Oldest known dinosaur nesting site discovered in South Africa
A fantastic discovery has been made in early Jurassic strata at the Golden Gate Highlands National Park, South Africa. The area has previously revealed many excellent fossils, including skeletons of prosauropod (early dinosaurs preceding Sauropods, and often bipedal) Massospondylus. In 1976, a block of siltstone was found to contain a partial egg clutch of this species with in ovo embryonic remains. A study led by Prof. Robert Reisz (University of Toronto), began in 2006 to investigate the site further and has uncovered a nesting site with several complete nests and fascinating evidence of complex reproductive behaviours. The nesting site is approximately 190 million years old, making it the oldest vertebrate nesting site of its kind, and 100 million years older than the previously oldest known nesting site with in ovo embryonic fossils.
Complex nesting sites of dinosaurs are not especially rare, but most are found in the late Cretaceous. For early dinosaurs, individual eggs have been discovered but actual nesting sites have proven extremely hard to come by. Only two Jurassic egg clutches with in ovo embryonic remains have ever been documented. There is the theropod Lourinhasaurus from the Upper Jurassic (discovered in Portugal), and Massospondyl from the Lower Jurassic strata found at Rooidraai (“Red Bend”) in South Africa. Rooidraai is a roadside exposure, where the first partial egg clutch was discovered in 1976. That discovery was the oldest dinosaur nest with embryos, but the recent excavations now also make it the earliest to demonstrate complex nesting behaviours. The cut is only 25 m long, and 10 nests have been found already, so the team believes many more may be hidden within the tons of muddy siltstone.
The egg clutches are quite remarkable for many reasons. Firstly, many are complete. Another interesting observation is that no individual eggs were discovered, all were within distinct nests consisting of a single layer of eggs (like modern birds) and arranged in rows. These egg clutches appear in at least three stratigraphically distinct layers, demonstrating a pattern that suggests several adults used the nesting area (colonial) and that the site possibly provides evidence for nesting site fidelity (several individuals returning to use the same site). Evidence of this type of reproductive behaviour in dinosaurs has mostly been restricted to later dinosaurs in the Cretaceous period, so the find is significant. The authors of the study point out that complex nesting behaviour could be a plesiomorphic characteristic considering the nesting behaviours observed in extant avian and crocodilian species.
The site also contains footprints that are consistent with young members of Massospondylus. Making comparisons between the skeletons of the embryos and the footprints found at the site, it appears the hatchlings stayed at the nesting area long enough to double in size. This is a fine example of how a fossil nesting site can provide a wealth of knowledge about the reproductive behaviours of a species. This large (25 m long) prosauropod seemed to lay large clutches of small eggs (maximum was around 34 eggs) and arrange them carefully in tight clutches, were the young would develop until they were large enough to leave the nest. It appears the adults would nest here in groups, and return to use the same site for many years. This is a fascinating insight into ancient vertebrate reproductive behaviour and I will be keeping my eyes on the research as I’m very interested to see what else is waiting to be discovered at Rooidraai.