A new international study has confirmed that a mysterious feature found in dinosaur eggshells—called secondary eggshell units (SEUs)—are natural biological structures, not just the result of geological processes over time.
This finding helps solve a long-standing puzzle in paleontology and sheds light on how dinosaur eggs formed and evolved.
Reptiles and birds have eggshells made mostly of calcite, a type of crystal. These eggshells grow in layers.
The first layer, which forms directly on the egg’s inner membrane, is made of what’s known as primary eggshell units (PEUs).
However, some eggs—especially dinosaur eggs—also have SEUs, which form deeper within the shell.
Scientists weren’t sure if these SEUs were created by the animal itself (biogenic) or if they formed later through mineral changes after the egg fossilized (abiogenic).
To answer this, a team led by Dr. Zhang Shukang and Dr. Choi Seung from the Institute of Vertebrate Paleontology and Paleoanthropology in China analyzed dinosaur eggshells using advanced imaging tools, including electron microscopes and polarized light.
They compared these fossils to the eggshells of modern birds, turtles, and crocodiles.
What they found was remarkable: the SEUs in dinosaur eggs had crystal structures nearly identical to the primary layers.
They also matched the SEUs found in modern turtles and crocodiles, suggesting that dinosaurs developed these structures just like their modern relatives.
Some of these SEUs even showed tiny grooves and bubble-like features, which the researchers believe were once filled with organic materials that later broke down—another sign that the structures were made by living creatures, not by fossilization.
In some dinosaur eggshells, SEUs were even found growing into natural holes in the shell, called pore canals. Despite their odd positions, the crystals still grew in the correct orientation.
This discovery challenges a previous theory, based on bird eggs, that the direction of crystal growth was due to competition between nearby crystals. Instead, this study suggests that the growth direction may have been guided by soft organic fibers in the eggshell.
Interestingly, SEUs were common in the eggs of large plant-eating dinosaurs like sauropods and hadrosaurs, but rarely seen in theropods, the group that includes modern birds. This suggests that the process of making eggshells changed as some dinosaurs evolved into birds.
The study also shows that SEUs appear in several ancient animal groups—turtles, crocodiles, and many types of dinosaurs—meaning these structures may have evolved more than once. But it’s also possible that they share a common origin deep in evolutionary history.
Published in Science Advances, this research gives us a better understanding of how eggs evolved over millions of years and how much we still have to learn from the tiny crystals that once protected baby dinosaurs.
