Early dinosaurs and marine reptiles left evidence of soft-shelled egg-laying, according to a new study published in Nature. The study includes analysis of findings from two early dino species—Protoceratops and Mussaurus—that were compared to other diapsids. Egg evolution helped reptiles spread further across land.
Scientists from Yale University, the American Museum of Natural History, and other institutions have discovered soft-shelled early dinosaur eggs.
“Here we present mineralogical, organochemical, and ultrastructural evidence for an originally non-biomineralized, soft-shelled nature of exceptionally preserved ornithischian Protoceratops and basal sauropodomorph Mussaurus eggs,” the study said. “Statistical evaluation of in situ Raman spectra obtained for a representative set of hard- and soft-shelled, fossil and extant diapsid eggshells clusters the originally organic but secondarily phosophatized Protoceratops and the organic Mussaurus eggshells with soft, non-biomineralized eggshells.”
In other words, the team determined that the state of the eggs had not changed from soft to hard, or vice-versa, due to any breakdown in matter over the millions of years since their inhabitants were born—they were laid soft and stayed soft.
How did the scientists determine this? By using the eggs of other diapsids—a broader grouping of reptile that includes some early dinosaurs—that lived during the same time. “Through an ancestral-state reconstruction of composition and ultrastructure, we compare eggshells from Protoceratops and Mussaurus with those from other diapsids, revealing that the first dinosaur egg was soft-shelled,” the study said.
As amniotic eggs evolved, they made it possible for reptiles like diapsids to spread further across land.
Reptiles evolved from amphibians approximately 300 million years ago. While amphibians had to spend considerable portions of their lives underwater, reptiles did not. Part of this was due to the eggs reptiles laid.
“[Reptiles] have internal fertilization with the female receiving sperm placed directly inside her by males,” said Dr. Anthony Martin, Professor of Practice in the Department of Environmental Studies at Emory University. “There’s growth of a membrane around each fertilized egg and development of other organs enclosed by this membrane. An extended time of embryonic growth happened inside each egg and that was followed by emergence through hatching.”
Unlike amphibians, reptiles undergo their physiological metamorphoses entirely inside their eggs. As Dr. Martin said, all that happens after they hatch is growth. So whereas amphibians depend regularly on water throughout their life cycles, reptiles could go much further away from bodies of water. Another important factor in reptiles’ abilities to thrive on land is the composition of the eggshells, which are most clearly distinguished by whether they’re leathery or mineralized.
“Leathery eggs have more of a tough, organically-based covering, whereas these mineralized eggs contain a little bit of calcite, which is calcium carbonate combined with organics,” Dr. Martin said. “Eggshells, regardless of whichever material is making them up, are porous and semipermeable. That allows for respiration of the developing embryo while also retarding water loss.”
Finally, hard-shelled eggs are obviously tougher and have broken down less in the last 300 million years. Dr. Martin said that some of the reason soft-shelled eggs have been so difficult to find is because of how much they tend to break down, leaving holes in the fossil record that are difficult to piece together. However, thanks to this new research, scientists may have another clue in assembling a clear picture of the past.
Dr. Anthony Martin contributed to this article. Dr. Martin is Professor of Practice in the Department of Environmental Studies at Emory University, where he has taught courses in geology, paleontology, environmental science, and evolutionary biology since 1990. He earned his BS in Geobiology from St. Joseph’s College (Indiana), MS in Geology from Miami University (Ohio), and PhD in Geology from the University of Georgia.