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. Buried so far back in time, there's still a lot we don't know about dinosaur eggs, but we nevertheless have learned a fair amount from the fossil record. The fossil record does show, for instance, that dinosaur eggs were laid in large batches, or 'clutches', likely because so few hatchlings survived the jaws of a predator.

01
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Female Dinosaurs Laid Multiple Eggs at the Same Time

dinosaureggs
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As far as paleontologists can tell, female dinosaurs laid anywhere from a handful (three to five) to a whole clutch of eggs (15 to 20) at a single sitting, depending on the genus and species. 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. Thus, little extra effort is required to lay multiple eggs at a single time.

02
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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. Lurking predators would immediately devour most of the dozen or so eggs laid by a female Apatosaurus, 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. A dinosaur would have to produce a lot of eggs to optimize (if not ensure) the survival of at least one baby dinosaur.

03
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Only a Handful of Fossilized Dinosaur Eggs Contain Embryos

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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.

04
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Fossilized Dinosaur Eggs Are Fantastically Rare

dinosaur egg
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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.

05
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Dinosaur Eggshell Fragments Are Fairly Common

dinosaur egg shell
Wikimedia Commons.

As might be expected, 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.

06
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Dinosaur Eggs Are Classified According to Their "Oogenus"

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

Unless a dinosaur egg is discovered in close proximity to an actual, fossilized dinosaur, it's virtually impossible to determine the exact genus or species that laid it. However, broad features of dinosaur eggs, such as their shape and texture, can at least suggest whether they were laid by theropods, sauropods, or other types of dinosaur. The term "oogenera" refers specifically to the taxonomy of dinosaur eggs. Some of these difficult-to-pronounce oogenera include prismatoolithus, macroolithus, and spheroolithus. 

07
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Dinosaur Eggs Didn't Exceed Two Feet in Diameter

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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. The discovery of a larger egg 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.

08
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Dinosaur Eggs Are More Symmetrical Than Bird Eggs

dinosauregg
Wikimedia Commons.

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

09
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Some Dinosaur Eggs Were Elongated, Rather Than Round

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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.

10
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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 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.