Considering how abundant they were when dinosaurs roamed the earth, dinosaur eggs are surprisingly rare on the ground today. Here are 10 facts you may (or may not) have known about these highly prized fossils.
1. Dinosaur females laid multiple eggs at the same time...
As far as we can tell, mother dinosaurs laid anywhere from a handful (three to five) to a whole clutch (15 to 20) eggs at a single sitting, depending on the genus and species. This is because the young of oviparous (egg-laying) animals engage in most of their development out of the mother's body, and eggs are thus "cheaper" in evolutionary terms than live births.
2. ...but most dinosaur eggs never got a chance to hatch.
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 predators, and of the remainder, most newborn hatchlings would be gobbled up as soon as they left the egg. That's why clutches evolved in the first place; you have to lay a lot of eggs to optimize the survival of at least one baby dinosaur!
3. Only a handful of fossilized dinosaur eggs contain embryos.
Even if an unhatched dinosaur egg managed to escape predation, and wound up buried in sediment, microscopic processes would usually destroy the embryo inside (for instance, bacteria can penetrate the shell and feast on the egg's contents). To date, the best-attested intact dinosaur embryos belong to a genus of prosauropod from the Triassic period, Massospondylus.
4. Dinosaur eggs are fantastically rare...
Billions of dinosaurs roamed the earth during the Mesozoic Era, laying literally trillions of eggs. Doing the math, you might think 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 a clutch of dinosaur eggs is discovered.
5. ...but eggshell fragments are fairly common.
As you might expect, the broken 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, though identifying the dinosaur they belonged to is another story. Most often, though, these fragments are ignored, since the dinosaur fossil itself is much more important.
6. Dinosaur eggs are classified by their "oogenus".
Unless a dinosaur egg is found in close proximity to an actual dinosaur, it's virtually impossible to determine the genus or species that laid it. However, there are broad features of these eggs that can at least determine whether they were laid by theropods, sauropods, or other types of dinosaur; three of these "oogenera" are prismatoolithus, macroolithus and spheroolithus.
7. Dinosaur eggs were more symmetrical than bird eggs...
There are various reasons bird eggs have distinctive oval shapes, including the reproductive anatomy of females (oval eggs are easier to lay), the structure of bird nests (oval eggs tend to cluster inward), 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!
8. ...but many of them were elongated rather than spherical.
As a rule, the eggs laid by theropod (meat-eating) dinosaurs were much longer than they were wide, while the eggs of sauropods and ornithopods tended to be more spherical. No one is quite sure why this is so, 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).
9. Dinosaur eggs didn't exceed two feet in diameter.
There are biological constraints on how big any given egg can be--and the 100-ton titanosaurs of the Cretaceous period certainly bumped up against that limit. It's reasonable to assume that no dinosaur egg exceeded two feet in diameter; if such an egg is ever found, that would have dire consequences for current theories about dinosaur metabolism and reproduction!
10. If you think you've discovered a dinosaur egg, you're probably wrong.
Are you convinced that you've found a fossilized dinosaur egg in your backyard? Well, you'll have a hard time making your case if no dinosaurs have 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.