10 Facts About Dinosaur Eggs

Which came first, the dinosaur or the egg?

Every dinosaur that ever lived during the Mesozoic Era hatched from an egg—but, even so,  there's still a lot we don't know about dinosaur eggs.

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Female Dinosaurs Laid Multiple Eggs at the Same Time

Getty Images.

As far as paleontologists can tell, female dinosaurs laid anywhere from a handful (three to five) to a whole clutch (15 to 20) of eggs at a single sitting, depending on the genus and species. Since the hatchlings of oviparous (egg-laying) animals experience most of their development outside the mother's body, from an evolutionary perspective, eggs are "cheaper" and less demanding than live birth—and little extra effort is required to lay multiple eggs at a single time.

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Most Dinosaur Eggs Never Got the Chance to Hatch

dinosaur eggs
Wikimedia Commons.

Nature was as cruel during the Mesozoic Era as it is today. Most of the dozen or so eggs laid by a female Apatosaurus would be immediately devoured by lurking predators, and of the remainder, most of the newborn hatchlings would be gobbled up as soon as they stumbled away from the egg. That's why the practice of laying eggs in clutches evolved in the first place; you have to produce a lot of eggs to optimize (if not ensure) the survival of at least one baby dinosaur!

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Only a Handful of Fossilized Dinosaur Eggs Contain Embryos

A Maiasaura hatchling emerging from its egg (Museum of the Rockies).

Even if an unhatched dinosaur egg managed to escape the attention of predators and wound up buried in sediment, microscopic processes would have quickly destroyed the embryo inside (for instance, small bacteria could easily penetrate the porous shell and feast on the contents within). For this reason, preserved dinosaur embryos are extremely rare; the best-attested specimens belong to Massospondylus, a prosauropod of the late Triassic period.

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Fossilized Dinosaur Eggs Are Fantastically Rare

dinosaur egg
Getty Images.

Literally billions of dinosaurs roamed the earth during the Mesozoic Era, and female dinosaurs laid literally trillions of eggs. Doing the math, you might come to the conclusion that fossilized dinosaur eggs would be much more common than fossilized dinosaur skeletons, but the opposite is true. Thanks to the vagaries of predation and preservation, it's always big news when paleontologists discover a clutch of dinosaur eggs!

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Dinosaur Eggshell Fragments Are Fairly Common

dinosaur egg shell
Wikimedia Commons.

As you might expect, the broken, calcified shells of dinosaur eggs tend to persist longer in the fossil record than the embryos they once protected. An alert paleontologist can easily detect these shell remnants in a "matrix" of fossils, though identifying the dinosaur they belonged to is practically impossible In the vast majority of cases, these fragments are simply ignored, since the dinosaur fossil itself is considered much more important.

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Dinosaur Eggs Are Classified According to Their "Oogenus"

dinosaur eggs
A clutch of "faveoolithus" dinosaur eggs (Wikimedia Commons).

Unless a dinosaur egg is discoered in close proximity to an actual, fossilized dinosaur, it's virtually impossible to determine the exact genus or species that laid it. However, there are broad features of dinosaur eggs (such as their shape and texture) that can at least determine whether they were laid by theropods, sauropods, or other types of dinosaur; some of these difficult-to-pronounce "oogenera" include prismatoolithus, macroolithus and spheroolithus. 

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Dinosaur Eggs Didn't Exceed Two Feet in Diameter

titanosaurus egg
The egg of a titanosaur dinosaur (Wikimedia Commons).

There are severe biological constraints on how large any given egg can be—and the 100-ton titanosaurs of late Cretaceous South America certainly bumped up against that limit. Still, paleontologists can reasonably assume that no dinosaur egg exceeded two feet in diameter; if such an egg is ever discovered, that would have dire consequences for our current theories about dinosaur metabolism and reproduction (not to mention for the female dinosaur that had to lay it!)

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Dinosaur Eggs Are More Symmetrical Than Bird Eggs

Wikimedia Commons.

There are various reasons bird eggs have distinctive oval shapes, including the reproductive anatomy of female birds (oval eggs are easier to lay), the structure of bird nests (oval eggs tend to cluster inward, thus reducing the risk of falling out of the nest), and, possibly, the fact that nature places a higher premium on the development of baby birds' heads. Presumably, these evolutionary constraints did not apply to dinosaurs, hence their much more symmetrical eggs, some of which were nearly spherical in shape.

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Some Dinosaur Eggs Were Elongated, Rather than Round

theropod eggs
A clutch of theropod dinosaur eggs (Wikimedia Commons).

As a general rule, the eggs laid by theropod (meat-eating) dinosaurs were much longer than they were wide, while the eggs of sauropods, ornithopods, and other plant-eaters tended to be more spherical. No one is quite sure why this is the case, though it probably has something to do with how the eggs were clustered in nesting grounds (perhaps elongated eggs were easier to arrange in a stable pattern, or more resistant to rolling away or being poached by predators).

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If You Think You've Discovered a Dinosaur Egg, You're Probably Wrong

dinosaur eggs
Getty Images.

Are you convinced that you've discovered an intact, fossilized dinosaur egg in your backyard? Well, you'll have a hard time making your case to your local natural history museum (or About.com dinosaur expert) if no dinosaurs have ever been discovered in your vicinity—or if the ones that have been discovered don't match the "oogenus" of your presumed egg. Most likely, you've stumbled on a hundred-year-old chicken egg or an unusually round stone!

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Strauss, Bob. "10 Facts About Dinosaur Eggs." ThoughtCo, Oct. 24, 2017, thoughtco.com/facts-about-dinosaur-eggs-1092047. Strauss, Bob. (2017, October 24). 10 Facts About Dinosaur Eggs. Retrieved from https://www.thoughtco.com/facts-about-dinosaur-eggs-1092047 Strauss, Bob. "10 Facts About Dinosaur Eggs." ThoughtCo. https://www.thoughtco.com/facts-about-dinosaur-eggs-1092047 (accessed February 19, 2018).