By Adam Bakewell, Co-host of Generic Drift
An 80-million-year-old nesting site in the eastern Gobi Desert of Mongolia is has provided the strongest evidence to date that some dinosaurs, like some birds and crocodiles, nested in colonies. In other words, Harvey was right:
15 clutches, of between 3 and 30 eggs per clutch, were found at the site – described in a recent publication in the journal Geology. All of the eggs in these clutches were similar, belonging to the same oofamily (a taxonomy classification for fossilized eggs): the Dendroolithidae.
This isn’t the first time that dinosaur nesting sites with multiple clutches of eggs have been unearthed. Famous fossil sites like Egg Mountain in Montana, where dozens of dinosaur nests have been found in a single area, have revolutionised thinking about dinosaur parental care and hinted at the possibility of colony nesting. The problem is identifying whether eggs and nests were laid simultaneously by dinosaurs living in a group or laid over multiple years by dinosaurs living alone.
Look at it this way: in the bottom of my mum’s wardrobe is a shoebox containing teeth, booties and other cradle memorabilia from my sister’s childhood and mine. Give it a few million years and how would a palaeontologist (or a disappointed alien burglar) finding the box judge whether my sister and I were deposited at the same time, or over successive breeding seasons?*
The Javkhlant nesting site is extraordinary because the quality of its fossil evidence allows just this kind of distinction to be made. The formation is made of layers of orange and grey rock, and running through it is a bright red streak – most likely created when flooding from a nearby river covered the site in sediment. This bright red band connects up the 15 dinosaur egg nests, indicating that they were all laid at a similar time to the flood and sediment washed over them and buried them together. In geological parlance we’d say that the eggs are associated with a common palaeosurface.
Also extraordinary is the apparent nesting success of these dinosaurs; at least 60% of the eggs in these clutches hatched, far more than we would expect if they had been laid and abandoned. Another unique feature of the site that it indicates that the eggs were buried for incubation in soil, or some other organic substrate, similar to the way that contemporary crocodilians or megapode birds like the Australian brushturkey build their nests. The overall picture is of a creature that not only laid eggs among its conspecifics but stayed to care for and protect them afterwards.
So what laid these eggs? The fragments left behind were identified, using their thickness and textures, as belonging to a non-avian theropod. These three-toed, bipedal, hollow boned dinosaurs are the ancestors of today’s birds, and the group includes famously fierce predators like Allosaurus, Velociraptor, and (of course) Tyrannosaurus rex.
Since no embryos or bones were found at the site to help the identification further, it’s still not exactly clear what laid them. The authors hypothesise that they were most likely a therizinosaur, a gigantic feathered dinosaur with claws up to a meter long. Importantly, the therizinosaurs are herbivores (they allegedly used their monstrous claws to pull down leaves from trees, or at worst tear open termite mounds) and may have bred together as a form of protection against predators.
*She’s 14 months older than me if anyone is interested.
Tanaka, K., Y. Kobayashi, D. K. Zelenitsky, F. Therrien, Y.-N. Lee, R. Barsbold, K. Kubota, H.-J. Lee, T. Chinzorig, & D. Idersaikhan. Exceptional preservation of a Late Cretaceous dinosaur nesting site from Mongolia reveals colonial nesting behaviour in a non-avian theropod. Geology 47.