Dinosaur 'clutches' discovered in Mongolia help researchers understand nesting habits

Alberta scientists are helping us better understand the nesting behaviour of dinosaurs and how it's evolved with other species thanks to a nesting site they helped uncover in Mongolia. 

Researchers from the Royal Tyrrell Museum, the University of Calgary and several foreign institutions discovered the site in Mongolia's eastern Gobi Desert, in the Javkhlant formation.

And, according to the Royal Tyrrell Museum in Drumheller, it reveals evidence that nest guarding behaviour first evolved among non-avian theropods before becoming widespread among living birds.

François Therrien, curator of dinosaur paleoecology at the Royal Tyrrell, says his colleagues were working in Mongolia between 2010 and 2015, when they came across the large accumulation of dinosaur eggs. 

These nests, known as clutches, generally consist of between three and 30 eggs, according to the Royal Tyrrell's website.

Therrien says the discovery of the eggs is very common in Mongolia — as it is in Alberta and Montana — but what was surprising was the sediment discovered in and around the 15 egg clutches they studied. 

"There was a very peculiar, prominent, thin red line through the rocks, a thin deposit of sand throughout the area, that could be traced all over the different clutches," he said. "It showed that at one moment in time millions of years ago (during the Cretaceous Period), a small flood buried all of the clutches in a single event."

Illustration by Masato Hattori

Nests built together

Therrien says that means all those dinosaur nests were at the surface at exactly the same time.

"Which was a demonstration that all of these clutches were actually a true dinosaur colony and that all those dinosaurs built their nests in the same area at the same time," he said.

Prior to this discovery, Therrien says, they did find lots of nests buried in the same area, but it was impossible to tell if they'd all been at the surface at the same time, or just in the same areas but decades or even centuries apart.

"Because, basically, all the rocks were so homogeneous that you couldn't tell," he said. 

The scientists say that among modern animals there are some, like crocodiles, that nest together but don't look after their nests. Instead, they abandon them. 

But, in the case of the Mongolian dinosaurs, they were able to find that the majority of the eggs hatched, Therrien says.

"We had little openings, and the window was left by the embryo when they hatched outside of the egg," he said. "And by doing a survey of old nests, we saw that approximately 60 per cent of the eggs had successfully hatched before being buried."

Parent dinosaurs guarded colony

Therrien says this is an extremely high success rate for hatching in these animals.

"If we compare that to modern animals, we see a very high hatching success like that around 60 per cent among species where one or several parents guard in their colony," he said. "Basically, if the adults leave — abandoned the nest — we have a much lower hatching success because the eggs either get trampled or get predated upon."

But, in this case, Therrien says they found clear evidence that there were parent dinosaurs guarding the nesting site.

"And this is the first time that we could demonstrate this type of behaviour in a dinosaur, so that was really exciting," he said. 

Photo courtesy of Kohei Tanaka, University of Tsukuba

Vegetarians, unable to fly

There are no fossils from the parent dinosaurs, but based on the internal egg shell, Therrien says they know the eggs are from non-avian theropod dinosaurs, a group that includes velociraptors.

He says the dinosaurs belonged to the meat-eating family, evolved to become vegetarian.

"They have a long neck, small head, but they have very, very large hands and very, very long claws on their four limbs suggesting that was for defence," he said.

"These animals were relatively big, they were around seven to nine metres in length, so way too big to fly. And they would have been covered with feathers, but very primitive types of feathers … hairy and light. They would not have had wings and would have been unable to fly."

Moving forward, Therrien says, they're on the lookout for fossils, but more that that, they want to know what the rocks have to say. 

"Because sometimes you have an indication of behaviours that may not be preserved by the fossils themselves, but the rock record can help you to to interpret the behaviours," he said. "In this case, being able to identify that these animals were nesting as a colony and that parents were guarding the colony."