An unusually long-necked marine reptile gave birth to live young 245 million years ago — the only known member of the dinosaur, bird and crocodile family to not lay eggs, researchers have found.
Archaeologists examining the fossil of a female Dinocephalosaurus from Yunnan Province in southwest China were amazed to discover the remains of a baby among the bones where her abdomen would have been.
“I was so excited when I first saw this embryonic specimen,” said Jun Liu of China’s Hefei University of Technology who co-authored a study published in Nature Communications.
“This discovery rewrites our understanding of the evolution of reproductive systems.”
Dinocephalosaurus was a member of the archosaur family, which includes extinct dinosaurs as well as today’s birds and crocodiles, which are all egg-layers.
The archosaurs’ sister clade of turtles also lays eggs, but a third group of reptiles called lepidosaurs, including lizards and snakes, contains some species that give birth to live young — including some sea snakes, boas, skinks and slow worms.
Live birth is usually associated with mammals, and egg-laying is considered the original, “primitive” state of animals.
Dinocephalosaurus was a strange-looking ocean-dweller with a neck almost twice the length of its trunk — some 3-4 metres in total.
It was a fish eater, snaking its long neck from side to side to catch prey. It had paddle-like flippers, a small head and a mouth with teeth, including large canines, perfect for snaring fish.
The baby Dinocephalosaurus, or what remained of it, was about a tenth of the mother’s size.
At first, “I was not sure if the embryonic specimen (was) the last lunch of the mother, or its unborn baby,” Mr. Liu said.
“Upon closer inspection and searching the literature, I realised that something unusual has been discovered” — an embryo providing “clear evidence for live birth”.
Unlike prey, which would ordinarily have been swallowed head-first, the young Dinocephalosaurus was facing forward in the abdominal cavity, said Mr. Liu.
The scientists also discounted the possibility that the tiny reptile had been inside an egg shell which simply eroded over time.
The specimen “demonstrates the curled posture typical for vertebrate embryos,” and there were no calcified shell bits found, said Mr. Liu.
Archosaurs are known to lay their eggs at a much earlier developmental stage, he added — long before the young one would had grown to this size.
The safety advantage
Not laying eggs provided advantages to Dinocephalosaurus, the researchers said. It indicated the creature was fully marine, not having to leave the ocean to lay eggs on land like sea turtles, exposing the eggs or hatchlings to land predators.
The new study pushes fossil evidence for the reproductive biology of archosaurs back by 50 million years, to the Middle Triassic, said the study. Montana State University evolutionary biologist Chris Organ said while some reptiles such as crocodiles determine the sex of their babies through the temperature inside the nest, Dinocephalosaurus determined its offspring’s sex genetically as mammals and birds do. — AFP/Reuters