Artist’s conception of a pregnant Dinocephalosaurus. (Dinghua Yang)

A quarter of a billion years ago, when a shallow sea covered what is now southwest China, a large, long-necked aquatic reptile got pregnant. That is an unusual fact by modern standards — many reptiles, such as birds, turtles and crocodiles, do not get pregnant, which is to say they do not incubate embryos within their bodies and give birth to live young; instead they lay eggs, as the average American, who eats 250 chicken eggs a year, could tell you.

What’s more, unlike the vast majority of the other 245-million-year-old and pregnant reptiles, one expectant mother of the species Dinocephalosaurus perished and became a fossil. As the Dinocephalosaurus‘s bones were preserved in rock, so too were those of the embryo that remained inside her. The scientists who examined the fossils, led by a team of Chinese paleontologists, concluded that this rare fossil embryo was the first evidence of live births in a vast group of species previously thought only to lay eggs.

First, however, the researchers had to determine if the tiny fossils-within-the-fossils, a specimen not unlike a reptilian nesting doll, represented an embryo and not a last meal.

“I was not sure if the embryonic specimen was the last lunch of the mother or its unborn baby,” said Jun Liu, a paleontologist at the Hefei University of Technology in China, in an email to The Washington Post. “Upon closer inspection and searching the literature, I realized that something unusual had been discovered.” Liu and his colleagues published their unusual discovery on Tuesday in the journal Nature Communications.

Dinocephalosaurus, which means “terrible-headed lizard” in honor of the toothy skull on the end of its skinny, five-foot-long neck, was not, properly speaking, a dinosaur. It predated all but the earliest dinosaurs by several million years. But it belonged to a group called archosauromorpha, a clade that includes the animals which later became crocodiles, alligators, birds — and, yes, the dinos.

Although other sorts of reptiles were known to give birth to live young (some snakes and lizards allow their eggs to hatch inside their bodies, for instance, and the babies slither out fully formed) it was assumed archosauromorpha always laid eggs.

To determine that the tiny fossil bones were gestating, not digesting, Liu and his colleagues analyzed the position of the embryo relative to the adult. Only a few bones of the embryo — bits of spine, forelimbs and some ribs — remained. But despite the limited amount of fossilized evidence, scientists concluded it was of the same species as the adult, only smaller, about 12 percent of the size of its mother.

Crucially, the embryo bones were aligned so that its body, though somewhat curved, would have pointed in the same direction as the adult’s. “The curled posture of the embryonic skeleton is also typical for vertebrate embryos,” Liu said.

They compared the forward-facing embryo to the bones of an ancient perleidid fish, which had been preserved inside another rare Dinocephalosaurus fossil. The partially digested fish had been consumed so that its head was pointed toward the rear of the 13-foot aquatic predator.

“The embryo is inside the rib cage of the mother, and it faces forward,” Liu told The Post. “Swallowed animals generally face backward because the predator swallows its prey headfirst to help it go down its throat.” (The backward swallowing behavior, face down the gullet, continues today; you can see it captured in the archosauromorpha-on-archosauromorpha violence in the tweet below.)

The ability to bear live young jibed with the concept of Dinocephalosaurus as a “fully marine reptile,” the scientists wrote. As aquatic reptiles cannot lay and incubate their eggs underwater, species like sea turtles must emerge on shore to lay eggs. The shape of Dinocephalosaurus, with a neck taking up more than a third of its body length, would have been poorly suited for even those short land excursions.

The discovery demonstrated that archosauromorphs do not have an ancient genetic excuse for their lack of live births. There are various hypotheses as to why living archosauromorphs, such as birds, only lay eggs; these include the “biomechanical demands of flight,” as Liu and his colleagues noted. But it appears that the question is no longer if archosauromorphs can get pregnant — but, rather, why birds and their closest relatives do not.

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