They were heavier than hippos, but dinosaurs like gigantoraptor were dedicated parents who sat for weeks on nests full of eggs just as today's birds do — and found a unique way to avoid crushing them, a Canadian-led study has found.
The researchers examined and measured about 40 fossil nests, mostly from China, each containing up to 30 eggs, of bird-like dinosaurs with parrot-like beaks called oviraptorosaurs. They found that the smallest were laid in a cluster like bird eggs, suggesting that the parents sat on top of them as birds did.
But larger nests — which could be up to 3.5 metres wide or the size of a small above-ground pool — took on a ring or donut shape.
"In the largest oviraptorosaur clutches (Macroelongatoolithus), the central opening represents most of the total clutch area, likely allowing giant-sized species to rest their entire weight on this area so as not to crush the eggs," reported the paper published Tuesday in the Royal Society journal Biology Letters.
"This adaptation may have allowed for an adult to sit on the nest and potentially even allow some contact with the eggs in the largest oviraptorosaurs."
Oviraptorosaurs were related to birds and to other two-legged meat-eating dinosaurs such as velociraptors. The first specimen, was found in China in the 1920s on a clutch of eggs, and was named "oviraptor" or "egg stealer" because the eggs were thought to belong to another kind of dinosaur.
Since then, many oviraptor eggs have been found, including some with embryos in them, and scientists now think the first oviraptor was a parent of the eggs it was found with.
Many species have been found, ranging from turkey sized to the huge gigantoraptor, which was eight metres long from beak to tail and would have towered over humans. Specimens have been found sitting on nests, females have been found with eggs inside them, and eggs have been found with embryos inside them.
While most nests have been found in Asia, Darla Zelenitsky an assistant professor of geoscience at the University of Calgary who co-led the new study, has done research on one nest found in Alberta.
The nests in the new study came from the late Cretaceous, ranging in age from 104 million to 67 million years old.
The eggs of oviraptorosaurs are elongated like baking potatoes, said Zelenitsky, who led the study with her PhD student Kohei Tanaka, now a postdoctoral researcher at Nagoya University.
Some of the eggs are as large as 47 centimetres long — about the size of a watermelon, making them the biggest dinosaurs eggs known, Zelenitsky added.
Nests contained up to 30, arranged in a "very neat ring configuration" with two or three layers carefully stacked on top of each other.
It's a unique structure. Zelenitsky says no other dinosaurs build their nests in that shape, and no living animals incubate their eggs this way.
"I just think it's really neat that we're able to say something more about the nesting behaviours and how they changed in these oviraptorosaur dinosaurs among the various species and species sizes," she said.
The researchers aren't sure why the oviraptorosaurs sit on their eggs. If it was to keep the eggs warm, those that sat in the middle of the ring probably couldn't transfer heat as effectively those ones that sat directly on the eggs. But they did have feathered wings, Zelenitsky said, so they could still have provided shelter and insulation, along with protection from predators.
The researchers don't know whether the parents cared for the young after they hatched.