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First Soft-Shelled Dinosaur Egg Fossils Found

Twin discoveries reveal that some ancient reptiles laid soft-shelled eggs, challenging long-held assumptions in paleontology

A fossilized Mussaurus egg that was the subject of one of two new studies documenting soft-shelled eggs at the time of the dinosaurs. Mussaurus was a long-necked, plant-eating dinosaur that grew to 20 feet in length and lived in modern-day Argentina between 227 and 208.5 million years ago. (Diego Pol)
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Two studies published this week in the journal Nature are upending paleontologists’ understanding of ancient reptile reproduction, reports Lucas Joel for the New York Times.

One study found evidence that some of the earliest dinosaurs laid soft-shelled eggs, and not eggs with hard shells as was previously thought. The second discovery is another soft-shelled egg that researchers have attributed to a kind of giant marine reptile called a mosasaur. The giant, nearly foot-long egg suggests mosasaurs didn’t give live birth as researchers once assumed, but instead laid leathery-shelled eggs like their closest living relatives monitor lizards and snakes.

The new evidence that some dinosaurs and their extinct reptilian contemporaries laid eggs without hard shells helps explain the rarity of eggs in the first half of the fossil record, according to the Times. Soft shells tend to rot away quickly, which would have made it less likely for them to fossilize. Both finds may have implications for the reproductive evolution of dinosaurs and ancient reptiles.

Chilean paleontologists found “The Thing” in Antarctica in 2011. It was a fossil the size of a football that had a crinkled exterior that made it look deflated. The researchers who had collected The Thing couldn’t tell what it was, reports Nell Greenfieldboyce of NPR. But when paleontologist Julia Clarke of the University of Texas at Austin saw its rumpled surface, she knew just what she was looking at.

Soft-shelled fossil egg
A side view of the fossil of the giant soft-shelled egg found in Antarctica. (Legendre et al. 2020)

It was a giant, soft-shelled egg, and it was also 66 million years old, the researchers report this week in Nature. No fossilized embryo or hatchling was available to positively identify what creature produced it. The researchers arrived at their suggestion that it came from a mosasaur based on the fossil egg’s size, shape and the fact that mosasaur remains were found nearby. The team estimates the mosasaur that laid the egg was more than 20 feet long, without including its tail, according to a statement from the University of Texas at Austin (UTA).

The find represents the first fossil egg ever found in Antarctica, the largest known soft-shelled egg and the second largest egg ever known to have existed, according to the UTA statement. The massive size of the egg also challenges assumptions about how big eggs with soft shells can get before collapsing, Lucas Legendre, a paleontologist at UTA and the study’s lead author, tells the Times.

baby mosasaur emerging from an egg
An artist’s interpretation of a baby mosasaur emerging from an egg in ancient Antarctic waters. (Francisco Hueichaleo)

Paleontologists don’t know for sure the egg came from a mosasaur. And in light of the other newly published paper documenting soft-shelled eggs in dinosaurs for the first time, the intriguing possibility exists that the giant ovum was produced by a dinosaur and somehow washed out to sea.

The first dinosaur eggs were found in the 1800s. Paleontologists went on to find communal nests and evidence of brooding that made dinosaur parents seem much like the creatures that are some of their closest living relatives: modern birds, write archaeologists Johan Lindreg of Lund University and Benjamin Kear of Uppsala University in a commentary published along with the new research.

As our understanding of dinosaur eggs deepened, so did certain inconsistencies. Most of the dino-eggs paleontologists uncovered were from the Cretaceous—which lasted from 66 million to 145 million years ago—leaving a more than 100-million-year gap in the fossil record during the Triassic and Jurassic periods, according to the commentary. The bulk of the eggs also came from a suspiciously small number of dinosaur groups.

“Over the last 20 years, we’ve found dinosaur eggs around the world. But for the most part, they only represent three groups—theropod dinosaurs, which includes modern birds, advanced hadrosaurs like the duck-bill dinosaurs, and advanced sauropods, the long-necked dinosaurs,” says Mark Norell, paleontologist at the American Museum of Natural History and lead author of the study, in the statement from the museum. “At the same time, we’ve found thousands of skeletal remains of ceratopsian dinosaurs, but almost none of their eggs. So why weren’t their eggs preserved? My guess—and what we ended up proving through this study—is that they were soft-shelled.”

Protoceratops eggs
Protoceratops eggs found in Mongolia's Gobi Desert. (M. Ellison/American Museum of Natural History)

The research that confirmed Norell’s hunch describes eggs from two different species of plant eating dinosaurs, per NPR. The twin finds come from Protoceratops, a sheep-sized relative of the more famous Triceratops that lived between 71 million and 145 million years ago, and Mussaurus, a long-necked dinosaur about 20 feet long that lived between 208.5 million and 227 million years ago, per the museum’s statement.

The Protoceratops fossil was a clutch of embryos found in Mongolia in the 90s, reports Michael Greshko of National Geographic. The embryos were all early enough in their development that the absence of hard egg shells from the fossil was conspicuous. Similarly, expeditions in 2012 and 2013 to Argentina unearthed a group of Mussaurus embryos that had no sign of calcified eggshells nearby, according to National Geographic.

The recognition that the fossils contained the remnants of soft-shelled eggs came from chemical analysis prompted by strange haloes surrounding the embryos, according to the Times. Molecular paleobiologist Jasmina Wiemann of Yale University compared the chemical composition of these haloes surrounding the fossil embryos to that of modern hard and soft-shelled eggs and found the chemical fingerprint of the fossils matched the eggs with soft shells, she tells the Times.

Finding out that some dinosaur groups laid soft-shelled eggs also explains puzzling differences that paleontologists have noted in the surfaces of previously discovered hard-shelled eggs, write Lindreg Kear in their commentary. Instead, Norell and his colleagues suggest that hard-shelled eggs evolved at least three times in separate lineages.

"The dinosaur calcified egg is something that is not ancestral, that is not sort of a primitive feature of all dinosaurs," Wiemann tells NPR.

The soft-shell revelation also suggests that many dinosaurs buried their eggs like some modern reptiles, since soft shells lose moisture quickly and couldn’t withstand the weight of a brooding parent.

“The idea that the ancestral dinosaur laid soft-shelled eggs like a turtle is a bold hypothesis, but I like it,” Stephen Brusatte, a paleontologist at the University of Edinburgh who was not involved in the study, tells the Times. “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.”

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