Paleontologists unearth fossil of a pregnant sea creature

The Dinocephalosaurus mother and embryo provide evidence of live birth.

A 250 million-year-old fossil of an ancient sea-dwelling reptile and its embryo has been uncovered by an international research collaboration. Dinocephalosaurus belonged to group of animals that today includes reptiles and birds. These animals were thought to reproduce only by laying eggs, but the newly analyzed fossils reveal that there were exceptions. “This discovery rewrites our understanding of the evolution of reproductive systems,” said lead author Jun Liu, a paleontologist at Hefei University of Technology.

Dinocephalosaurus was a marine animal that lived in the shallow seas of what is now South China during the Middle Triassic epoch. They used their long necks to snake from side to side and snatch fish as prey. When the fossils were first discovered during a large-scale excavation in Yunnan Province, Liu and his colleagues weren’t entirely sure the smaller skeleton was an embryo, speculating that it might be the remains of some prey that had been swallowed. “We were so excited, but we were not sure if the embryonic specimen is the last lunch of the mother or its unborn baby,” Liu said.

Further analysis has since shown that the smaller specimen is indeed an embryo. “There is clear evidence that the embryo is the same species as the mother, and it demonstrates the curled posture typical for vertebrate embryos,” Liu explained. It is also significant that the embryo faces forward; swallowed prey tends to face backwards, because it goes down the throat head-first.

The discovery has significant implications for scientists’ understanding of the evolution of reproductive systems. Live birth is an indication that Dinocephalosaurus determined the sex of its offspring genetically, as mammals and birds do. This sets it apart from crocodiles and turtles, its closest living relatives, in which sex is determined by the temperature of the nest in which eggs incubate. The authors say that genetic sex determination and live birth seem to have been necessary for animals like Dinocephalosaurus to become aquatic. “Our discovery pushes back evidence of reproductive biology in the Archosauromorpha by 50 million years,” said Liu.


Pregnant Dinocephalosaurus
The fossilized Dinocephalosaurus embryo and its placement in relation  to the adult fossil. Courtesy of Jun Liu.

Featured image: An artist's representation of the pregnant Dinocephalosaurus biting a fish. Credit: Dinghua Yang & Jun Liu.