Scientists know little about the details of dinosaur mating and
reproduction. Dinosaurs may have utilized a variety of mating habits,
including but not limited to pair-bonding, herding and harem-building,
and chance mating.
Fossil dinosaur nests clearly show that dinosaurs laid hard-shelled
eggs, like birds. In some coelurosaur theropods, these
eggs were laid in pairs within the nest, demonstrating that these
dinosaurs retained two functioning oviducts. This is similar to
the condition in most reptiles, but unlike birds which lost one
oviduct in their early evolutionary history. Nearly all dinosaur
eggs appear to have been laid in nests, but only those of theropods
show any regular arrangement.
Dinosaur nests are often isolated, especially those of carnivorous dinosaurs. However, some herbivores (such as sauropods, hadrosaurs, and hypsilophodontids) appear to have made their nests in large colonies, forming nesting grounds with hundreds or perhaps thousands of individuals. These "nesting horizons" have been found as egg-rich fossil layers in Argentina, India, France, and Montana.
Living archosaurs (crocodilians and birds) both afford some care to their eggs and newborns, and paleontologists accept that dinosaurs may have engaged in similar levels of parental care. Discoveries of the theropod Oviraptor preserved sitting on its nests reveal that this dinosaur may have incubated its eggs. Other dinosaurs may have guarded their nests, much as crocodilians do, and protected the newly hatched young.
Once hatched, baby dinosaurs probably remained in the nest for at least a short period of time. Some dinosaur nest fossils include the bones of newborn dinosaurs, suggesting that the young did not leave the nest immediately. Bones from very young (but not newborn) dinosaurs are also found in other deposits, however, indicating many young animals could and did leave the safety of the nest quite early.