The first dinosaurs were huge softies, judging by the shells of the eggs they laid, a new study reports.

Researchers from the American Museum of Natural History and Yale University analyzed the eggs of two different non-avian dinosaurs to find that they resemble those of turtles in terms of microstructure, composition, and mechanical properties.

Meaning that early dinosaurs likely laid soft-shelled eggs. Under pressure from predators or the environment, dinosaurs evolved hard-shelled eggs at least three independent times, the team adds.

Getting an egg up

The assumption has always been that the ancestral dinosaur egg was hard-shelled,” said lead author MarkNorell, chair and Macaulay Curator in the Museum’s Division of Paleontology. “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, advanced hadrosaurs like the duck-bill dinosaurs, and advanced sauropods.”

“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.”

Amniotes are a group of animals including birds, mammals, and reptiles, which reproduce by laying eggs with an inner membrane or “amnion”. This membrane helps keep the embryo from drying out. Some amniotes, the team explains, including many turtles, lizards, and snakes, lay soft-shelled eggs, whereas others (mostly birds) lay hard-shelled eggs. Their hardness comes from high levels of calcification in the shell.

Harder eggshells provide better protection from the environment and was a big development for amniotes, as it allowed for more eggs to survive until hatching in more varied environments.

Modern crocodilians and birds, the closest living relatives to the dinosaurs, lay hard-shelled eggs. The fact that soft-shelled eggs rarely fossilize also helped as it made it extremely difficult to study the transition from soft to hard eggshells. So, it was assumed that non-avian dinosaurs used this type of shell.

But this was not the case, a new study reports. The authors studied embryo-containing fossil eggs of Protoceratops (a sheep-sized herbivore) and Mussaurus (a long-necked, big herbivore dino).

The skeletons of six among the Protoceratops embryos have been preserved surrounded by a black-and-white, egg-shaped halo, according to the team. Two of them (potentially hatched) were largely free of this halo. A closer analysis showed that the shapes were created from chemically-altered residues of the membrane lining the inside of all modern archosaur eggs. The same was true for the Mussaurus embryos.

Comparing the minerals that made up these shells with those in the eggshells of modern species, the team determined that the Protoceratops and Mussaurus eggs were indeed non-biomineralized, meaning they were leathery-soft.

“It’s an exceptional claim, so we need exceptional data,” said study author and Yale graduate student Jasmina Wiemann. “We had to come up with a brand-new proxy to be sure that what we were seeing was how the eggs were in life, and not just a result of some strange fossilization effect.”

“We now have a new method that can be applied to all other sorts of questions, as well as unambiguous evidence that complements the morphological and histological case for soft-shelled eggs in these animals.”

Using chemical composition data and the mechanical properties of eggshells from 112 other extinct and living relatives, the team determined that calcified eggs evolved independently at least three times in dinosaurs, probably from a soft-shelled type.

Soft eggshells are more sensitive to water loss and offer little protection against mechanical stress, so Protoceratops and Mussaurus probably buried them in sand or soil and incubated them with heat from decomposing plant matter, as some reptiles do today.

The paper “The first dinosaur egg was soft” has been published in the journal Nature.