The world's earliest known dinosaur eggs - dating back 195 million years - have revealed new information about the evolution of reproduction in the gigantic animals, scientists say. Researchers led by the University of Toronto Mississauga in Canada studied the fossilised remains of eggs and eggshells discovered at sites in Argentina, China and South Africa. At 195 million years old, they are the earliest known eggs in the fossil record, the researchers said. They were all laid by a group of stem sauropods - long-necked herbivores that ranged in size from four to eight metres in length and were the most common and widely spread dinosaurs of their time. "Reptile and mammal precursors appear as skeletons in the fossil record starting 316 million years ago, yet we know nothing of their eggs and eggshells until 120 million years later," said Robert Reisz from the University of Toronto. "It's a great mystery that eggs suddenly show up at this point, but not earlier," Reisz said in a statement. According to Koen Stein, a post-doctoral researcher at Ghent University in Belgium, the eggs represent a significant step in the evolution of dinosaur reproduction. Spherical, and about the size of a goose egg, these dinosaur eggshells were paper-thin and brittle, much thinner than similar-sized eggs of living birds. "We know that these early eggs had hard shells because during fossilisation they cracked and broke, but the shell pieces retained their original curvature," said Stein, lead author of the project. The researchers analysed shell thickness, membrane, mineral content and distribution of pores, looking for clues about why these early eggs might have developed hard shells. The study shows that hard-shelled eggs evolved early in dinosaur evolution with thickening occurring independently in several groups, but a few million years later other reptiles also developed hard-shelled eggs. One possibility is that hard and eventually thicker shells may have evolved to shield fetal dinosaurs and other reptiles from predators. "The hard shells would protect the embryos from invertebrates that could burrow into the buried egg nests and destroy them," Reisz added.