Palaeontologists found 10 separate nests, each containing clutches of up to 34 eggs measuring 6-7cm.
The fossils are of the prosauropod Massospondylus, a relative of the long-necked sauropods such as Diplodocus.
They suggest that Massospondylus returned to the site repeatedly, laying their eggs in groups in the earliest-known case of “colonial nesting”.
The 190-million-year-old finds also included embryonic dinosaur skeletons, and are described in Proceedings of the National Academy of Sciences.
They were found in a 25m stretch of rock in South Africa’s Golden Gate Highlands National Park.
The researchers suggest that many more sites remain embedded in the rock, which will be exposed as natural weathering processes continue.
But the current find already vastly extends what is known about dinosaurs in their earliest days on Earth.
“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 David Evans, associate 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.”