Paleontologists Find Fossils of a Pregnant, 245-Million-Year-Old Sea Reptile

It's the first time live birth has been observed in an archosauromorph, the reptilian parent group from which dinosaurs, birds, and crocodilians derive.

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Feb 14 2017, 4:08pm

Some 245 million years ago, at the dawn of the Triassic period, a pregnant aquatic reptile from the genus Dinocephalosaurus perished in the seas covering what is now the Luoping region of southwest China. But as luck would have it, the expectant mother was fossilized in the seabed—along with her developing embryo.

This extraordinary specimen presents the first evidence of live birth observed in an archosauromorph, which is the reptilian parent group from which dinosaurs, birds, crocodilians, and marine animals like Dinocephalosaurus all derived. The find is described in new research published Tuesday in Nature Communications, led by Hefei University of Technology paleontologist Jun Liu.

"I was not sure if the embryonic specimen was the last lunch of the mother or its unborn baby"

Though the pair's remains were originally discovered in 2008, it was many years before the baby Dinocephalosaurus was recognized as an embryo, and not a tasty sea creature digesting in the mother's belly.

"I was so excited when I first saw this embryonic specimen in 2011 after the lab preparation finished, but I was not sure if the embryonic specimen was the last lunch of the mother or its unborn baby," Liu told me over email.

Indeed, Dinocephalosaurus is thought to have been a devastatingly effective hunter that used its bizarrely elongated neck to creep up on unsuspecting prey, like a kind of predatory periscope. These necks stretched out to lengths of nearly two meters, twice as long as the animal's actual body, allowing Dinocephalosaurus to strike at its prey while its body remained camouflaged by the shallow seabed.

There are also fossils of a perleidid fish inside this mother Dinocephalosaurus, showing that she had successfully snagged some grub in the leadup to her death.

READ MORE: Dinosaurs Took a Surprisingly Long Time to Hatch

This specimen fascinated Liu, but he had to sideline this research for a few years as he pursued his PhD and fielded other projects in Australia and Hong Kong. Finally, in 2014, he returned to the tantalizing mystery of the larger fossilized creature inside the adult Dinocephalosaurus.

"Upon closer inspection and searching the literature, I realized that something unusual [had been] discovered," he said.

The new paper identifies several lines of evidence supporting the idea that the smaller animal was a developing baby. For starters, it has the same long neck as its mother, suggesting conspecificity, plus it is curled into a standard fetal position. It is also oriented with its vertebrae aligned in the same direction as its mother, unlike the digested fish, which is facing the rear end of the hunter that consumed it. This is a common orientation for aquatic prey, since those unlucky enough to be eaten are normally swallowed head first.

Diagram visualizing the anatomy of Dinocephalosaurus. Image: David Peters

Given those factors, Liu's team concludes that the encased animal was most likely a Dinocephalosaurus pup in utero, and that it was fairly far along in its development when its parent was killed, perhaps by an algae bloom or volcanic eruption. The team estimates the baby was about 12 percent the size of its mother.

Modern archosauromorphs, like crocodiles and birds, reproduce by laying eggs, and the abundance of eggshells in the fossil record suggests that the majority of dinosaurs shared the same reproductive strategy. But Liu's team has upended the notion that egg-laying and archosauromorph biology are always coupled.

"Our discovery of a pregnant Dinocephalosaurus demonstrates that ancestrally there was no genetic or developmental impediment to evolve live birth in this diverse group," the team concluded in the paper. "The reasons why live birth is absent in extant archosauromorphs may therefore be because of lineage specific constraints and adaptations rather than an attribute of the wider groups' underlying biology."

It's kind of sad to think of this mother and her embryo dying together; forgotten for 245 million years as oceans subsided, continents shifted, and whole biological kingdoms were wiped out and replaced. But because of the fortuitous preservation conditions of the pair's watery grave and the paleontologists who finally freed this skeletal pair from it, we know now their story.

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