The evolution of the cleidoic egg was an important milestone in the history of the vertebrates, an innovation that enabled amniotes to colonize land. The complex structure the cleidoic egg added extramembryonic membranes (the chorion and the ammnion) and a shell that provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The shell may be either leathery or calcified. Early amniotes and more primitive tetrapods laid soft eggshells. Lizards, snakes, and pterosaurs, also laid soft eggs. Modern crocodilians and birds lay hard-shelled eggs. This feature has been interpreted as a key factor in their survival through the Cretaceous–Palaeogene extinction (approximately 66 million years ago).
Paleontological studies of this critical event have been greatly hampered by the poor early record of fossil eggs. Eggs from ornithopods, sauropodomorphs, titanosaurs and tetanuran have been reliably identified but most of these fossils are from the Cretaceous period. The bias in the egg fossil record cannot be explained solely by preferential preservation of certain nesting sites, as previously hypothesized. A new study by an international team of scientists lead by Mark Norell found that hard-shelled eggs evolved at least three times independently in dinosaurs.
The first description of dinosaur eggshells was made in 1859 by Jean-Jacques Pouech, a Catholic priest and amateur naturalist. Although he did not identify the eggshell as dinosaurian, but from a gigantic bird. The egg architecture of non-avian dinosaurs, crocodilian, extant birds, and turtles, is the same: an innermost shell membrane, a biomineralized protein matrix (both arranged in multiple layers), and an outer cuticle. The new study analyzed eggs from two very different non-avian dinosaurs: Protoceratops, a small plant-eater, and Mussaurus, a long-necked herbivore.
The Protoceratops specimen, from the the Ukhaa Tolgod locality (Campanian/Upper Cretaceous) in Mongolia, comprises a clutch of at least 12 eggs and embryos. The researchers found the presence of a diffuse black and white egg-shaped halo. Raman spectroscopy revealed the presence of protein fossilization products (PFPs) and phosphate (the white layer) in the Protoceratops eggshell. Additionally, they found that the PFPs in the Protoceratops eggshells contain relatively high amounts of S-heterocycles, which are characteristic of eggshell-derived organic matter.
Mussaurus patagonicus was originally described from several well-preserved post-hatchling specimens associated with egg remains found at Laguna Colorada Formation (Late Triassic/Early Jurassic) in Argentina. The histological evaluation of the Mussaurus eggshells revealed a dark brown, semi-transparent, apparently multilayered carbonaceous film, comparable to the Protoceratops soft eggshell.
The discovery of the soft nature of Protoceratops and Mussaurus eggs provides direct evidence for the independent evolution of calcified eggs in dinosaurs. This finding is supported by the recent description of several reproductives traits in theropod dinosaurs that differs considerably from that of derived ornithischians and sauropods, and may have played a key part in the Cretaceous–Palaeogene survival and radiation of modern birds.
Norell, M.A., Wiemann, J., Fabbri, M. et al. The first dinosaur egg was soft. Nature (2020). https://doi.org/10.1038/s41586-020-2412-8