Some of the first dinosaurs likely laid soft-shelled eggs, challenging previous work suggesting that all dinosaurs laid hard eggs. The findings, which are published in Nature, are cluing researchers into the evolution of amniotes.
Amniotes are a group of animals that includes birds, mammals, and reptiles that produce eggs with an inner membrane known as an “amnion,” which prevents the embryo from drying out. Until now, it was believed that early dinosaurs laid hard-shelled eggs.
"The assumption has always been that the ancestral dinosaur egg was hard-shelled," said lead author Mark Norell, chair and Macaulay Curator in the American Museum of Natural History's Division of Paleontology, in a statement.
"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."
Some animals lay soft-shelled eggs, like turtles, while others lay hard, calcified eggs, such as birds, which suggest divergent evolutionary paths between the animals. Most dinosaur eggs are characterized by an innermost membrane, a calcite shell, and an outer waxy cuticle. To date, just three different dinosaur clades “differ markedly” in the eggs that they lay, and, just like egg-laying animals today, characterizing eggs can be an important indication in the role of species survival and evolution. But until recently, soft-shelled eggs were rarely found in the fossil record because they do not preserve as easily as those with hard shells.
The team studied embryos found in the eggs of two different avian dinosaurs. A fossilized Protoceratops clutch with at least a dozen eggs and embryos with preserved, nearly complete skeletons was found in Mongolia and compared against several Mussaurus eggs with preserved juvenile specimens found in the Laguna Colorada Formation of Patagonia, Argentina. Close examination of the insides of the eggs using a petrographic microscope and a chemical characterization found that the eggshell membrane of both species was the same as all eggs from modern archosaurs, the ancestors of birds, and crocodiles. When further compared against modern eggs from lizards, crocodiles, birds, and turtles, the researchers find that the eggs were “non-biomineralized” and likely leathery and soft.
Using this data, the researchers then constructed a “supertree” of evolution tracking how eggshells have changed throughout time. The “exceptional claim” challenges the idea of a “single evolutionary origin of hard-shelled dinosaur eggs” suggesting that hard-shelled eggs evolved independently at least three times in dinosaurs, and very likely evolved from soft-shelled eggs.
"From an evolutionary perspective, this makes much more sense than previous hypotheses, since we've known for a while that the ancestral egg of all amniotes was soft," said study author and Yale graduate student Matteo Fabbri. "From our study, we can also now say that the earliest archosaurs – the group that includes dinosaurs, crocodiles, and pterosaurs – had soft eggs. Up to this point, people just got stuck using the extant archosaurs – crocodiles and birds – to understand dinosaurs."
However, soft eggshells are more sensitive to water and offer less protection against environmental stressors, which not only means that they were likely buried in sediment and incubated there but were also less likely to be preserved as fossils for modern study.