An 80-million-year-old nesting site found in the Gobi desert in Mongolia confirms that some dinosaurs nested in colonies like birds. It consists of at least 15 clutches of eggs laid during the same season and buried by a flood.
Several other dinosaur nesting sites with multiple clutches have been discovered, so it has long been suspected that some species nested together, as many birds do today. However, at these sites it is not clear that all the clutches were laid during the same season – they could instead be a result of dinosaurs returning to the same site year after year.
The Mongolian site, discovered in 2011, is unique in that there is clear evidence that all the eggs were laid during the same season, say Kohei Tanaka at the University of Tsukuba and his colleagues.
Each clutch at the site contains between 3 and 30 eggs that are 13 centimetres wide on average. They appear to have been buried in soil or organic material to keep them warm – as megapode birds and some crocodiles do today – rather than being brooded by the parents.
No embryos have been found inside the eggs, but based on similar finds the team thinks they were laid by therizinosaurs – feathered dinosaurs with massive claws on their forearms, which they may have used to pull down branches to feed on the leaves.
Many of the eggs are partly eroded, but ten that are complete have a large opening in the upper half – thought to be the hatching window through which the young therizinosaurs clambered out.
The team think at least 60 per cent of the eggs hatched successfully. “We do not know the fate of the rest,” says Tanaka.
At some point after most of the eggs had hatched, a small flood covered the site in a thin layer of red mud. Crucially, this red mud is inside eggs in different clutches.
“If some eggs were laid in earlier years, we would expect different types of sediments filling the eggs,” says Tanaka.
Later a bigger flood buried the site entirely, preserving it.
Journal reference: Geology, DOI: 10.1130/G46328.1
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