Some vertebrates reproduce through eggs laid on land, such as most modern reptiles and birds, as well as monotreme mammals. Extinct organisms that left eggs in the geologic record are represented by reptiles, birds, and dinosaurs; I do not know of any un disputed fossil mammal eggs. Eggs are either represented in the geologic record as individuals or are arranged in definite patterns, as seen in some dinosaur egg assemblages. Definite patterns of eggs probably indicate 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 contras t, 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 e mbryo to "breathe," thus offering protection but also allowing an exchange with the surrounding environment.
Eggs are preserved as oblate to semispherical structures that also show distinctive shell microstructures. In some cases, body fossils can identify the egg layer; for example, dinosaur eggs have preserved parts of embryonic dinosaurs, which can help to c orrelate a dinosaur egg with a species of dinosaur (i.e., Maiasaurus, 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 wi th one another in the fossil record is often regarded as part of a clutch, meaning that these eggs represent one egg-laying episode.
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, which may be evident as a bowl-like depression. Nests most likely to have been preserved in the geologic record were on the ground and excavated in soft sediment. Some modern reptiles and a few birds (i.e., penguins) use this strategy and at least some dinosa urs used it, too. Nests made in trees, like those made by many modern birds, and as vegetative piles on the ground, such as those made by some crocodilians, would have had low preservation potential.
Nests are sometimes found containing remains of juveniles or parents, which assists in the identification of the tracemakers. Community structures and social interactions for some fossil species, such as the dinosaur Maiasaurus, are indicated by m ultiple 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.
Further Reading on Eggs and Nests
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.
Varricchio, D. J., Jackson, F., Borkowski, J. J., and Horner, J. R. 1997. Nest and egg clutches of the dinosaur Troodon formosus and the evolution of avian reproductive traits. Nature, 385, p. 247-251.
Dinosaur Eggs and Nests - Link to the Dinosaur Trace Fossil page.