Dinosaur Eggshells Had a Secret Until This Study Clutched It Opened

An extraordinary study in Science confirms that fossil eggshell structures once dismissed as geological quirks were actually a unique biological trait lost in birds

During my master’s at the Institut Català de Paleontologia in Sabadell, I got a rare behind-the-scenes look at how dinosaur eggshells are studied: sliced, scanned, and analyzed under microscopes that can see not just the texture, but the direction of every single crystal.

These weren’t just dusty museum pieces. They were biological records. And Catalonia, with its rich fossil beds, offered a front-row seat to one of the best-preserved dinosaur nesting grounds on the planet.

So when I saw the latest Science Advances paper by Dr. Zhang, Choi, and colleagues, I knew I had to cover the story.

The team had confirmed that strange, lesser-known structures found inside dinosaur eggshells, so-called “secondary eggshell units,” or SEUs, were not artifacts of fossilization. They were real, biological features. Formed by living dinosaurs. 

What makes these structures so unique is that modern birds no longer make them.

And just like that, something I’d once seen under the microscope had taken center stage in a story about evolutionary loss, physiology, and what it really meant to lay an egg 80 million years ago.

But what are SEUs, and why do they matter?

Let’s break it down. When you think of an eggshell, you probably imagine a single, continuous layer. But in reality, eggshells, especially in reptiles and birds, are built in units. The ones that form directly from the membrane (closer to the cute little growing embryo) are called primary eggshell units, or PEUs. These are the “normal” ones, the ones we see in birds, too.

SEUs, on the other hand, are a bit of a mystery. They’re crystalline structures that show up embedded within the calcified layer, sometimes superimposed over the primary units, sometimes tucked into pore canals (how embryos get oxygen for the growing dinosaur). 

In modern birds, they’re vanishingly rare. But in many dinosaur eggs, they’re everywhere.

For years, paleontologists debated whether SEUs were biological or just weird mineral patterns that formed as the eggs fossilized. But this paper closes the case.

But let’s take a step back to see how the researchers did the study. 

Dr. Zhang’s team took a wide array of fossil eggshells, including sauropods, hadrosaurs, and other non-avian dinosaurs, and compared them with modern turtle, crocodile, and bird eggs using a suite of high-resolution imaging tools: polarized light microscopy, scanning electron microscopy, and electron backscatter diffraction (EBSD).

Think of EBSD as a way to map the grain of a crystal, like seeing how the wood fibers run in a plank. This allowed the team to determine how these crystals formed, whether they were all aligned (a sign of biology) or messy and random (a sign of geology).

They also looked for grooves and vesicles, which are tiny cavities left by decayed organic fibers.

And what did they see?

SEUs weren’t random at all. Their crystal orientations matched the primary units. They showed signs of biological growth, including the grooves left by organic matter and porous textures only found in living tissues. They even had nucleation centers, tiny spots from which growth radiated outward, just like in biologically formed structures.

And here’s where things get interesting: the SEUs showed up in turtles and crocs, too. That means these weren’t exclusive to dinosaurs. But in birds, the only living dinosaurs, they’re mostly gone.

In other words, somewhere along the evolutionary line, birds lost this eggshell-building process. And this study gives us a rare glimpse of when, and possibly why, that happened.

I know what you’re thinking: why would evolution toss SEUs out?

The researchers suggest that early dinosaurs might have had less tightly controlled mineralization during egg formation. SEUs may have been a backup system: forming when PEUs stopped growing or when conditions were less than ideal. In some eggs, SEUs even filled in gaps, possibly to reinforce weak spots or regulate gas exchange through pores.

But as the bird lineage evolved, their eggshells became more refined: structured in distinct layers, with more precise timing and spatial control. In other words, the modern bird egg is a highly specialized product. And that shift might’ve made SEUs unnecessary or even incompatible with the new setup.

To put it simply, birds optimized their egg-making. Dinosaurs still had a bit of improvisation in the process.

And why does this matter?

This isn’t just about eggshells. It’s about biomineralization: how life turns raw materials into bone, shell, and armor. It’s a window into physiological evolution, showing us how complex systems simplify, specialize, and sometimes lose useful tools along the way.

And it challenges how we interpret fossils. For decades, paleontologists saw SEUs and wondered if they were even real. That hesitation makes sense: fossilization processes can play tricks on you. But now we know they’re not just real. They’re biologically meaningful.

The story also highlights how far methods have come. The fact that something invisible to the naked eye, something you need EBSD to confirm, can rewrite our understanding of egg-laying in dinosaurs is, frankly, a little wild.

I think back to those eggshells back in Sabadell. Holding one felt like holding a piece of a story too big to tell. Knowing that a little dinosaur once grew behind those layers,

But here we are, finally clutching open another chapter.

SEUs might sound like a minor detail, but they remind us of something much bigger: how evolution works in fits and starts, how not all progress is linear, and how sometimes, what gets lost is just as important as what’s gained.

And all of that was hiding in the shell of a long-lost egg.

Published in Fossils et al. Follow to learn more about Paleontology and Evolution.

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I’m thrilled you’re here. Stay curious, and thank you for sharing this journey with me!

Best,

Sílvia P-M, PhD Climate Ages