Troodon formosus, a small North American theropod dinosaur, incubated its eggs in a similar way to brooding birds, according to paleontologists at the University of Calgary and Montana State University.

Using egg clutches found in Alberta and Montana, the researchers closely examined the shells of fossil Troodon eggs. In a paper published in the journal Paleobiology, they concluded that Troodon, which was known to lay its eggs almost vertically, would have only buried the egg bottoms in mud.

“Based on our calculations, the eggshells of Troodon were very similar to those of brooding birds, which tells us that this dinosaur did not completely bury its eggs in nesting materials like crocodiles do,” said study co-author Dr Darla Zelenitsky of the University of Calgary.

“Both the eggs and the surrounding sediments indicate only partial burial; thus an adult would have directly contacted the exposed parts of the eggs during incubation,” said lead author Prof David Varricchio from Montana State University.

“While the nesting style for Troodon is unusual, there are similarities with a peculiar nester among birds called the Egyptian Plover that broods its eggs while they’re partially buried in sandy substrate of the nest.”

Researchers have always struggled to answer the question of how dinosaurs incubated their eggs, because of the scarcity of evidence for incubation behaviors. As dinosaurs’ closest living relatives, crocodiles and birds offer some insights.

Paleontologists know that crocodiles and birds that completely bury their eggs for hatching have eggs with many pores or holes in the eggshell, to allow for respiration. This is unlike brooding birds which don’t bury their eggs; consequently, their eggs have far fewer pores.

The team counted and measured the pores in the shells of Troodon eggs to assess how water vapor would have been conducted through the shell compared with eggs from contemporary crocodiles, mound-nesting birds and brooding birds. They are optimistic their methods can be applied to other dinosaur species’ fossil eggs to show how they may have been incubated.

“For now, this particular study helps substantiate that some bird-like nesting behaviors evolved in meat-eating dinosaurs prior to the origin of birds. It also adds to the growing body of evidence that shows a close evolutionary relationship between birds and dinosaurs,” Dr Zelenitsky said.

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Bibliographic information: David J. Varricchio et al. Porosity and water vapor conductance of two Troodon formosus eggs: an assessment of incubation strategy in a maniraptoran dinosaur. Paleobiology, vol. 39, no. 2, pp. 278-296