A Canadian-led team of international researchers has unearthed the 190-million-year-old nesting site of the prosauropod dinosaur Massospondylus — predating previously known nesting grounds by 100 million years — at an excavation site in South Africa.
The finding was published Monday in the Proceedings of the National Academy of Sciences.
The study led by Robert Reisz, a paleontologist and professor of biology at the University of Toronto's Mississauga campus, describes clutches of eggs, many with embryos, as well as tiny dinosaur footprints.
They provide the oldest known evidence that the hatchlings remained at the nesting site until doubling in size. At least 10 nests were discovered at several levels at the site, each with up to 34 round eggs in tightly clustered clutches.
The distribution of the nests in the sediments reveals a couple of key nesting instincts, providing the oldest known evidence of such behaviour in the fossil record.
One is nesting fidelity, meaning that these early dinosaurs returned repeatedly to the site. Second, they also likely assembled in groups to lay their eggs, known as colonial nesting.
The mother's large size — six metres long — and the small size of the eggs, about six to seven centimetres in diameter, along with the highly organized nature of the nest suggest that the mother may have arranged them carefully after she laid them, said Reisz.
"The eggs, embryos and nests come from the rocks of a nearly vertical road cut only 25 metres long," he said. "Even so, we found 10 nests, suggesting that there are a lot more 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 discovery provides important clues about how the complicated reproductive behaviour of early dinosaurs evolved, said co-author David Evans of the Royal Ontario Museum.
"Even though the fossil record of dinosaurs is extensive, we actually have very little fossil information about their reproductive biology, particularly for early dinosaurs," said Evans, the ROM's associate curator of vertebrate paleontology.
"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."
The fossils were found in sedimentary rocks from the early Jurassic period in the Golden Gate Highlands National Park in South Africa. The previously oldest known embryos belonging to Massospondylus, a relative of the giant, long-necked sauropods of the Jurassic and Cretaceous periods, were also found at the site.