Clever, bird-like dinosaurs that lived 74 million years ago got cozy in communal nests, study suggests

An illustrated reconstruction of Troodon, a nonavian theropod dinosaur that lived in the late Cretaceous. (Image credit: Alex Boersma/PNAS)

Bird-like dinosaurs that lived up to 74 million years ago did not have pig nests; Instead, these beaked dinosaurs shared communal nests where several female mates often laid more than 20 eggs together, which these feathered dinosaurs then woke up to keep warm, a new study suggests.

Most dinosaurs are thought to have laid eggs “in bulk” and buried them in the ground to incubate, as crocodiles and other cold-blooded reptiles do today. But not Troodon, a small predatory dinosaur that was closely related to modern birds and lived in the late Cretaceous, 74 million to 66 million years ago. Researchers were already aware of scratching behavior in some theropods — a group of bipedal, mostly carnivorous dinosaurs that includes Tyrannosaurus rex and Velociraptor — and recorded an ultra-rare dinosaur fossil digging on its eggs in 2021.

Now, fossilized eggshells have revealed that Troodons were endothermic, meaning they were warm-blooded and could self-regulate their body temperature. This confirms that dinosaurs could maintain their body temperature high enough to lay their eggs.

“Our study shows that Troodon had a high body temperature, like birds, so it would probably have been able to provide heat for incubation while sitting on its eggs,” Darla Zelenitsky (opens in new tab), a paleontologist at the University of Calgary. in Canada and co-author of the study in which the finding was published, told Live Science in an email.

Additionally, these dinosaurs could probably switch between a warm-blooded state and a cold-blooded state—a strategy common in modern birds, called heterothermy. Troodon maintained a body temperature of about 107.6 degrees Fahrenheit (42 degrees Celsius), but could drop to 29 degrees Celsius to cope with limited food or harsh weather, Zelenitsky said.

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In the study, published Monday (April 3) in the journal Proceedings of the National Academy of Sciences (opens in new tab), the researchers examined patterns of eggshell mineralization in eggs belonging to the species Troodon formosus and compared it with those of modern birds. and reptiles. They used a technique that analyzed the calcite content of the eggshells and determined the speed and temperature at which they formed.

“Birds and reptiles have two different patterns of mineralization when it comes to eggshell production,” study lead author Mattia Tagliavento (opens in new tab), a postdoctoral researcher in paleontology at Goethe University, told Live Science in Frankfurt, Germany. “Since reptiles and birds are, respectively, the ancestors and descendants of dinosaurs, dinosaurs must have either one of the two, or be somewhere in between.”

An illustration of Troodon, a bird-like dinosaur that lived in the late Cretaceous, 74 million to 66 million years ago. (Image credit: Masato Hattori)

Birds have a functional ovary, having lost the other early in their evolution, likely to limit their weight and facilitate flight. To counterbalance this loss, they evolved the ability to precipitate calcite and quickly form eggshells, producing several relatively large eggs per clutch. On the other hand, reptiles have two functional ovaries that produce several small eggs at a time. The mineralization process is much slower than in birds, however, which limits the number of eggs they can lay.

The researchers were surprised to find that T. formosus retained its slow, reptile-like mineralization of the eggshell, despite having already evolved bird-like traits such as heterothermy. “With our work, we show that Troodon was probably in between, producing just two eggs at a time and laying them — not a proper ‘mass’ production, but at a rate that’s still slower than modern birds,” Tagliavento said.

An illustration of Troodon, a non-avian theropod dinosaur from the Late Cretaceous that probably laid eggs in communal nests. (Image credit: Masato Hattori)

The team also used estimates of Troodon’s weight and the amount of calcite per eggshell to calculate that a female could have laid four to six eggs per clutch, which did not match the number of eggs typically found in their nests. “A single individual could not have laid more than 20 eggs in a reasonable amount of time to ensure the survival of the eggs and embryos,” Zelenitsky said.

This suggests that the females were nestmates, laying and caring for their eggs in the company of others.

The discovery sheds light on the evolutionary transition from cold-blooded to endothermic in dinosaurs, Tagliavento said. “We may have a chance to explore the whole lineage of dinosaurs and see exactly where the change happened,” he said.