Dinosaurs reproduced through eggs laid on land, just like most modern reptiles and birds, rather than giving live birth (such as most mammals). Dinosaur eggs are either represented in the geologic record as individuals or are arranged in definite patterns, indicating the former presence of nests.
An egg is an enclosed, mineralized structure containing an amniote (yolk sac) that helps to nourish the developing embryo. The structure is a type of protection for the embryo that also keeps all of its nutrients in a restricted space. In contrast, amphibians require a water source for their eggs, hence times of drought (and consequent shrinkage of aquatic habitats) can be detrimental to amphibian reproduction. Amniotic eggs also have a porous and permeable structure that allows the developing embryo to "breathe," thus offering protection but also allowing an exchange with the surrounding environment.
Dinosaur eggs are preserved as oblate to semispherical structures that also show distinctive shell microstructures. In some cases, dinosaur eggs have preserved parts of embryonic dinosaurs, which can help to correlate a dinosaur egg with a species of dinosaur, such as Maiasaurus and Oviraptor. However, the egg itself is the trace fossil, whereas any bodily remains of an embryo constitute a body fossil. An assemblage of eggs in close association with one another in the fossil record is often regarded as part of a clutch, meaning that these eggs represent one egg-laying episode.
This dinosaur egg, attributed to the theropod Oviraptor, is partially crushed but still shows the asymmetrical and oval shape associated with this species' eggs. Shell fragments are to the right of the egg. Samples are in Western Colorado's Museum of Dinosaur Valley, Grand Junction, Colorado, USA.
This pair of partially preserved dinosaur eggs come from the Late Cretaceous Nanxiong Basin of southeastern China, a locality that has yielded thousands of dinosaur eggs. The specimens are in the Löwentor Museum, Stuttgart, Germany.
A nest is a biogenic structure typically containing a clutch and commonly represented by an arrangement of eggs in a semicircular or spiraled pattern. In some instances, a raised area surrounding the eggs will denote the border of the nest. Dinosaur nests were on the ground, like some modern reptiles and a few birds (i.e., penguins), rather than in trees, like many modern birds. No evidence exists indicating that any dinosaurs lived in trees, let alone built nests in trees. Nests are sometimes found containing hatchlings or other remains of juveniles, which assists in the identification of the parental species. Community structures and social interactions for some dinosaurs, such as Maiasaurus, are indicated by multiple nests preserved on the same sedimentary horizon. The significance of this inferred interaction is that certain dinosaurs behaved much more like birds, rather than like most reptiles.
One of the more spectacular dinosaur fossil finds of recent years was of a Late Cretaceous specimen of Oviraptor that was found in a sitting position directly over its nest. This find, a wonderful combination of trace fossils and a body fossil, represents one of the most compelling pieces of evidence for brooding behavior in dinosaurs. This fossil find is currently on display at the American Museum of Natural History in New York and was illustrated in a National Geographic article.
This dinosaur nest shows a spiraled arrangement of the eggs, which were probably oriented in the nest by the mother dinosaur after they were laid. Specimen is in the Fruita Paleontogical Museum, Fruita, Colorado, USA.
Further Reading on Dinosaur Eggs:
Hirsch, K. F. 1989. Interpretations of Cretaceous and Pre-Cretaceous eggs and shell fragments. In Gillette, D. D., and Lockley, M. G. (eds.) 1989. Dinosaur Tracks and Traces. Cambridge University Press, Cambridge, p. 89-97.
Horner, J. R., and Gorman, J. 1988. Digging Dinosaurs. New York, Workman Publishing, 210 p.