Soft-shelled eggs from dinosaurs and Antarctica

It’s a big day for big egg stories – as in both the eggs and the stories are big.

In separate papers in the journal Nature, one team of scientists reports on what it says is the second-largest egg of any known animal ever found, and another suggests that, contrary to established thought, the early dinosaurs laid soft-shelled eggs.

Dinosaur eggs
An artist’s interpretation of a baby mosasaur emerging from an egg in the shallow waters of Late Cretaceous Antarctica. Credit: John Maisano, UTA

The egg in the first paper is also soft-shelled, and at around 28 by 18 centimetres it pushes the limits of how big scientists thought such eggs could grow.

It is the first fossil egg ever found in Antarctica and the authors believe it was laid by an extinct giant marine reptile, such as a mosasaur – a discovery that challenges the prevailing thought that such creatures did not lay eggs.

“It is from an animal the size of a large dinosaur, but it is completely unlike a dinosaur egg,” says lead author Lucas Legendre from the University of Texas at Austin (UTA), US. “It is most similar to the eggs of lizards and snakes, but it is from a truly giant relative of these animals.”

Co-author David Rubilar-Rogers, from Chile’s National Museum of Natural History, was one of the scientists who discovered the fossil in 2011, describing it as looking like a deflated football. Neither Chilean nor visiting geologists could work out what it was until UTA’s Julia Clarke suggested it might in fact be a deflated egg.

Studies with a suite of microscopes confirmed this and revealed a structure similar to transparent, quick-hatching eggs laid by some modern snakes and lizards. Comparisons with data from 259 living reptiles suggested an ancient marine reptile, an idea supported by the number of skeletons of baby mosasaurs and plesiosaurs in the area.

“Many authors have hypothesised that this was sort of a nursery site with shallow protected water, a cove environment where the young ones would have had a quiet setting to grow up,” Legendre says.

The paper does not discuss how the ancient reptile might have laid the eggs, but the researchers suggest two possible options: the eggs hatched in open water, which is how some species of sea snakes give birth, or the mother deposited the eggs on a beach and hatchlings then scuttled into the ocean like baby sea turtles.

Dinosaur egg
The fossil egg, its parts and relative size. Arrows point to its folds and surrounding sediment, shown as light grey. Credit: Legendre et al. (2020)

In the second study, researchers led by the American Museum of Natural History (AMNH) and Yale University used a suite of geochemical methods to analyse the eggs of two vastly different non-avian dinosaurs and found that they resembled those of turtles in their microstructure, composition, and mechanical properties.

This leads them to suggest that the first dinosaurs laid soft-shelled eggs and that hard-shelled eggs evolved at least three times independently in the dinosaur family tree.

“The assumption has always been that the ancestral dinosaur egg was hard-shelled,” says AMNH’s Mark Norell, the lead author.

“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.

“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.”

Dinosaur eggs
The clutch of fossilised eggs and embryos in the Protoceratops specimen. Credit: M Ellison/ AMNH

Norell and colleagues from the US, Argentina and Canada studied well-preserved embryo-containing fossil eggs belonging to two species of dinosaur: Protoceratops from Mongolia and Mussaurus from Argentina.

When they chemically characterised the samples, they found chemically altered residues of the proteinaceous eggshell membrane that makes up the innermost eggshell layer of all modern archosaur eggshells.

When they compared the molecular biomineralisation signature of the dinosaur eggs with eggshell data from other animals, including lizards, crocodiles, birds, and turtles, they determined that the Protoceratops and Mussaurus eggs were leathery and soft.

“[T]he non-biomineralised, soft nature of both Protoceratops and Mussaurus eggs provides direct evidence for the independent evolution of calcified eggs in dinosaurs,” they write in their paper.

“This discovery ties in with recent findings of several reproductive traits, such as egg colour, paternal nest car and open nest structures, that are confined to theropod dinosaurs, representing an independent lineage of eggshell evolution. “

Because soft eggshells are more sensitive to water loss and offer little protection against mechanical stressors, such as a brooding parent, the researchers propose that they were probably buried in moist soil or sand and then incubated with heat from decomposing plant matter, similar to some reptile eggs today.

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