While we don’t know a whole lot about dinosaur romance and reproduction, we have much more information about what happens after mating. That’s because the fossil record of eggs and nests is quite good. Like living reptiles, dinosaurs buried their eggs, which appear to have had long incubation periods—up to half a year. Scientists figured this out by counting growth lines in the teeth of well-developed duck-billed and horned dinosaur embryos that were close to hatching. Sauropod and duck-billed dinosaurs appear to have laid a mass of spherical eggs in their nest all in one go and buried it much like a crocodylian or turtle.
Some carnivorous dinosaurs closely related to birds had large eggs compared to their body size and would have laid them asynchronously; that is, a pair a day. This egg pairing can be seen in the nests of dinosaurs like Citipati and Troodon. Unlike birds, but like lizards and other reptiles, these dinosaurs would still have had two functional ovaries, whereas birds only have one (so we don’t see pairing in the nests of living birds). Skeletons of oviraptorosaurs like Citipati have been found lying on nests of eggs.
Indeed, the first discovery of such a skeleton on eggs was mistaken as an act of nest predation, leading to the misnomer Oviraptor (“egg thief” in Latin) being applied to these animals. Although later finds of oviraptor embryos in the eggs exonerated these animals, the rules of zoological nomenclature mean they are stuck with the name.
There is a surprising diversity of nest types in theropod dinosaurs and early birds. Whereas theropods that are only remotely related to birds have buried nests with lots of eggs lying helter-skelter like an alligator nest, Oviraptor laid large nests with the eggs arranged in a wide ring so that there was space for the adult to sit in the middle. Sometimes these rings have up to three layers of eggs and a total of 30 to 40 eggs. That’s probably too many for one female to lay, suggesting that these animals may have been like ostriches with multiple females laying eggs in one nest that would be guarded by a male parent.
Troodon may also have guarded its eggs since a partial skeleton was found with a nest, which had a different structure from oviraptorosaurs. Troodon eggs are closely grouped in the nest, and all are arranged nearly vertically with the pointy end down into the sediment. The nests are smaller with fewer eggs but appear to have a dirt ring around them.
Early birds that were contemporaneous with other dinosaurs exhibit a wide range of nesting strategies, from single buried eggs to nesting colonies. All appear to have buried their eggs, which in turn suggests that egg incubation through direct contact with the parent evolved quite late in birds, probably close to the origin of the last common ancestor of all living birds. So, if you think about it, the next time you’re enjoying an omelet, that’s the accumulation of millions of years of dinosaur reproductive evolution.