An extraordinary fossil unearthed in southwestern China shows a pregnant long-necked marine reptile that lived millions of years before the dinosaurs. Called dinocephalosaurus, the animal includes a developing embryo, indicating the creature gave birth by delivering live babies rather than laying eggs.

Scientists say the fossil of the unusual fish-eating reptile, which lived about 245 million years ago, changes the understanding of the evolution of vertebrate reproductive systems.

Mammals and some reptiles, including certain snakes and lizards, are viviparous, which means that they give birth to live young.

Dinocephalosaurus is the first member of a broad vertebrate group called archosauromorphs known to give birth this way, paleontologist Jun Liu of China’s Hefei University of Technology said. The group includes birds, crocodilians, dinosaurs and extinct flying reptiles known as pterosaurs.

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Dinocephalosaurus boasted one of the longest necks relative to body size of any animal that ever existed. The specimen unearthed in Yunnan province was an estimated 13 feet long, including a slender neck that was about 5½ feet long, Liu said. It had paddle-like flippers, a small head and a mouth with teeth, including large canines, perfect for snaring fish.

“I think you’d be amazed to see it, with its tiny head and long snaky neck,” said University of Bristol paleontologist Mike Benton, who also participated in the research about the find that was published in the journal Nature Communications.

Its body plan was similar to plesiosaurs — long-necked marine reptiles akin to Scotland’s mythical Loch Ness Monster — that thrived later during the dinosaur age, although they were not closely related.

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Not laying eggs gave advantages to dinocephalosaurus, the researchers said. It indicated the creature was fully marine, not having to leave the ocean to lay eggs on land as sea turtles do, exposing the eggs or hatchlings to land predators.

Many animal fossils have been found with intact stomach contents — whole fish, for example. Several factors showed that in this case, the embryo was the fossil’s baby, not its breakfast.

Liu said it was found in a curled posture typical of vertebrate embryos. The embryo faces forward relative to the mother, while swallowed animals generally face backward because a predator will gulp prey head first to help it get down the throat.

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