Dinosaurs laid eggs with fascinating shells that hold clues to how these creatures lived. In many non-avian dinosaur eggs, scientists have noticed extra little crystal clusters called secondary eggshell units (SEUs). Unlike the main eggshell structures that grow from the membrane, these SEUs appear within the shell itself. For years, researchers debated whether these weird formations were created by the dinosaurs themselves or simply formed later through geological processes.
Eggshells, like bones and teeth, are made of minerals and proteins, and they tend to fossilize very well. That means paleontologists can study their tiny features to learn about an animal’s biology long after it has disappeared. Modern turtles and crocodiles sometimes have SEUs too, but bird eggs almost never do.
Finding SEUs in dinosaur eggs raised questions: were these features a normal part of how dinosaurs built their shells, or did they appear after burial, thanks to mineral-rich waters seeping through the fossils?
An international team led by Dr. Zhang Shukang and Dr. Choi Seung from the Chinese Academy of Sciences decided to settle the debate. They examined dinosaur eggshell fragments under powerful microscopes, including electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM), to map out how the crystals were oriented inside each unit.
To have a comparison, the researchers also studied eggshells from living birds, turtles, and crocodiles. By looking at these modern examples, they could see whether the SEUs in dinosaur eggs behaved like the biogenic units in today’s reptiles or like random mineral deposits.
What they found was striking: the crystal orientations in dinosaur SEUs matched almost exactly those in primary eggshell units (PEUs) and in SEUs from modern turtles and crocodiles. In other words, these secondary units looked like they had grown under biological control, not just formed by chance. The team also spotted tiny grooves and vesicles, small holes left behind when organic fibers decayed, just as you would see in bird eggs. Those features strongly suggest that SEUs began as living parts of the eggshell, anchored by organic fibers, rather than as post-mortem mineral growths.
Another clue came from looking at shells where pore canals (tiny channels for gas exchange) were well preserved. Sometimes SEUs were found growing right inside these canals or overlapping the main shell units. Even so, the crystals’ long axes pointed in the same direction as the rest of the shell, which goes against the idea that they simply competed for space with neighboring crystals. Instead, it seems that organic matrix fibers guided their growth, hinting that dinosaurs used a different eggshell-building strategy than modern birds, where such secondary units are extremely rare.
When the team surveyed various dinosaur families, they noticed that SEUs appeared often in sauropods and hadrosaurs but nearly disappeared in maniraptoran theropods—the group that eventually gave rise to birds. This pattern suggests an evolutionary shift in how eggshells were made. Early dinosaurs might have formed multiple crystal centers at once, but over time, as birds evolved, they refined their eggshell chemistry to avoid creating extra units.
Whether SEUs popped up independently in turtles, crocodiles, and different dinosaur branches or share some deeply rooted genetic toolkit remains an open question, but the study’s findings tip the scale toward a shared, ancient origin.
The study has been published in Science Advances.
