Dinosaur eggs have been known since antiquity, the material used for jewelry and basic cutting tools. Many classic dinosaur books credit American George Olson with the modern discovery of dinosaur eggs back in 1923 during an expedition in Mongolia. While Olson's find was the first documented egg find, the real honor goes to 19th century French Catholic Priest Father Jean-Jacques Pouech. In 1859, Father Pouech came across the shell fragments of what he believed to be a giant bird. As it turns out, those eggs were really from Hypselosaurus.

Hypselosaurus was first described by P.E. Matheron in 1869 based on a selection of fossilized bone fragments. Matheron initially concluded the remains came from a huge crocodilian, but in 1890 Charles Depéret proposed Hypselosaurus was actually a sauropod dinosaur. The definition of Hypselosaurus has continued to shift from a mid-sized sauropod perhaps 12m (40ft) in length, to current theories that assign the remains to more firmly defined titanosaurs.

Matheron’s Hypselosaurus genus may be dubious, but the discovery of dinosaur eggs has been anything but. Research into Egg Mountain, located in Montana’s Two Medicine formation, has been foundational not just to our understanding of dinosaur eggs, but to the wider dinosaur social structure. Prior to Jack Horner’s work there in the 1980s, it was assumed that baby dinosaurs were left to themselves to survive after hatching. With the discovery of a massive egg colony at Two Medicine, a picture dawned of a complex social structure with massive herds of Maiasaura protecting their eggs as they incubated.

This discovery was a revelation, challenging long held doctrine of dinosaur behavior. When it was discovered in 1923, the Oviraptor was thought to prey on unhatched eggs, as its remains were found nearby a fossilized egg nest. In 1994, after the discovery in Two Medicine, embryonic studies of the nearby eggs were revealed to actually be Oviraptor eggs themselves. The genus’ holotype was not preying on the eggs, it was brooding over them. These two discoveries fundamentally changed our understanding of dinosaurs and how they cared for their young.

How exactly dinosaurs cared for their eggs during incubation varied widely between genera. Maiasaura and others would use rotting vegetation to heat their eggs, freeing up the herd to scavenge and protect the colony. Some sauropod eggs have been found near what were once geothermal hotspots, suggesting these dinosaurs used the Earth’s heat to protect their eggs. Others simply brooded over their eggs, carefully laying them in a set pattern to maximize heat distribution. Some genera did simply abandon their eggs to hatch on their own, but this seems to be the exception and not the rule.

The study of dinosaur eggs allows insight into the biological development of a given genus of dinosaur, especially if the embryo has been preserved. Beyond maturation rates, an egg is a valuable source into speculating on the wider genus. How a group cares for their young is a fundamental life process that can tell us more about social structures, mating rituals, and other interconnected behaviors that would otherwise be a mystery. Study of these eggs is the bedrock that much of dinosaur paleontology rests on.