Life Hatched From Soft Eggs, Some a Foot Long, in Dinosaur Era

There’s a problem with dinosaur eggs: A lot of them are missing.

Dinosaurs dominated land from about 245 million years ago until an asteroid extinguished them some 66 million years before our time. But their eggs, a lot like those of other reptiles that lived on the planet during that time, are mostly absent from the first half of their fossil record. A new study published Wednesday in Nature, showcasing baby dinosaur remains from Mongolia and Argentina, offers a reason: The very first dinosaurs laid soft eggs like turtles do today, and their eggs decomposed long before they could ever turn into fossils.

In a second study also published in Nature, paleontologists announced the first known fossil egg found in Antarctica. The egg, also soft-shelled, looks like a deflated football. It’s bigger than any dinosaur egg ever found, and the team that unearthed it thinks it might be the egg of a mosasaur. These rocket-size marine reptiles patrolled the ancient oceans during the dinosaur era and, until now, were thought to give birth to live young, not lay eggs.

Both studies scramble scientific understanding of ancient reptile reproduction. The dinosaur find explains a gap in the fossil record. But it also reveals how natural forces most likely guided the evolution of dinosaur reproduction over time, ultimately leading dinosaurs to evolve a completely different kind of egg-laying ability. At the same time, the Antarctic egg find expands the known size limits that life can reach at birth, inviting questions about how big living things can truly grow.

The idea behind the dinosaur egg research incubated for a long time.

“This is actually an idea I had about 15 years ago,” said Mark Norell, a vertebrate paleontologist at the American Museum of Natural History in New York, who led the team behind the dinosaur study.

Dr. Norell was in Mongolia excavating dinosaur fossils in the Gobi Desert. There, he unearthed the fossil babies of a dinosaur called Protoceratops — a beaked, herbivorous dinosaur in the same group as Triceratops.

The Protoceratops babies died sometime between 75 and 71 million years ago, and they’re curled in fetal positions. They look as if they should still be sheltering in their eggs. But when Dr. Norell found them, there were no fossilized eggshell fragments. Instead, a thin film surrounded the animals.

That’s when it hit Dr. Norell: The films could be residues of decomposed soft shells. Today, reptiles like turtles, snakes and lizards will lay soft-shelled eggs, which are easier to hatch from, but offer little protection from the elements or some predators.

“But I couldn’t ever really prove it,” Dr. Norell said.

Dr. Norell said the idea bugged him, because paleontologists would regularly unearth large hauls of dinosaur eggs at excavation sites. But these eggs were always younger than the middle of the Jurassic Period, which runs from about 200 to 145 million years ago.

Dr. Norell thought something important had happened around that time. But until there was more evidence, the question of the missing eggs would be “put down to the normal vagaries and biases and inadequacies of the fossil record, nothing more specific than that,” said Stephen Brusatte, a vertebrate paleontologist at the University of Edinburgh.

So, the idea gestated. It wasn’t until Jasmina Wiemann, a molecular paleobiologist at Yale University, joined Dr. Norell’s team that the idea finally hatched.

Ms. Wiemann analyzed the chemistry of hard and soft eggs from animals like chickens and turtles, and she found that each shell type produces a unique chemical fingerprint. She then looked at the chemistry of the Protoceratops films, as well as films from the egg of another dinosaur from Argentina called Mussaurus, and found that the dinosaur eggs matched the soft shell fingerprint. “That was incredibly exciting,” Ms. Wiemann said.

The team now knew that some dinosaurs laid soft eggs. After sorting out where the soft-shelled trait fit on the dinosaur family tree, they realized that the common ancestor of all the dinosaurs must have laid soft eggs. The ability to lay the hard-shelled eggs that turned up later in the fossil record evolved tens of millions of years later, closer to their extinction than when non-avian dinosaurs first emerged.

“The idea that the ancestral dinosaur laid soft-shelled eggs like a turtle is a bold hypothesis, but I like it,” said Dr. Brusatte, who was not involved in the study. “It’s a stunning revelation — and it’s remarkable to think of these giant dinosaurs, larger than buses and in some cases airplanes, starting out as little pipsqueaks tearing their way out of a soft egg.”

While not a dinosaur, the mosasaur that tore out of the soft-shelled egg from Antarctica was one such pipsqueak. Adult mosasaurs might have reached lengths of nearly 60 feet. The egg, discovered in 2011 and the first mosasaur egg ever found, is about a foot long. The only larger egg ever found came from the elephant bird, which went extinct in the 17th century.

“That’s what really surprised us, because we didn’t think that soft-shelled eggs could grow that big without collapsing,” said Lucas Legendre, a paleontologist at the University of Texas at Austin who is the lead author of the new study. “We didn’t think there were animals that large that might have laid soft-shelled eggs.”

There’s no embryo inside the egg, so it’s hard to say for sure what kind of animal it was from. But dozens of mosasaur fossils have been discovered in Antarctica, Dr. Legendre explained, which has led other researchers to propose that the waters around the continent once served as a nursery for the large aquatic beasts.

Jingmai O’Connor, a vertebrate paleontologist at the Chinese Academy of Sciences who wasn’t involved in the study, saw Dr. Legendre give a talk about the egg at a conference in South Africa.

“When he gave that talk I was like, ‘Whoa! That is so cool!’” Dr. O’Connor said. “It’s the first time an egg from a marine reptile has ever been found,” and now, she explained, paleontologists will know what to look for when they seek marine reptile fossils in the future.

The same goes for dinosaur hunters, explained Matteo Fabbri, a paleontologist at Yale and co-author of the dinosaur study. Now that we know dinosaurs could lay soft-shelled eggs, paleontologists can no longer automatically assume that fossilized dinosaur embryos incubated in a hard-shelled egg.

For Dr. Norell, such an evolution in understanding is surprising — but that’s par for the course in his field. “I think the more we look at the evolution of basically anything, the more it just kind of blows your mind,” he said. “That’s just the way it works.”

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