How Did Dinos Lay Eggs and What Were Dinosaur Eggs Like?

When the first fossilized fragments of dinosaur eggshells were discovered and described around 1860, most individuals — including most scientists — were wholly unaware of what the dinosaurs were, not to mention the ways that the dinosaurs were born. It wasn’t until 1920 or so that scientists described dinosaur eggshells as dinosaur eggshells for the first time.

In the years since then, the occasional discoveries of eggs and eggshells have become one of the best windows into the way that baby dinosaurs burst into the world, many millions of years ago.

Paleontologists are inclined to think that every one of the extinct dinosaurs emerged out of an externally laid egg, whether a triceratops, a tyrannosaurus, a stegosaurus or any other type of dinosaur. In fact, though there aren’t any sure signs in the fossil record that any of these animals produced live progeny, there are an abundance of indications that they laid embryo-filled eggs.

So, here's all the dirt that paleontologists have discovered about dinosaur eggs, based on their fossil finds.

Today’s paleontologists tend to agree that different dinosaur species laid different amounts of eggs. While some species produced around 1 to 5 in a single sitting, others produced around 15 to 20. Paleontologists have even discovered one type of dinosaur, the psittacosaurus, that laid around 35 eggs at a time.

Laying large clutches of offspring constituted a common practice among the dinosaurs, and an advantageous one at that. Because predatory dinosaurs had a habit of devouring almost any eggs that they could find, laying a lot of eggs all at once became one of the best ways for female dinosaurs to ensure that at least one hatched successfully.

Read More: How Did Dinosaurs Mate?

Even despite the egg-eating habits of predatory dinosaurs, the prevalence of such big batches of offspring would suggest, in theory, that the world is filled with the fossils of intact dinosaur eggs. But paleontologists find these specimens only occasionally in practice, since they tended to shatter during the process of preservation.

As a result, much more common than intact dinosaur eggs are the broken bits of dinosaur eggshells, which are strewn throughout the sediment of paleontological sites throughout the world.

Read More: Why One Extraordinary Collection of Dinosaur Embryos Remains Locked Away From Science

Whether they are intact or in fragments, scientists struggle to determine the identity of dinosaur eggs when they aren't discovered in the immediate vicinity of their fossilized parents. (Even then, identification can still pose problems, with the parents of the potential offspring sometimes being mistaken as predators foraging for food.)

That being said, the basic traits of an egg can provide paleontologists with broad clues as to its parentage, with the traits of theropod and sauropod eggs often sitting at opposite ends of the structural and aesthetic spectrums.

Take, for instance, the size of the eggs. Scientists say that adult dinosaurs laid eggs in a wide variety of sizes, with those of the theropods being both smaller and larger than those of the sauropods.

Oddly enough, though it may seem sensible to assume that the largest dinosaurs — the lofty, leaf-loving sauropods — laid the largest eggs, there isn't always a clear connection between the size of the dinosaurs and the size of their eggs. Instead, the theropods from the oogenesis, Himeoolithus and Macroelongatoolithus, laid the smallest and largest dinosaur eggs ever discovered, which are around 2 and 24 inches in length.

And that isn't the only thing that distinguished the different dinosaurs-to-be. In fact, scientists say that adult dinos also laid eggs in a wide variety of shapes, too, with those of the theropods being oblong and those of the sauropods being spherical. According to these scientists, this difference was probably due to the different terrains that the theropods and sauropods inhabited.

Size and shape aren't even the extent of the differences between these fossilized specimens. In 2018, an analysis from Nature found an array of different colors and coloration patterns in the eggshells of an assortment of different dinos. And while the theropods’ eggs tended to be bright blue and green, the sauropods’ tended to be brown, beige and white.

According to the authors of this analysis, these colors did much more than differentiate between the offspring of the theropods, sauropods and several other types of dinosaurs. They also protected the progeny from predators, with some shades acting as a form of camouflage.

There’s also the question of texture.

Paleontologists traditionally thought that the dinosaurs produced porous, hard-shelled eggs, and there was no wonder why. For years, the only eggshells that they’d discovered had been porous and hard. But studies have since shown that the texture of these shells varied substantially, depending on the type of dinosaur that produced them.

Though dinosaur eggs are all porous, allowing for the transfer of oxygen and other sustaining substances through the surface of the shell, these pores show serious diversity in terms of their shape, size and density. In fact, a study from 2015 says that the preserved eggshells of the theropods are much more porous than those of the sauropods, due, once again, to the distinctions between their surroundings.

Even more muddled is the malleability of dinosaur eggs. According to an analysis from 2020, an abundance of dinosaurs actually laid leathery, soft-shelled eggs, in spite of their rarity in the fossil record. And though these soft-shelled specimens were less likely to survive the preservation process, the analysis went as far as suggesting that they were the standard throughout the early evolution of the dinosaurs.

Read More: Here’s What Dinosaurs Really Looked Like

All in all, scientists suppose that these traits allude to the ways that the dinosaurs behaved toward their broods. For instance, the roll-resistant, oblong shapes, the camouflage colors and the semi-porous shells of the theropods’ eggs suggest that these dinosaurs organized their offspring in open nests. But the absence of these traits among the sauropods’ eggs suggests that they arranged their offspring in closed cavities, instead.

Furthermore, while the hard-shelled structure of some species’ eggs would have been able to withstand the weight of a parent, the soft-shelled structure of others wouldn’t have, preventing those parents from participating in an assortment of brooding behaviors.

Read More: What Species Today Are Descendants of Dinosaurs?

Dinosaur parents were just as diverse as dinosaur babies. Different dinos dedicated different amounts of time and attention to their eggs throughout the incubation period, which took anywhere from three to six months.

Some species tended to their eggs tirelessly. Take, for instance, the oviraptors. In addition to finding an abundance of these theropods near their nests, paleontologists have also found their fossils on top of their nests. Situated in what seem to be sustained sitting positions, these oviraptors braved the threats of predation and starvation to shelter their broods, making them the picture of protective parenting.

At the other extreme, some species seemed to abandon their eggs immediately after laying them. The fossil record reveals that several types of sauropods, including the apatosaurus, buried their babies instead of brooding them, leaving them to emerge from their shells all on their own.

Read More: Dinosaur Hatchery With 92 Nests And Over 250 Eggs Uncovered In India