Dinosaurs grew in the egg in a similar fashion to modern birds, a study suggests.
Scientists made the discovery after examining a cache of more than 200 fossilised bones from embryonic dinosaurs.
They were found strewn among fragments of eggshell and are all believed to belong to the same species, the 26-foot long-necked sauropod Lufengosaurus.
The Early Jurassic fossils, dating back nearly 200m years to the start of the dinosaurs’ reign, originated from several different nests. The scientists believe they were washed by floodwater into the excavation site in Yunnan Province, southern China.
Crucially, the embryos were at different stages of growth, providing scientists with a rare opportunity to study how Lufengosaurus developed before hatching.
Focusing on the thigh bone, they found evidence of rapid growth within the egg. Before hatching, the bones doubled in length from 12 to 24 millimetres, indicating a short incubation time.
Analysis of the bones’ anatomy and internal structure showed that, as in birds, muscles became active inside the egg and helped to shape the skeleton.
“This suggests that dinosaurs, like modern birds, moved around inside their eggs,” said lead scientist Dr Robert Reisz, from the University of Toronto in Canada. “It represents the first evidence of such movement in a dinosaur.”
The findings, reported in Nature, revealed organic material inside the bones that may be fibres of collagen connective tissue.
Dr Reisz said: “We are opening a new window into the lives of dinosaurs. This is the first time we’ve been able to track the growth of embryonic dinosaurs as they developed.
“Our findings will have a major impact on our understanding of the biology of these animals.”