There has been a lingering mystery in the fossilized dinosaur eggs. Although the structure superficially resembles that of modern reptiles' eggs, there has been ongoing debate as to whether it has always looked like this or if it formed during the fossilization process. Scientists have now put a period on this mystery.
An international research team, including Seoul National University, announced on the 31st that "after precisely analyzing dinosaur egg fossils excavated in China, Spain, and the United States, we have revealed that the unique eggshell structure was formed through a biological process." This means that dinosaur eggs were structurally similar to today's reptilian eggs from the beginning. The research findings were published in the international journal 'Science Advances' on the same day.
Eggshells are formed from the minerals that accumulate starting from the membranes surrounding the embryo. In contrast, the eggshells of typical birds grow as mineral crystals spread out in a branch-like shape from the membrane.
However, a unique structure called the "Secondary Eggshell Unit" was discovered in the dinosaur egg fossils. In the secondary eggshell unit structure, the branch-like mineral crystals initiate growth not only from the membrane but also from other locations. It resembles a 'fractal' structure, where a simple form is repeated to create a complex shape.
Such structures are also found in the eggs of some turtles and crocodiles. Naturally, the eggs of today's species are formed biologically. However, it was uncertain whether the secondary eggshell unit structure of dinosaur eggs was also biologically formed. Co-author Choi Seung, a research professor at Seoul National University's Institute for Basic Science, noted, "Researchers in East Asia, centered around China, viewed it as a biological structure, while European researchers interpreted it as a structure resulting from fossilization."
The research team compared dinosaur egg fossils with today's eggs of turtles and crocodiles using an electron microscope. Upon analyzing the crystal structure, it was revealed that the unique structure of dinosaur eggs is formed as mineral crystals grow on organic fibers. This is similar to the method of modern reptiles like turtles and crocodiles. The research team explained that if the structure had emerged during fossilization, the crystals would have grown randomly; however, all the crystals of the dinosaur eggs exhibited a consistent directionality in their growth.
Although the eggshell structures of turtles and crocodiles are similar, the types of minerals within them differed. The research team suggested, "While dinosaurs and modern reptiles share the same biological principles, the results may have varied depending on environmental and species characteristics."
In the egg fossils of the non-avian small dinosaur Maniraptora, the secondary eggshell unit structure decreased over time. All dinosaurs that are not ancestors of birds are referred to as non-avian dinosaurs. The research team interpreted that "as dinosaurs evolved into birds, the mechanism of eggshell formation also appeared to have become more sophisticated and systematic," suggesting that as a result, the secondary eggshell unit structure may have evolved in the direction of disappearing.
This discovery further supports the fact that dinosaurs are evolutionarily deeply connected to today's birds and reptiles. Shukang Zhang, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, stated that "the structure of dinosaur eggshells was confirmed to be not merely a result of fossilization, but part of a biological developmental process," adding that "this is an important clue in understanding the evolutionary relationships between dinosaurs and modern animals."
Professor Choi Seung said, "This research began during discussions with researchers at a dinosaur egg conference in China," adding that "despite many challenges due to the COVID-19 pandemic, we confirmed that analyzing modern organisms alongside paleontological samples can provide deeper insights, and we plan to continue with this type of interdisciplinary research in the future."
References
Science Advances (2025), DOI: https://doi.org/10.1126/sciadv.adt1879