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A Protoceratops nest with embryos curled up inside soft-shelled eggs

M. Ellison/© AMNH

Researchers say they’ve solved the mystery of the missing dinosaur eggs

Fifteen years ago, Mark Norell came across a clutch of dinosaur eggs in southern Mongolia. Frozen in place were at least a dozen embryos of the Triceratops relative Protoceratops. But “something weird” was going on, says the paleontologist, who works at the American Museum of Natural History. It was as if the tiny dinosaurs were curled up inside invisible eggs. In the rock immediately surrounding each miniskeleton was an enigmatic white halo. Now, more than 10 years later, he and colleagues say they have solved the mystery: The dinosaurs had laid soft-shelled eggs.

Their new study of the Protoceratops clutch helps explain why dinosaur eggs are relatively rare—many laid soft-shelled eggs that were unlikely to fossilize. Their work may also have implications for how dinosaurs grew and tended their young. And a second study in Nature suggests large marine predators called mosasaurs went soft at breeding time, too.

The Protoceratops paper is convincing, says Ricardo Araújo, a paleontologist at the Technical Univesity of Lisbon’s Institute of Plasma and Nuclear Fusion. It emphasizes “how little we know about the incredible diversity of dinosaur reproduction strategies.”

Norell and molecular paleobiologist Jasmina Wiemann at Yale University analyzed two clutches of fossil dinosaur eggs—that of the 75-million-year-old Protoceratops from Mongolia, and a clutch laid about 215 million years ago by Mussaurus, a relative of long-necked giants like Diplodocus.

A crumpled soft-shelled egg that may have been laid by a mosasaur

Legendre et al. (2020)

They used a new technique that bathes the samples in laser light and records how the light changes as it interacts with the sample’s surface, offering clues to the chemical composition of the eggshells. The molecular fingerprint of modern soft eggshells differs from that of hard eggshells. The halos around the Protoceratops and Mussaurus eggs both showed a fossilized version of this “soft eggshell” fingerprint.

Mussaurus lived early in the age of the dinosaurs, so Norrell and Wiemann say it’s likely that the first dinosaurs laid soft-shelled eggs. But the Protoceratops nest shows some species were still laying soft eggs toward the end of the age of dinosaurs 66 million years ago, even as others evolved to lay hard-shelled eggs.

Dinosaurs laying soft eggs may have buried them, given that a several-ton adult dinosaur that attempted to sit on such a fragile clutch would probably end up with scrambled eggs. Burying soft-shelled eggs also keeps them from drying out, says ecologist Ricky-John Spencer at Western Sydney University.

If dino moms did bury their eggs, the clutches might experience lower temperatures and develop more slowly—leading to “a more advanced hatchling,” that may have required less parental care, suggests Charles Deeming, a biologist at the University of Lincoln. There’s some evidence for that already in pterosaurs, winged reptiles related to dinosaurs that also laid soft-shelled eggs. Last year, Deeming and his colleague, David Unwin at the University of Leicester, argued that baby pterosaurs were so well-developed that they could fly immediately after hatching.

The second Nature study suggests some ancient marine giants laid soft-shelled eggs that may have hatched just minutes after they were laid. In 68-million-year-old rocks in Antarctica that formed on the sea floor, paleontologist Julia Clarke at the University of Texas, Austin, and colleagues discovered a 30-centimeter-long object that has the size and appearance of a deflated football. Under a microscope, the flimsy and crumpled walls of the object have the layered structure of a reptile egg—which is what Clarke thinks the object is.

It is the second largest fossil egg ever found and may have belonged to a mosasaur, a 10-meter-long marine predator. Some snakes and lizards, living mosasaur relatives, lay eggs with thin shells that hatch almost as soon as they are laid. The new find suggests mosasaurs may have, too.