Unearthing the oldest dinosaur nesting site
Fig. 1. Reconstruction of a Massospondyus nesting site. Courtesy J. Csotonyi
Today, an international team that includes leader University of Toronto at Mississauga palaeontologist Dr. Robert Reisz and myself announced the discovery of the oldest known dinosaur nesting site, detailed in a article published in the Proceedings of the National Academy of Sciences. Here we document an amazing series of newly discovered dinosaur nests, all found in 195 million year old red rocks of a small road cut in Golden Gate Highlands National Park, in South Africa. This wealth of fossils and the documentation of the environment in which they were laid gives us our first detailed glimpse at dinosaur reproduction early in dinosaur history, and allows us to better constrain the evolution of reproductive traits dinosaurs as a whole.
Fig. 2. The nesting site locality at the Rooidraai road cut, or Red Corner, Golden Gate Highlands National Park, South Africa. Photo credit: Ian Morrison
Like all stories of discovery in palaeontology, this one has a long history. The first eggs from this nesting site were actually discovered by famed South African fossil finder James Kitching in 1977, in the rubble of the freshly constructed road cutting now in Golden Gate Highlands National Park. In 2003, Robert Reisz was shown the eggs. He instantly recognized their significance and borrowed the block for preparation. A year later, expert preparation by Diane Scott had uncovered perfect embryonic skeletons that were unmistakably dinosaurian. They were so well preserved that they could be identified to the sauropodomorph Massospondylus- a 6 meter long plant–eater with a small head, long neck, and long tail. Not only were these by far the oldest know dinosaurs embryos, but the skeletons revealed that Massospondylus started life on four legs and became bipedal as it grew. In 2005, we published our results in the journal Science, and made headlines.
This is where the story of the nesting discovery actually begins- as amazing as the fossil embryos were, they were just an isolated occurrence in 2005. The outstanding quality of the fossils suggested to us that there had to be more eggs hidden in that road cut, and we became determined to find them. In 2006, with the help of a grant from the National Geographic Society, we set out to find more. This first field season was difficult, because it took us a while to figure out just how to find what we were looking for. After three frustrating days of finding nothing, I got to be the lucky one, and made the first big find- the faint outlines of half a dozen eggs in a neat row in the cliff face. After excavating the nest from the hard red rock, work in the lab revealed that I had found an almost complete clutch of 34 eggs, which matched those that contained the Massospondylus embryos exactly.
Fig. 3. The first nest found by our team at the nesting site in 2006. Photo credit: Diane Scott
This discovery encouraged us to mount a series of three more expeditions to the small road cut. Now knowing what to look for, we found another 8 nests. Work by our team’s geologist Dr. Eric Roberts showed that all of the nests came from a restricted rock layer, and that the nests were preserved in fine muds resulting from very low energy flood events from a nearby river. The amazing density of nest and their distribution in the rocks showed that Massopondylus mothers laid their eggs close together, perhaps together at the same time in organized colonies, and that the same species used this exact area to lay their eggs year after year.
ROM paleontologist David Evans (left) and ROM technician Ian Morrison collect dinosaur nests in white plaster jackets. Photo credit: N. Wong-Ken
Just why the dinosaurs used this spot to lay their eggs we may never know, but we can presume that the environmental conditions were just right for this particular species, because it used the site over and over again, over a period of at least several hundred years. Incredibly, our study also uncovered tiny footprints of baby Massospondylus in the nesting horizons, indicating that the babies stuck around the nesting site for some time after hatching. The evolutionary context and size of the egg clutches suggest that it is likely that Massospondylus even cared for its nests and hatched young, at least to the level that crocodiles do today.
Fig. 5. Tiny footprints of baby Massospondylus found in the fine layers of mud from the nesting interval. Photo credit: Robert Reisz.
How dinosaurs reproduced and cared for their nests and young, has fascinated scientists and the general public since the discovery of dinosaur nests in Mongolia were publicized 90 years ago. The new discoveries reveal the most ancient evidence to date for seemingly complex reproductive behaviors, including the repeated use of a nesting site through time by a single species, which have only been well documented in Late Cretaceous dinosaurs that lived over 100 million years after Massospondylus. Although these discoveries paint an incredible picture of dinosaur life in the Early Jurassic, our knowledge of dinosaur reproduction remains incredibly scant, and many more important discoveries of dinosaur nests are surely on the horizon, and our project in South Africa will continue.