American Scientist

A long-standing mystery in paleontology is why we have discovered thousands of eggs from some types of dinosaurs, such as the duck-billed hadrosaurs, but none at all from other types of dinosaurs, such as the armored stegosaurs. For more than a century, most paleontologists hypothesized that all dinosaurs laid hard-shelled eggs. This assumption seemed like a safe one because the closest living relatives of dinosaurs, crocodilians and birds, also lay hard-shelled eggs. In 2020, however, that assumption was completely overturned, opening an exciting new realm of research on dinosaur reproduction.

That revelation—that some dinosaurs laid soft-shelled eggs—and other recent discoveries about dinosaur nests offer vivid glimpses into the lives of long-dead dinosaurs, from egg to parenthood, and show the complicated ways that dinosaurs both resembled and differed from their modern relatives. Narratives about dinosaur research are often framed in the context of new discoveries revealing more and more birdlike anatomy and behaviors. In the case of dinosaur reproduction, the true story is much blurrier.

The Case of the Missing Eggs

The road to our current understanding of dinosaur nesting began long ago. When the first species of dinosaur, Megalosaurus bucklandi, was named in 1821, paleontologists knew almost nothing about how dinosaurs reproduced. In 1921, the first major clue emerged: Acclaimed fossil hunter Roy Chapman Andrews discovered intact dinosaur nests in Mongolia. In fact, dinosaur eggs had already been found in France in 1859, but were mistakenly thought to belong to giant birds at the time.

Studying the beautiful elongated oval eggs, Andrews concluded the small ceratopsian dinosaur Protoceratops must have laid them. A relative of much larger horned ceratopsians such as Triceratops, Protoceratops was far and away the most common dinosaur around the excavation site, so it seemed likely the nests belonged to that group. When a skeleton of an unusual beaked theropod dinosaur was found next to one nest, Andrews assumed that it had died pilfering Protoceratops eggs. This unusual dinosaur was dubbed Oviraptor, meaning “egg thief.” For the next 70 years, Oviraptor stood falsely accused of stealing eggs.

Then in 1993, a team led by Mark Norell of the American Museum of Natural History returned to Andrew’s old fossil hunting grounds and uncovered a truly remarkable skeleton of Citipati, a close cousin of Oviraptor, brooding a clutch of elongated eggs. The alleged Protoceratops eggs turned out to have belonged to oviraptorid theropods the whole time, and the formerly unpopular Oviraptor was reimagined as a caring parent.

This discovery, however, created a new mystery: If all those eggs belonged to relatively rare oviraptorids, why had we never found a single egg from the much more abundant Protoceratops? Almost a century after Andrews’s discovery of the misidentified oviraptorid nest, Norell finally uncovered the first real Protoceratops eggs. But something about the eggs puzzled him. Well-preserved embryos were curled up in each egg, but the eggs themselves were barely visible, appearing almost like halos encircling the tiny skeletons. Norell invited molecular paleontology expert Jasmina Wiemann, now at the University of Chicago, to study the chemical structure of the Protoceratops eggs.

In 2020, they found that the eggs originally had soft, nonmineralized shells similar to most modern turtle eggs. Parts of the soft eggshell had become phosphatized after burial in the sediment, preserving them from destruction and resulting in the halolike appearance. This discovery explained the mystery of the missing Protoceratops eggs: Soft-shelled eggs were very unlikely to be preserved in the fossil record in most circumstances.

All of a sudden, it was clear why the fossilized eggs of some types of dinosaurs were common, but no eggs at all had been discovered for other types of dinosaurs. Hadrosaurs, sauropods, and many theropods laid eggs with hard, calcite-rich shells. The calcite portion of a hard-shelled egg is essentially “prefossilized” since the mineral calcite can remain stable for hundreds of millions of years. By contrast, the organic components of soft eggshell degrade quickly under most conditions. Hence, an Oviraptor egg had a much better chance of showing up in the fossil record than a Protoceratops egg.

People are never going to see a live nonavian dinosaur, but thanks to discoveries like these we are now learning previously unimaginable details about dinosaur family life. Dinosaurs were remarkable creatures. Although we tend to emphasize the shared features of dinosaurs and birds, dinosaurs in fact possessed a mixture of birdlike traits, reptilian traits, and unique traits not seen in either group. In the case of Protoceratops, finding out that its eggs were soft was completely unexpected since both birds and crocodilians lay hard-shelled eggs. This discovery also unlocked new details of Protoceratops nesting behavior. Soft-shelled eggs are more sensitive to the environment, because they lose moisture easily in dry conditions. In addition, parents could not sit directly on top of them without risking a crushed shell. Given these limitations, Protoceratops likely buried its eggs in moist sediment and left them to be incubated by external heat sources such as decaying plants or sunlight.

As Norell and Wiemann studied more eggs from species on different branches of the dinosaur family tree, a startling implication emerged: The very first dinosaur egg was probably soft as well. Norell and Wiemann observed that soft eggs were laid by the ancestors of the giant long-necked sauropod dinosaurs, as well as by the winged pterosaurs, which are considered by most paleontologists to be close relatives of dinosaurs. This finding suggests that the earliest dinosaurs were limited to moist nesting environments. Other groups of dinosaurs evolved a hard calcite shell that locked in moisture and allowed them to nest in a wider range of environments. This development would have offered a major advantage, and at least three separate dinosaur lineages evolved hard-shelled eggs independently: theropods, sauropods, and hadrosaurs.

Colorful Clutches

Hard-shelled dinosaur eggs also have impressive stories to tell, revealing the complex diversity of reproductive biology and behaviors among dinosaurs. Today’s bird eggs have a remarkable array of patterns. Sandpipers lay eggs with exuberant spots and swirls reminiscent of a Jackson Pollock painting. Robins lay baby blue eggs the same shade as the sky on a pleasant spring day. The brilliant lacquered green eggs of the elegant crested tinamou look almost ceramic. By contrast, crocodilians lay plain, unpatterned eggs. Colorful eggs make no difference in buried nests, where they can’t be seen. But for birds that lay eggs in open nests, color can camouflage the clutch from predators or differentiate parents’ eggs from those of other species. For much of the field’s history, paleontologists had no way of knowing whether dinosaur eggs shared the plain egg palette of crocodilians or the more colorful palette of bird eggs. Then, Wiemann discovered that secret traces of color were locked deep in the dozens of blanched and stained fossil eggs in museum drawers, awaiting the day scientists figured out how to unlock them.

Using mass spectroscopy, Wiemann identified pigments called protoporphyrin and biliverdin in the eggs of oviraptorids and other dinosaurs. These two versatile pigments combine in different ways to make up the palette of colors found in modern bird eggs. Examining chemical signatures in layers of eggshell, Wiemann reconstructed the color patterns of dozens of dinosaur eggs. Her work yielded startling results that showed that dinosaur egg morphology and nesting behavior had diversified prolifically from the simple buried nests full of plain, soft-shelled eggs that Protoceratops had left behind.

The oviraptorid Heyuannia and the famous sickle-clawed dromaeosaur Deinonychus turned out to have laid colorful blue-green eggs. Some troodontid eggs that Wiemann tested were found to have been brown, while others were white with speckled or spotted patterns. By contrast, all non-theropod dinosaurs seem to have lacked eggshell pigments. The study’s reconstructions of sauropod and hadrosaur eggs showed their color was plain white.

At least three separate dinosaur lineages evolved hard-shelled eggs independently.

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The fact that eggshell pigments are detected only in theropods provides another piece of evidence (in addition to features like feathers and a wishbone) that this group of dinosaurs gave rise to modern birds. Birds are the only living amniotes that lay colored eggs, making it likely that eggshell pigments evolved a single time in an ancestor of birds and advanced theropods.

Fossil egg color also holds clues to nesting behaviors. The evolutionary journey from the simple nest of an early dinosaur (essentially a hole scooped in soil) to the meticulously constructed nests of modern birds such as weaver finches is a long one. Oviraptorids represent the first big step—they differed from earlier dinosaurs in leaving their eggs partially exposed in a shallow excavation scraped into the ground.

The spectacular nesting Citipati fossil mentioned earlier provides some of the most remarkable evidence of how these dinosaurs incubated their eggs. The large adult skeleton is preserved at the center of a ring of eggs, with its arms wrapped around the precious clutch. This Citipati parent was shielding the eggs when it perished in a sandstorm. But, the eggs are widely spaced, and it appears the adult avoided sitting directly on top of them, possibly to prevent crushing them. Oviraptorids like Citipati seem to have covered their nests with their feathered arms to insulate them, but avoided direct body contact.

Troodontids, which occupy a branch closer to birds on the evolutionary tree, developed even more advanced nesting strategies. Troodontid nests from North America show an arrangement with the eggs closer to the center. This would allow the brooding parent to cover the entire clutch directly with its belly, warming the eggs using a behavior known as contact incubation just as most modern birds do.

The new incubation strategies explored by oviraptorids and troodontids required the eggs to be partially exposed at the surface, leaving them visible for the first time in dinosaur evolution. The appearance of colored eggs coincides with the evolution of these partially open nests, and so may have been driven by new selective pressures. Brown speckled eggs, for example, may have been better camouflaged from predators when the parents left the nest to feed.

The advantage of baby blue eggs is a bit more uncertain. Some modern birds use egg colors to help fight brood parasites such as cuckoos, by making any imposter eggs more obvious so that they can be discarded by the parents. For now, we can only speculate as to the existence of dinosaur brood parasites, but the idea of a furtive oviraptorid sneaking an egg into another dinosaur’s nest is satisfying to ponder. (See the 2022 digital feature “Putting Eggs in Many Baskets.”)

Fossil Egg Timers

With the discovery of more dinosaur eggs and embryos, paleontologists can start to reconstruct what was happening while those eggs sat in nests millions of years ago. Higher incubation temperatures speed up embryonic development, which shortens the length of time when eggs are vulnerable. Birds are masters of speeding up hatching, using their body heat and rotating their eggs in the nest to usher their hatchlings into the world more quickly. Accounting for body size (larger eggs take longer to incubate), birds complete egg incubation roughly twice as fast as crocodilians. For a long time, paleontologists lacked a reliable way to determine whether dinosaurs had long incubation times like crocodiles or short incubation times like birds.

Parental care becomes especially important in challenging environments, and some dinosaurs raised their young in extreme conditions.

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A cutting-edge idea recently unlocked a hidden timestamp on dinosaur embryos. As teeth grow, cells called odontoblasts inject a tiny sliver of dentin below the outer enamel layer each night. The dentine hardens overnight, forming a ring called a von Ebner line. The number of these lines thus indicates the age of a tooth. For adult dinosaurs, von Ebner lines can reveal how quickly teeth were replaced. But for embryonic dinosaurs, they can reveal the age of the embryo itself. Like crocodiles, dinosaurs began growing their teeth while still inside the egg. So, counting up the von Ebner lines in the teeth of nearly hatched dinosaur embryos—and adding a little extra time to account for the fact that teeth usually start forming several weeks after the egg is laid—can reveal how many days a fossil embryo had been inside the egg.

Greg Erickson of Florida State University had studied von Ebner lines in adult dinosaurs before, and recently he has turned his focus to babies. Thanks to increasingly powerful CT scanners and new fossil embryo discoveries, Erickson’s team was able to count von Ebner lines in those precious Protoceratops fossils that had revealed the soft-shelled nature of the ancestral dinosaur egg. In 2017, Erickson’s tooth scans revealed that the eggs would have hatched approximately 83 days after being laid. Erickson’s team found that the giant duck-billed hadrosaur Hypacrosaurus took about twice as long to incubate its eggs at 173 days, which follows the general pattern of larger animals having longer incubation times. These plant-eating dinosaurs were thus more similar to crocodilians in their growth patterns than to modern birds.

Evolution doesn’t always align with easy assumptions about early and modern traits.

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Using similar methods, another team led by Dave Varricchio of Montana State University found in 2018 that troodontids were a bit faster, with a 74-day incubation time that falls right between the times predicted for a bird or crocodile of the same body size. What about Oviraptor? Unfortunately, von Ebner lines cannot help us study this dinosaur, because it had no teeth.

Dinosaur Day Care?

Even though Protoceratops laid soft-shelled eggs in turtle-like nests, it also showed hints of attentive nesting behaviors like modern birds exhibit. This mosaic of traits is part of what makes dinosaurs so interesting and surprising: Evolution doesn’t always align with easy assumptions about early and modern traits. David Fastovsky of the University of Rhode Island reported a Protoceratops nest that lacked eggs, but contained 15 juveniles tumbled together in a nesting burrow. The young Protoceratops were relatively large compared to hatchlings, and so must have been hanging around the nest together for many weeks, rather than scattering to the wind like many modern reptiles do after emerging from the egg. A similar aggregation of babies is also known for Psittacosaurus, an earlier relative of Protoceratops. In the case of Psittacosaurus, a five-year-old subadult was found in a nest burrow with 24 smaller babies. The larger individual was too young to be a parent, and so may have been an older sibling watching over the young.

Parental care becomes even more important in challenging environments, and new discoveries show that some dinosaurs raised their young in extreme conditions. One of the ultimate frontiers in understanding dinosaur nesting is the High Arctic. Back when all dinosaurs were assumed to have ectothermic metabolisms, paleontologists concluded they would not be able to survive high latitude winters. Today, we know many dinosaurs thrived farther north than we ever imagined was possible. During the Cretaceous Period, North America was even closer to the North Pole. Dinosaurs ranged into the northernmost reaches of present-day Alaska, reaching latitudes beyond 80 degrees north, far above the Arctic Circle (66.33 degrees north).

Since 2009, field teams led by Patrick Druckenmiller of the University of Alaska, Fairbanks, have collected a treasure trove of High Arctic fossils including hadrosaurs, ceratopsians, and troodontids. Even more remarkably, the team discovered bones of juveniles from each of these groups. In many cases, the tiny teeth and bones fit on the head of a pin, indicating that the dinosaurs were truly recent hatchlings, some no bigger than a guinea pig.

During the Cretaceous, northern Alaska would experience more than 80 days of continuous darkness during the winter. Because of this harsh season, paleontologists had proposed that the Arctic dinosaurs enjoyed the sunny summer months but migrated south before the Sun disappeared in October. The discovery of juvenile fossils in Alaska suggests that many species instead stayed put for the polar winter.

Druckenmiller and Erickson teamed up to combine polar daylight models and incubation estimates and concluded there simply wasn’t enough time for the polar dinosaurs to undertake a major migration. The Alaskan hadrosaur Ugrunaaluk needed to incubate its eggs for nearly six months. Even if this species began nesting at the very start of spring, the calendar left almost no time between hatching and the onset of winter darkness. Druckenmiller’s team concluded that its young would have been too tiny to trek the roughly 2,500 kilometers required to get south of the Arctic Circle before the Sun vanished.

Combined with the evidence from other hadrosaur species that parents tended to their hatchlings, a remarkable picture emerges: Ugrunaaluk herds may have carefully tended their babies through many weeks of polar darkness, helping them forage on “leftover” bark, ferns, and moss under the northern lights. Such a lifestyle may have been possible due to the warmer Cretaceous climate, which would have meant snowy conditions for part of the year but less extreme Arctic temperatures than those that occur today.

Hard Eggs to Crack

The evolution of dinosaur nesting shows the diversity of strategies that arose during the millions of years that these animals walked the Earth. Some recent discoveries emphasize the similarities between theropods and birds, such as colored eggs and evidence of tending nests and young. On the other hand, some dinosaurs were far from avian in their nesting ecology, burying soft-shelled eggs in the ground like sea turtles to incubate on their own over extended periods. One thing is certain—at the rapid pace of research innovation seen over the past few years, we can expect to learn more secrets of how dinosaurs nested and raised their young in the near future.