Next time you come across a group of zebra, wildebeest, blesbok and springbok typically found in the grasslands at Golden Gate National Park, picture a creature at least six metres in length carrying a small head at the end of a long neck, with a huge body and a really long tail walking on four legs towards the hallmark Brandwag rock. Today she is known as a prosauropod dinosaur Massospondylus. She lived there about 190 million years ago and scientists have excavated her nesting site, which will tell us more about the complex way early dinosaurs reproduced.
The fossils were found in sedimentary rocks from the Early Jurassic Period. 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.
The researchers, led by Canadian paleontologist Robert Reisz, together with Drs. Hans-Dieter Sues (Smithsonian Institute, USA), Eric Roberts (James Cook University, Australia), and Adam Yates (Bernard Price Institute for Paleontological Research, University of the Witwatersrand, South Africa) published their findings in the international journal, Proceedings of the National Academy of Sciences (PNas).
The newly unearthed dinosaur nesting ground is more than 100 million years older than previously known nesting sites. It 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.
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 organized 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, a professor of biology at University of Toronto Mississauga. “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.”
“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 Mississauga alumnus and a curator of Vertebrate Paleontology at the Royal Ontario Museum.