaurus: they are greatly elongated, have very low neural spines, and bear long cervical ribs extending across at least three intervertebral articulations. No other protorosaur is known from the Anisian of South China, further supporting the interpretation that the smaller individual belonged to the same genus, and most likely the same species, as the adult.

The orientation of the skeleton provides another important clue. Aquatic amniotes generally swallow prey head-first, and this orientation can remain recognizable even as the prey is digested and its skeleton becomes disarticulated. LPV 30280 conveniently preserves a comparison: a partially digested fish is also present in the abdominal region of the adult Dinocephalosaurus, with its head directed toward the rear of the adult's body. The small Dinocephalosaurus, by contrast, has its neck directed toward the front of the adult. Its neck is also curved toward the forelimbs and ribs, producing the curled posture typical of vertebrate embryos. Together, these observations led the researchers to identify the small individual as an embryo.

The embryo had already reached a relatively advanced stage of development. Both forelimbs are preserved, and its humerus measures about 1.5 cm in length. In an adult Dinocephalosaurus, the humerus measures approximately 12.1 cm, while the fibula is about 7.5 cm long. From these proportions, the researchers estimated that the embryo was approximately 12% of the body size of its mother. The distal parts of the limbs are absent. These elements may not yet have ossified at this developmental stage, although they could also have been lost after death.

One of the strongest pieces of evidence linking the specimen to viviparity is the absence of any trace of an eggshell around the embryo. Birds, crocodilians, and other known egg-laying archosauromorphs produce well-calcified eggshells. The Luoping biota is exceptionally well preserved, and delicate calcareous fossils occur within the same horizon, yet no eggshell is preserved around the embryo.

The degree of ossification of the embryo provides additional support for live birth. Living birds and crocodilians lay their eggs at very early stages of embryonic development, while turtles and the tuatara lay their eggs even earlier. If Dinocephalosaurus reproduced in a similar manner, it would be unusual for an embryo with such extensively ossified bones to remain inside the mother. Taken together, these lines of evidence led the researchers to interpret LPV 30280 as a pregnant Dinocephalosaurus.

Viviparity also fits well with the highly aquatic anatomy of Dinocephalosaurus. Its paddle-like limbs, hyperphalangy in the feet, and extremely elongated neck indicate extensive adaptation to marine life, and the animal was probably poorly suited to moving on land. Amniote embryos developing inside shelled eggs must exchange respiratory gases with the environment through the eggshell, and gas exchange is far slower in water than in air. Reptilian eggs therefore cannot simply be left underwater to incubate. Live birth would remove the need to return to land to lay eggs by allowing the embryo to develop inside the mother. In Dinocephalosaurus, the sacral ribs were also not firmly attached to the sacrum, producing a relatively mobile pelvis—a feature that has likewise been associated with viviparity in several other Mesozoic marine reptiles.

Reconstruction of a pregnant indeterminate Dinocephalosaurus（Image source：Liu J et al. (2017)， CC BY 4.0 ）

The complete absence of viviparity among living birds and crocodilians does not mean that Archosauromorpha as a whole lacked the genetic or developmental capacity to evolve live birth. At least once in the Triassic seas, a long-necked archosauromorph followed a reproductive pathway very different from that of its living avian and crocodilian relatives.

Author: Shui-Ye You

Reference:

Liu J et al. (2017). Live birth in an archosauromorph reptile. Nature Communications.