An excavation at a site in South Africa has unearthed the 190-million-year-old dinosaur nesting site of the prosauropod dinosaur Massospondylus - revealing significant clues about the evolution of complex reproductive behaviour in early dinosaurs.

A new study, entitled Oldest known dinosaurian nesting site and reproductive biology of the Early Jurassic sauropodomorph Massospondylus and published in the prestigious international journal Proceedings of the National Academy of Sciences (PNAS), was led by Canadian palaeontologist Prof. Robert Reisz, a professor of biology at the University of Toronto at Mississauga, and co-authored by Drs. Hans-Dieter Sues (Smithsonian Institute, USA), Eric Roberts (James Cook University, Australia), and Adam Yates (Bernard Price Institute (BPI) for Palaeontological Research at Wits).

The study reveals clutches of eggs, many with embryos, as well as tiny dinosaur footprints, providing the oldest known evidence that the hatchlings remained at the nesting site long enough to at least double in size.

Prof. Bruce Rubidge, Director of the BPI at Wits, says: "This research project, which has been ongoing since 2005 continues to produce groundbreaking results and excavations continue. First it was the oldest dinosaur eggs and embryos, now it is the oldest evidence of dinosaur nesting behaviour."

The authors say the newly unearthed dinosaur nesting ground is more than 100 million years older than previously known nesting sites.

At least ten nests have been discovered at several levels at this site, each with up to 34 round eggs in tightly clustered clutches. The distribution of the nests in the sediments indicate that these early dinosaurs returned repeatedly (nesting site fidelity) to this site, and likely assembled in groups (colonial nesting) to lay their eggs, the oldest known evidence of such behaviour in the fossil record.

The large size of the mother, at six metres in length, the small size of the eggs, about six to seven centimetres in diameter, and the highly organised nature of the nest, suggest that the mother may have arranged them carefully after she laid them.

"The eggs, embryos, and nests come from the rocks of a nearly vertical road cut only 25 metres long," says Reisz. "Even so, we found ten nests, suggesting that there are a lot more nests in the cliff, still covered by tons of rock. We predict that many more nests will be eroded out in time, as natural weathering processes continue."

The fossils were found in sedimentary rocks from the Early Jurassic Period in the Golden Gate Highlands National Park in South Africa. This site has previously yielded the oldest known embryos belonging to Massospondylus, a relative of the giant, long-necked sauropods of the Jurassic and Cretaceous periods.

"Even though the fossil record of dinosaurs is extensive, we actually have very little fossil information about their reproductive biology, particularly for early dinosaurs," says David Evans, a University of Toronto at Mississauga alumnus and a curator of Vertebrate Palaeontology at the Royal Ontario Museum.

"This amazing series of 190 million year old nests gives us the first detailed look at dinosaur reproduction early in their evolutionary history, and documents the antiquity of nesting strategies that are only known much later in the dinosaur record," says Evans.